# A holographic bottom-up description of light nuclide spectroscopy and
stability

Miguel Angel Martin Contreras<EMAIL_ADDRESS>Escuela de Ciencias
Universidad Viña del Mar,
Viña del Mar, Chile  Corporación Universitaria Minuto de Dios–UNIMINUTO
Unidad de Ciencias Básicas
Cundinamarca, Colombia  Alfredo Vega<EMAIL_ADDRESS>Instituto de Física y
Astronomía,
Universidad de Valparaíso,
A. Gran Bretaña 1111, Valparaíso, Chile  Saulo Diles<EMAIL_ADDRESS>Campus
Salinópolis,
Universidade Federal do Pará,
68721-000, Salinópolis, Pará, Brazil

###### Abstract

This work explores a holographic proposal to describe light nuclide
spectroscopy by considering extensions to the well-known bottom-up AdS/QCD
proposals, the hardwall and softwall models. We also propose an alternative
description inspired by the Woods-Saxon potential. We find the static dilaton
associated with this potential in this Wood-Saxon-like model. We compute the
nuclide spectra finding that, despite their pure AdS/QCD origin, hardwall and
softwall, as monoparametric models, have good accuracy and precision since the
RMS error is near 11$\%$ and 4 $\%$ respectively. In the case of the Wood-
Saxon model, the RMS was around 1$\%$. We also discuss configurational entropy
as a tool to categorize which model is suitable to describe nuclides in terms
of stability. We found that configurational entropy resembles a stability
line, independent from nuclear spin, for symmetric light nuclides when
considering softwall and Wood-Saxon-like models. For the hardwall case,
configurational entropy, despite increasing with the constituent number,
depends on the nuclear spin. Thus, the Woods-Saxon-like model emerges as the
best choice to describe light nuclide spectroscopy in the bottom-up scenario.

##  I Introduction

Holography nowadays is one of the most used tools to describe non-perturbative
phenomena. It is not restricted to hadronic physics only (glueballs, mesons,
and baryons[1, 2, 3, 4, 5, 6, 7]). There are applications to heavy ion
collisions [8, 9], condensed matter [10], neutron stars [11, 12], or fluid
mechanics [13, 14, 15, 16]. After more than two decades from Maldacena’s
seminal work [17], holography seems to provide a fruitful soil to develop
effective models to approach non-perturbative phenomena.

Among the non-perturbative systems available in nature, one of the most
challenging is the nuclear realm. The nuclear force observed in nucleon
systems is a low energy strong force residual approximately, thus implying it
is non-perturbative. At the holographic level, this hypothesis allows using
bottom-up models to describe such phenomenology. This manuscript will focus on
describing the light nuclide spectrum using holographic tools.

There are significant challenges in describing composite nucleus spectroscopy
candidates in holography. The holographic spectrum should describe the nuclear
mass as a function of the atomic $Z$ and mass $A$ numbers. In the case of
hadrons, the spectra are organized by defined mass poles, the so-called Regge
Trajectories, where each excited state defines a new hadron in the family, as
in the AdS/QCD bottom-up radial case [18, 19, 5]. In the nuclear case, the
excited nuclear states come from transitions defining metastable states that
do not differ so much from the nucleus ground state mass. Thus, the
holographic nuclear mass spectrum should not have a large mass gap compared to
the ground state mass.

Another ingredient to take into account is the holographic dictionary. In the
hadronic case, the conformal dimension is connected with the operator that
creates hadrons at the boundary. When we consider the twist operator, the
conformal dimension associated with the bulk dual field is translated into the
hadronic constituent number [20, 21]. In the nuclear case, we expect the same
behavior. Since atomic and mass numbers characterize the nucleus, the
conformal dimension of the dual bulk field should carry this constituent
information. Therefore a sensible AdS/QCD model of light nuclei should capture
the constituent nucleon dependence on the dual bulk field conformal dimension.

The discussion on nuclei stability analyses is a more subtle problem. Nuclei
are composite states of two different components, i.e., protons and neutrons.
Each one is composed of three first-generation valence quarks. So, their
baryonic numbers are far from vanishing, and it is not consistent with
associating nuclear stability with leptonic annihilation. This fact excludes
the possibility of making a stability analysis through electromagnetic decay
constants, as mesonic holography literature proves [3, 7, 22]. Nuclear
stability is associated with the arrangement of nucleons. Thus, looking for
the associated information with the different configurations is natural. In
this context, the concept of configurational entropy (CE) emerges as a natural
observable to account for the nuclear stability [23, 24]. Fortunately, in the
last years, the holographic calculation of configurational entropy has been
extensively discussed in the context of bottom-up models [25, 26, 27, 28, 29].

The summary of this work is: in section II we provide a holographic model for
light nuclei in the context of bottom-up models. We discuss how computing the
mass spectrum for light-nuclides in the context of the so-called hardwall and
softwall models, and a new approach, the hybrid Woods-Saxon model. Section III
addresses nuclear stability from the perspective of configurational entropy.
Finally, Section IV presents conclusions for this work.

##  II Holographic model for light nuclide

In the AdS/QCD _a la_ bottom-up, it has been established that colored medium
properties observed at the boundary system are encoded into bulk fields and
the bulk metric. In nuclear systems, considered as a system of nucleons
interacting through a strong residual force, we can extend the hypothesis used
in hadronic holography to the nuclear medium. The intensity of this force,
responsible for keeping the nucleus cohered, depends on the mass number $A$
and the atomic number $Z$, i.e., the constituent number. Thus, it is quite
natural to extend the bottom-up confinement procedure to mimic the nuclear
force.

Following the ideas above, a given nucleus endowed with an atomic number $Z$
and $A$ nucleons ($Z$ protons and $A-Z$ neutrons), i.e., a nuclide, will be
dual to normalizable bulk mode. Thus, nuclides in this formulation are
considered fundamental objects composed by $A$ nucleons, i.e., we neglect the
nuclide inner configuration. For simplicity, we will focus on symmetric
nuclides, i.e., those with $A=2\,Z$.

Our starting point is to consider the nuclide, defined by a mass number $A$,
and the spin $p$, characterized by a $p$-form bulk field
$A_{p}(\zeta,x^{\mu})$. In the bulk, these $p$-forms obey the following
general action principle

$I_{\text{Bulk}}=\int{d^{5}x\,\sqrt{-g}\,e^{-\Phi(\zeta)}\,\mathcal{L}_{\text{Nuclide}}},$
(1)

where the dilaton field $\Phi(\zeta)$ defines the confinement mechanism, and
$\mathcal{L}_{\text{Nuclide}}$ defines the $p$-form Lagrangian density that
sets the dual physics in the bulk. This lagrangian density is defined as

$\mathcal{L}_{\text{Nuclide}}=\frac{\left(-1\right)^{p}}{2}\,\times\\\
\left[\frac{1}{g_{p}^{2}}\,g^{mn}\,g^{m_{1}\,n_{1}}\ldots\,g^{m_{p}\,n_{p}}\nabla_{m}\,A_{m_{1}\ldots
m_{p}}\,\nabla_{n}\,A_{n_{1}\ldots n_{p}}\right.\\\
\left.-M_{5}^{2}\,g^{m_{1}\,n_{1}}\ldots g^{m_{p}\,n_{p}}\,A_{m\dots
m_{p}}\,A_{n\ldots n_{p}}\right].$ (2)

Notice that $g_{\text{p}}$ is a coupling that sets units in the bulk action.

We will consider the bulk manifold described by the five-dimensional AdS space
parametrized by the Poincaré Patch, defined as

$dS^{2}=\frac{R^{2}}{\zeta^{2}}\left(d\zeta^{2}+\eta_{\mu\nu}\,dx^{\mu}\,dx^{\nu}\right),$
(3)

where $R$ is the AdS curvature Radius and $\zeta$ represents the holographic
coordinate and the Greek indices label the minkowskian coordinates. The
conformal boundary lies at $\zeta\to 0$ as usual.

After choosing a transverse gauge [1], i.e.,

$\displaystyle g^{m_{1}m_{2}}\,A_{m_{1}\,m_{2}\ldots m_{p}}$ $\displaystyle=$
$\displaystyle 0$ (4) $\displaystyle\nabla^{m_{1}}\,A_{m_{1}\,m_{2}\dots
m_{p}}$ $\displaystyle=$ $\displaystyle 0,$ (5)

and performing a Fourier decomposition, the action defined above brings the
following set of equations of motion

$\partial_{\zeta}\left[e^{-B(\zeta)}\,\psi^{\prime}(\zeta)\right]+M_{0}^{2}\,e^{-B(\zeta)}\,\psi(\zeta)-\frac{M_{5}^{2}\,R^{2}}{\zeta^{2}}\,e^{-B(\zeta)}\,\psi(\zeta)=0,$
(6)

where we have defined $A_{m_{1}\ldots m_{p}}(\zeta,q)=A_{m_{1}\ldots
m_{p}}(q)\,\psi(\zeta)$, with $A_{p}(q)$ being the Schwinger source of the
operators that create nuclides at the boundary as composite objects and
$\psi(\zeta)$ defines the bulk eigenmode. The function
$B(\zeta)=\Phi(\zeta)+\beta\,\log\left(R/\zeta\right)$ encloses the
geometrical effect associated to the geometry and the confining dilaton, and
$\beta=-(3-2\,p)$ fixes the bulk field spin: for scalar fields we have
$\beta=-3$ and for vector fields, $\beta=-1$. Notice that we have considered
the on-shell mass $-q^{2}=M_{0}^{2}$, defining the nuclide mass. The bulk mass
$M_{5}^{2}$ defines the _nuclide identity_.

When we apply the confinement criterium, the normalizable ground state
$\psi(\zeta)$ will be dual to the eigenstate associated with the nuclide. The
eigenvalue of this bulk $p$-form defines the nuclide mass through the
holographic Schrodinger-like potential $V(\zeta)$ constructed by applying the

$\displaystyle V(\zeta)$ $\displaystyle=$
$\displaystyle\frac{1}{4}B^{\prime}(\zeta)^{2}-\frac{1}{2}B^{\prime\prime}(\zeta)+\frac{M_{5}^{2}\,R^{2}}{\zeta^{2}}.$
(7)

This particular picture, provided by the bottom-up models, summarizes the
nucleon many-body phenomena in the behavior of geometrically confined bulk
fields. In bottom-up models, the confinement mechanism is defined by the
procedure to transform the continuum spectrum into a discrete one. We can do
this by deforming the geometry or adding a dilaton profile.

Following the holography recipe, the conformal dimension of the bulk $p$-form
field, i.e., $\Delta$, is dual to the dimension of the operator creating the
nuclide, $\text{dim}\,\mathcal{O}$, living at the conformal boundary at
$\zeta\to 0$. This is captured into the holographic dictionary as

$A_{p}(\zeta,q)\propto A_{p}(q)\,\zeta^{\Delta-p}.$ (8)

We can write the conformal dimension $\Delta$ in terms of the twist, which
accounts for the nuclide constituents, as

$\Delta\to\text{dim}\,\mathcal{O}=\tau+L,$ (9)

where $L$ is the angular momentum number.

We will consider nucleons as _constituent objects_. Thus, their associated
twist is one. In this context, a given light nucleus should be identified with
a _bag_ with $N$ nucleons, symmetric under SU$(2)$ as in the Heisenberg
Isospin model. Then, the main difference between the bottom-up formulation for
QCD and nuclear spectroscopy lies in how we consider the twist. In the former
case, twist comes from constituent quarks. In the latter, twist comes from
nucleons. As a first approximation, we will consider all nucleons in s-wave,
i.e., $L=0$.

For a general $p$-form, spanned in Fourier space as
$A_{p}(\zeta,q)=A_{p}(q)\,\psi(\zeta)$, we have in the limit $\zeta\to 0$ that
the confinement mechanism is not relevant. Thus, the equations of motion
reduce to those in pure AdS:

$\left(\frac{\zeta}{R}\right)^{-\beta}\,\partial_{\zeta}\left[\left(\frac{\zeta}{R}\right)^{\beta}\,\psi^{\prime}\left(\zeta\right)\right]+(-q^{2})\,\psi(\zeta)-\frac{M_{5}^{2}\,R^{2}}{\zeta^{2}}\,\psi(\zeta)=0,$
(10)

where the prime denotes derivative with respect to $\zeta$, the parameter
$\beta=-(3-2\,p)$ accounts for $p$-form index effect in the equations, and
$M_{5}^{2}$ defines the $p$-form bulk mass.

The solution for this equation is written in terms of Bessel functions of
first kind $J_{n}(x)$ as

$\psi(\zeta)=A(q)\,\zeta^{\frac{1-\beta}{2}}\,J_{\left|\Delta-2\right|}\left(M_{0}\,\zeta\right)$
(11)

and for the bulk mass we have

$\displaystyle M_{5}^{2}\,R^{2}$ $\displaystyle=$
$\displaystyle\left(\Delta-p\right)\left(\Delta-p-1+\beta\right)$ (12)
$\displaystyle=$ $\displaystyle\left(\Delta-p\right)\left(\Delta+p-4\right).$

This expression plays a fundamental part since it will modify the behavior of
the holographic nuclide ground state in terms of the constituent mass.

Once we have defined the bulk mass in terms of the conformal dimension, which
carries information about nuclide composition and spin, we will write the
holographic potential as

$V\left(\zeta\right)=\frac{15-16\Delta+4\Delta^{2}}{4\,\zeta^{2}}+\frac{(3-2\,p)}{2\,\zeta}\,\Phi^{\prime}+\frac{1}{4}\,\Phi^{\prime
2}-\frac{\Phi^{\prime\prime}}{2}.$ (13)

Solving the potential above, we obtain the nuclide mass spectrum $M_{0}(Z)$
and the Schrodinger-like modes $\phi_{0}^{Z}(\zeta)$ associated with nuclides
at the conformal boundary labeled by their atomic number $Z$, recalling that
the constituent number is $\Delta=A=2\,Z$ for symmetric nuclides.

Holographic light nuclide spectrum
---
Experimental nuclide data | Hard Wall Model | Soft Wall Model | Wood-Saxon-like Model
$Z$ | Nuclear Spin |  $M_{0}^{\text{Exp}}$ (u) |  $M_{0}^{\text{Th}}$ (u) | Rel. Error (%) |  $M_{0}^{\text{Th}}$ (u) | Rel. Error (%) |  $M_{0}^{\text{Th}}$ (u) | Rel. Error (%)
2 | 0 | 4.00260 | 4.00260 | 0.00 | 4.002603 | 0.00 | 4.00391 | 0.01
3 | 1 | 6.01511 | 5.91421 | 1.68 | 5.480791 | 8.88 | 6.10883 | 2.34
4 | 0 | 8.00530 | 7.74401 | 3.26 | 8.005206 | 0.01 | 8.16760 | 2.70
5 | 3 | 10.0129 | 9.52800 | 4.84 | 8.372045 | 16.4 | 10.2040 | 2.39
6 | 0 | 12.0000 | 11.2819 | 5.98 | 12.00781 | 3.59 | 12.2212 | 2.21
7 | 1 | 14.0030 | 13.0143 | 7.06 | 13.49952 | 3.59 | 14.2297 | 1.89
8 | 0 | 15.9949 | 14.7303 | 7.91 | 16.01302 | 0.09 | 16.2301 | 1.68
9 | 1 | 18.0009 | 16.4333 | 8.71 | 17.50424 | 2.76 | 18.2244 | 1.40
10 | 0 | 19.9949 | 18.1259 | 9.34 | 20.01301 | 0.10 | 20.2137 | 1.23
11 | 3 | 21.9944 | 19.8096 | 9.93 | 20.45835 | 6.98 | 22.1989 | 1.02
12 | 0 | 23.9850 | 21.4859 | 10.4 | 24.01561 | 0.13 | 24.1805 | 0.89
13 | 5 | 25.9869 | 23.1558 | 10.9 | 23.38185 | 10.1 | 26.1593 | 0.72
14 | 0 | 27.9769 | 24.8201 | 11.3 | 28.01822 | 0.15 | 28.1374 | 0.61
15 | 1 | 29.9783 | 26.4794 | 11.7 | 29.51496 | 1.54 | 30.1092 | 0.47
16 | 0 | 31.9721 | 28.1343 | 12.0 | 32.02082 | 0.15 | 32.0811 | 0.36
17 | 0 | 33.9738 | 29.7853 | 12.3 | 34.02213 | 0.14 | 34.0512 | 0.24
18 | 0 | 35.9675 | 31.4328 | 12.6 | 36.02349 | 0.15 | 36.0196 | 0.15
19 | 3 | 37.9691 | 33.0770 | 12.9 | 36.49290 | 3.89 | 37.9867 | 0.05
20 | 0 | 39.9626 | 34.7183 | 13.1 | 40.02603 | 0.16 | 39.9523 | 0.03
21 | 0 | 41.9655 | 36.3569 | 13.3 | 42.02733 | 0.15 | 41.9168 | 0.12
22 | 0 | 43.9597 | 37.9929 | 13.6 | 44.02863 | 0.16 | 43.8801 | 0.18
23 | 0 | 45.9602 | 39.6267 | 13.8 | 46.02994 | 0.15 | 45.8425 | 0.27
24 | 0 | 47.9542 | 41.9540 | 14.0 | 48.03124 | 0.16 | 47.8038 | 0.33
25 | 0 | 49.9542 | 42.8880 | 14.1 | 50.03254 | 0.16 | 49.7643 | 0.40
26 | 0 | 51.9481 | 44.5158 | 14.3 | 52.03384 | 0.16 | 51.7240 | 0.45
27 | 0 | 53.9484 | 46.1418 | 14.5 | 54.03644 | 0.16 | 53.6829 | 0.51
28 | 0 | 55.9421 | 47.7663 | 14.6 | 56.03644 | 0.17 | 55.6411 | 0.56
29 | 1 | 57.9445 | 49.3891 | 14.8 | 57.53525 | 0.71 | 57.5987 | 0.62
30 | 0 | 59.9418 | 51.0106 | 14.9 | 60.03905 | 0.16 | 59.5555 | 0.67
Table 1: This table summarizes the light nuclide spectrum running from $Z=2$
(He) up to $Z=30$ (Ca) symmetric nuclei. We have used $\Lambda_{N}=0.7794$ u
for the hardwall, $\kappa_{0}=1.0006$ u for the softwall and
$A_{1}=A_{2}=1.863$ u GeV and $B=2.5$ u for the Woods-Saxon-like model.
Experimental masses read from [30].

###  II.1 Bottom-up holographic models for the light nuclide masses

Among the AdS/QCD models, the most successful in describing hadronic
properties, particularly hadronic spectra, are the so-called hardwall and
softwall models. In such models, the main idea is to place confinement as a
geometrical deformation. Confinement in the bulk implies the emergence of
bounded states, which will be dual to hadronic states living at the boundary.
The dilaton field $\Phi(\zeta)$ (see eqn. (6) and (13) for the holographic
potential) categorizes these bottom-up models.

In the first place, we will consider the so-called hardwall model [31, 32]. We
fix the dilaton to zero and place a hard cutoff $\Lambda_{N}$ (a D-Brane) in
the holographic coordinate to raise bounded states in a similar form as the
infinite square well does in quantum mechanics. In this situation, the
spectrum is given in terms of Bessel function zeroes $\alpha_{n,m}$, depending
on the hard cutoff, and the number of constituents as:

$M_{0}(\Delta)=\Lambda_{N}\,\alpha_{\Delta-2,1}\,\,\,\text{with}\,\Lambda_{N}=\frac{M_{{}^{4}_{2}\text{He}}}{\alpha_{2,1}}.$

Masses for the light nuclide spectrum are summarized in table 1.

Another well-known bottom-up approach we will discuss in this manuscript is
the softwall model [33]. This approach considers setting the dilaton field as
$\Phi(\zeta)=\kappa^{2}\,\zeta^{2}$. This choice ensures that the mass
spectrum is linear with the excitation number. The dilaton slope $\kappa$
carries information about the quark-antiquark strong interaction. In AdS/QCD,
the emergence of the linear spectrum consistent with the Regge theory at the
boundary is a clear signal of confinement in bulk. In the case of the softwall
model, the quadratic dilaton brings a holographic potential that resembles the
2-dimensional radial harmonic potential with a general linear spectrum given
generically by $M_{n}^{2}=A\,\kappa^{2}(n+B)$. Thus, $\kappa$ also defines
holographically the Regge slope.

For the light nuclide case, following the intuition that nuclear force depends
on the mass number $A$ and that $\kappa$ carries information about the
interaction, an educated guess is to consider that nuclear $\kappa$ scales
with the mass number as $\kappa=\sqrt{\frac{\Delta}{2}}\,\kappa_{0}$. The
scale $\kappa_{0}$ is an energy scale associated with the proton mass that
fixes energy units. Thus, the mass spectrum for light nuclides is given by

$M_{0}^{2}(\Delta)=\Delta\,\kappa_{0}^{2}\left(\Delta-p\right).$ (14)

The calculation of the light nuclide masses using the spectrum given above is
summarized in table 1.

Notice that in both hardwall and softwall, we consider that the constituent
number $\Delta$ instead of the excitation number (as in AdS/QCD), fixed to the
ground state, defines light nuclide masses.

In order to quantify the accuracy and precision exhibited by the models, we
will follow the RMS analysis. For a model with $N$ parameters used to fit $M$
observables $\mathcal{O}_{i}$, having relative deviation
$\delta\,\mathcal{O}_{i}$ with the model outcomes, the RMS error is calculated
as

$\delta_{\text{RMS}}=\sqrt{\frac{1}{M-N}\,\sum_{i}^{M}\left(\frac{\delta\,\mathcal{O}_{i}}{\mathcal{O}_{i}}\right)^{2}}.$
(15)

For the hardwall model the RMS error fitting 29 states with one single
parameter $\Lambda_{N}$ is $11.6\,\%$. In the case of the softwall model, the
RMS associated with 29 masses fitted with one parameter $\kappa_{0}$ is
$4.4\,\%$.

Hardwall and softwall models accurately describe the light nuclei mass
spectrum. However, both potentials have infinite bounded states,
holographically dual to light nuclei. In principle, this mass tower is stable
and does not decay, implying that heavy nuclei, with large values of $\Delta$,
are stable, which phenomenologically is not accurate. Also, the shifting
between excited states keeps constant, which is not expected in the nuclear
case, where the excited states of a given nuclide are connected with decay
processes. These decays do not change the ground mass drastically.

Thus, to introduce a finite set of stable ground states regarding the
constituent number $\Delta$ with a small enough mass shifting with the ground
state mass, we will formulate a hybrid holographic potential with a Wood-
Saxon-like profile. We will provide a deeper discussion in the next section.
Then, we will reconstruct the associated dilaton associated with this
potential. As in the softwall model case, the dilaton will be dependent on the
mass number and nuclear spin. It is interesting to notice that Woods-Saxon
potential emerges as a good tool to describe nuclear spectra, however, coming
from a holographic perspective, different from the original nuclear shell
model. Such a model has been quite successful in describing the nuclear
structure and provides properties of bound-state and continuum single-particle
wavefunctions such as nuclear single-particle energies or nuclear radii
calculations. However, WS potential (or any other single-particle) does not
help compute total binding energies since it is not based on a specific two-
body interaction. This single-particle potential proposed in [34] has been
used as a less complicated alternative to other multi-particle approaches, as
the standard Hartree-Fock calculation (see for example [35, 36]). WS-like
potential for heavy nuclei provides a good description of nucleon energy
levels. Their uses have been extended to other physics branches beyond nuclear
physics, such as confined systems in condensed matter [37].

The key point of the proposed inverse holographic engineering is the dilaton
reconstruction. At sufficient large values of $\zeta$, we expect that the
holographic potential acquires a softened profile, flowing asymptotically to a
constant value, opposite as in the AdS/QCD soft-wall model, where the
potential goes asymptotically to infinite. The dilaton controls the asymptotic
evolution of the holographic potential. Thus, by fixing the asymptotic form of
the potential (7) with a Woods-Saxon profile, according to the expression

$\left[A_{1}-\frac{A_{2}}{1+\text{exp}\left(\frac{\zeta-B}{\Delta}\right)}\right]\Delta^{2}=\frac{(3-2\,p)}{2\,\zeta}\,\Phi^{\prime}+\frac{1}{4}\,\Phi^{\prime
2}-\frac{\Phi^{\prime\prime}}{2},$ (16)

we can compute the dilaton field for each nuclide. We have supposed that the
depth size in the potential depends on the constituent number encoded in
$\Delta$.

With this profile we can compute the holographic potential and the holographic
light nuclei mass. The table 1 summarizes the numerical results in this model.
It is remarkable that the holographic reconstruction of the Woods-Saxon
potential provides a very precise model for the nuclear masses. It is a clear
case where the dynamics of the bulk modes with respect to the holographic
direction captures the spectral properties of the boundary modes dynamics with
respect to the radial direction in coordinate space.

In the Woods-Saxon-like model, having three parameters $A_{1}$, $A_{2}$, and
$B$ to control the potential well size, modeling 29 light nuclide masses
brings an RMS error, following eqn. (15), around $1.2\,\%$.

###  II.2 Holographic nuclide spectra

Holographic ${}_{20}^{40}$Ca excited states
---
Hardwall | Softwall | Woods-Saxon-like
$n$ |  $M_{n}$ (u) |  $\Delta\,M_{n}$(u) | $n$ |  $M_{n}$ (u) |  $\Delta\,M_{n}$(u) | $n$ |  $M_{n}$ (u) |  $\Delta\,M_{n}$(u)
0 | 34.7183 | 0 | 0 | 40.02603 | 0 | 0 | 39.9523 | 0
1 | 38.8472 | 4.1289 | 1 | 41.0145 | 0.9884 | 1 | 40.0085 | 0.0562
2 | 42.4237 | 7.7054 | 2 | 41.9796 | 1.9536 | 2 | 40.0641 | 0.1117
3 | 45.7299 | 11.012 | 3 | 42.9231 | 2.8971 | 3 | 40.1187 | 0.1665
4 | 48.8703 | 14.152 | 4 | 43.8463 | 3.8203 | 4 | 40.1728 | 0.2206
5 | 51.8971 | 17.179 | 5 | 44.7505 | 4.7244 | 5 | 40.2262 | 0.2740
Table 2: This table summarizes the ${}^{40}_{20}$Ca ground state (in bold
font) with the first five excited radial states for each holographic model
considered. In the hardwall and softwall models, the energy shifting between
radial levels and the ground state $\Delta\,M_{n}=M_{n}-M_{0}$ grows with the
excitation level beyond the single nucleon mass. In the case of the Woods-
Saxon-like approach, the energy shifting is less than the nucleon mass.

Let us devote a few comments on the holographic nature of the calculated
nuclide spectra, summarized in table 1. Recall that masses in table 1 are
composed by ground states of each holographic potential, characterized by
$\Delta$, according to eqn. (13). In the case of hardwall and softwall models
coming from AdS/QCD, it is interesting to wonder about the validity of these
models applied to the nuclear realm and then extrapolate to the Woods-Saxon-
like approach.

In the three models discussed above, the key ingredient is the holographic
potential that gives rise to the radial eigenvalue spectrum, i.e., defined in
terms of the excitation level. In the hadronic case, the energy shifting
between levels is high enough to consider each excitation as a metastable
hadronic state. However, the energy shift is not high enough in the nuclear
context compared with nuclide mass. Thus, excitation levels in the radial case
correspond with the energy transitions between nucleons in the nuclide that,
in essence, do not change the nucleus mass far from a nucleon mass, depending
on the nature of the energy transition, which would imply changing the atomic
number $Z$, leaving the mass number $A$ untouched, or gamma transitions
leaving both $Z$ and $A$ intact. These last nuclides, which are not in the
ground state, with nucleons in levels above the ground energy, but leaving
unchanged the atomic number and mass number, are called _isomers_. Recall that
we are considering transitions that leave the nucleon number in the nuclide
unaltered. We will consider this last affirmation as a criterion to test the
validity of a given holographic model describing nuclear spectroscopy.

Table 2 summarizes the ground state and the first five excited radial states
calculated in each holographic model considering the Ca nuclide. In the case
of hardwall and softwall models, the energy shifting between radial states and
the ground state grows with the excitation number of several nucleon masses.
Thus, the excited states cannot be considered the same nuclide described by
the ground state. From the nuclear phenomenology, these sorts of transitions
are not allowed.

In the case of the Wood-Saxon-like model, the energy shifting between radial
level and the ground state is less than 20 $\%$ of the nucleon mass. Thus, in
the first five excited states, the nuclide mass does not change beyond one
nucleon mass in the case of He and remains almost the same for the Ca and Zn
cases. Thus, the Woods-Saxon-like model is more suitable for describing
nuclide spectroscopy than the AdS/QCD counterparts at the holographic level.

##  III Nuclear stability

Recently, it has been proposed that the configurational entropy (CE) of the
hadronic state works as a measure of its stability: the smaller the CE, the
more stable the hadron [38, 39, 40, 41]. In the holographic approach, the
spectral properties of a given hadron encode in the holographic bulk mode.
Thus, bulk modes contain the wave-function holographic necessary to compute
the differential configurational entropy of such a hadronic state. So, it is
natural to associate the configurational entropy of the holographic bulk mode
describing the nuclear state with its stability. Here we will perform the
holographic computation of the CE of the light nuclei in each of the three
different holographic models presented in the previous section.

In our case we restrict to bosonic nuclei state, that are integer spin
arrangements of many spin $1/2$ baryons. The dual description of bosonic
nuclei is encoded in a $p$-form field in the deformed $AdS_{5}$ space with
bulk action given by eq. (2). In this sense, the configuration of the bulk
mode appears in the functional dependence on the holographic direction
$\zeta$.

Configurational entropy measures the relationship between the informational
content of the physical solutions regarding their equations of motion. CE is
also a logarithmic measure of how spatially-localized solutions with given
energy content have spatial complexity. Thus, it measures information content
in the solutions to the equations of motion. CE has a connection with the
relative abundance related to the abundance of the hadronic states. We expect
the CE of atomic nuclei to grow with the atomic number.

In the original formulation, coming from information theory, CE can be
interpreted as measuring how much information is necessary to describe
localized functions, i.e., e.o.m. solutions, concerning their parameter set.
In general, dynamical solutions come from extremizing an action. CE measures
the available information in those solutions.

The association between CE and complexity is translated into stability. Since
CE measures the complexity of a given physical system, physical states with
higher CE require more energy to be produced in nature than their low CE
counterparts. More energy also implies more modes conforming such a physical
state, indicating CE increases with the coarseness degree. In this sense, CE
is also a measure of stability. Recall that CE measures the relative ordering
in field configuration space, showing how energy is related to coarseness. The
higher the constituents, the higher the energy and relative configurational
entropy. In addition, studies of active matter systems show that the
stationary final state of a many-particle system minimizes the configurational
entropy [42, 43]. In this sense, dynamical stability is related to
configurational entropy: the smaller the CE, the more stable the system.

Configurational entropy for a discrete variable with probabilities $p_{n}$ is
defined from the Shannon entropy as follows [44, 39, 4]

$S_{C}=-\sum_{n}\,p_{n}\,\text{log}\,p_{n}.$ (17)

In the case of continuous variables, we have the _differential configurational
entropy_ (DCE) defined as

$S_{C}\left[f\right]=-\int{d^{d}\,k\,\tilde{f}\left(k\right)\,\text{log}\,\tilde{f}\left(k\right)},$
(18)

where $\tilde{f}\left(k\right)=f\left(k\right)/f\left(k\right)_{\text{Max}}$
defines the modal fraction, $f(k)_{\text{Max}}$ is the maximum value assumed
by $f(k)$. Also we have that $f(k)\in\,L^{2}\left(\mathbb{R}^{2}\right)$ i.e.,
the square-integrable space of functions on the plane. This ensures that
$f(k)$ has a defined Fourier transform. Usually, this $f(k)$ function is
associated with the energy density in momentum space, $\rho(k)$. Thus, to
compute the DCE for a given physical system, we must address the following
algorithm:

  1. 1.

Obtain the localized solutions to the equations of motion.

  2. 2.

Evaluate the on-shell energy density.

  3. 3.

Transform to momentum space.

  4. 4.

Calculate the modal fraction.

  5. 5.

Evaluate the DCE integral given in the expression (18).

We will follow this prescription to compute the DCE for the three holographic
models discussed above.

---
Figure 1: Differential Configurational Entropy (DCE) for holographic models
considered as a function of the atomic number $Z$. In the left panel we plot
the hard wall DCE. The middle panel depicts the DCE for the soft wall model.
In the left panel, we depict the Woods-Saxon-like DCE result.

In the AdS/CFT context, the holographic approach to configurational entropy in
bottom-up and top-down AdS/QCD models was made in [38]. For hadronic states,
it was introduced in [39, 25, 45, 29, 46, 47] and references therein. In the
context of heavy quarkonium stability, DCE was used as a tool to explore
thermal behavior in a colored medium [26], in presence of magnetic fields [48]
or at finite density [49]. Recently, in [50] was used DCE to address the
holographic deconfinement phase transition in bottom-up AdS/QCD.

In our case, we will compute the DCE for the holographic nuclide starting from
the associated bulk stress-energy tensor

$T_{mn}=\frac{2}{\sqrt{-g}}\,\frac{\partial\left[\sqrt{-g}\,\mathcal{L}_{\text{Nuclide}}\right]}{\partial\,g^{mn}},$
(19)

which holds since the action does not depend on metric tensor derivatives.
From the action principle (2) we can compute the stress-energy tensor for
holographic nuclides:

$T_{mn}=-g_{mn}\,\mathcal{L}_{\text{Nuclide}}+(-1)^{p}\,e^{-\Phi}\,\left[\frac{1}{g_{p}^{2}}\,g^{m_{1}\,n_{1}}\ldots
g^{m_{p}\,n_{p}}\,\nabla_{m}\,A_{m_{1}\ldots
m_{p}}\,\nabla_{n}\,A_{n_{1}\ldots n_{p}}\right.\\\
\left.+p\,g^{m_{2}n_{2}}\ldots
g^{m_{p}n_{p}}\left(\frac{1}{g^{2}_{p}}\,g^{\sigma\rho}\,\nabla_{\sigma}\,A_{m\,m_{2},\ldots
m_{p}}\,\nabla_{\rho}\,A_{n\,n_{2}\ldots n_{p}}-M_{5}^{2}A_{m\,m_{2}\ldots
m_{p}}\,A_{n\,n_{2}\ldots n_{p}}\right)\right]$ (20)

The $p-$form bulk field can be spanned in terms of plane waves as

$A_{m_{1}\ldots m_{p}}(\zeta,x)=\epsilon_{m_{1}\ldots m_{p}}\,e^{-iq\cdot
x}\,\psi(\zeta).$ (21)

Once we define a polarization for the $p-$form field, since nuclides are
supposed to be at rest, we will choose a rest frame, i.e.,
$q=(M_{0},\vec{0})$. It is important to remark that eq.(20) consider a real
p-form field, while the plane wave modes are complex valued. The complex phase
is absorbed in the $\Omega$ factor and does not contribut to the DCE. By
taking the $00$-component, associated with the energy density
$\rho_{p}(\zeta)$, we obtain

$\rho(\zeta)\equiv
T_{00}=\frac{e^{-\Phi(\zeta)}}{2}\left(\frac{\zeta^{2}}{R^{2}}\right)^{p}\times\\\
\left\\{\left[\frac{1}{g_{p}^{2}}\left(M_{0}^{2}\,\psi^{2}+\psi^{\prime
2}\right)-\frac{M_{5}^{2}\,R^{2}}{\zeta^{2}}\psi^{2}\right]\right\\}\,\Omega,$
(22)

where $\Omega$ is a factor carrying plane wave and polarization contraction
factors. This factor becomes irrelevant during the modal fraction calculation.

The Fourier transform reads as

$\bar{\rho}(k)=\int_{0}^{\infty}d\zeta e^{ik\zeta}\rho(\zeta).$ (23)

The modal fraction is defined following [23] as

$f(k)=\frac{|\bar{\rho}(k)|^{2}}{\int dk|\bar{\rho}(k)|^{2}}.$ (24)

The differential configurational entropy for the holographic light nuclide is
then written as

$S_{DCE}=-\int dk\,\tilde{f}(k)\log\,\tilde{f}(k)$ (25)

where $\tilde{f}\left(k\right)=f\left(k\right)/f\left(k\right)_{\text{Max}}$.
The results for the holographic light nuclide DCE, calculated in each model
considered, are summarized in the figure 1. Notice that each nuclide is
defined by the ground state calculated from the holographic potential. This
localized ground state is characterized by its nuclear spin and mass number
encoded into $\Delta$. Thus, although we are not summing over different
states, we increase the particle content, i.e., the coarseness degree. Thus,
the calculated DCE is a holographic measure of stability.

It is essential to make a difference with the hadronic DCE at this stage. The
energy density $\rho(\zeta)$, in general, is a function of the mass spectrum
$M^{2}(n)$ and the particle content. In the hadronic case, the particle
content is fixed to the valence quarks while the hadronic mass increases,
implying that DCE increases with the excitation number $n$. In the nuclear
case considered here, the energy is fixed by the ground state mass while the
nucleon content is incremented. The energy density comes from bulk modes
calculated from the holographic potential. The potential carries the
confinement information inherited by the dilaton field. Thus, in the case of
hadrons, the direct consequence is the emergence of Regge trajectories. The
bulk mass $M_{5}$ controls the particle content in such a hadronic scenario
via $\Delta$, which is dual to the dimension of the operators creating
hadrons, fixed by the number of valence quarks. In the nuclear case, we extend
this idea to consider that the bulk mass carries the nucleon number $A$
(coarseness degree) of the nuclide at hand. In this sense, the DCE measures
nuclear stability in the holographic context. However, if the energy and
configuration (given by the bulk mass) are degenerate, the CE is also
degenerate since there is no quantity in holography that directly measures the
inner structure or configuration. According to the holographic dictionary, the
state identity (hadronic or nuclear) is defined by the conformal dimension
since it is connected with the operator creating these states at the conformal
boundary, which does not consider the constituent inner configuration. In this
sense, at the holographic level, nuclides can be understood as a bag filled
with constituent interacting nucleons.

In the case of the hardwall model (left panel in figure 1), DCE does not
provide evidence of a single entropy evolution with the atomic number $Z$.
Instead, the DCE tends to organize by spin-labeled structures, emulating Regge
trajectories for hadrons. This particular behavior comes from the AdS/QCD
naturalness that inherited the hardwall since this model was done initially to
address hadronic spectra. As was expected, when restricting for only one
nuclear spin DCE increases with the nucleon number.

For soft-wall-like model results, summarized in the central panel in figure 1,
all nuclide states define a single trajectory despite their spin, despite
their AdS/QCD inherit behavior. DCE increases with the atomic number $Z$, as
was expected. We can consider this trajectory a holographic stability line
since most of the symmetric light nuclides are stable. Thus, we can expect
that at some $Z$, when the nuclide becomes heavier, instability may arise.
This observation suggests that asymmetric nuclides can exhibit differences in
their relative DCE with their local partners, making them out of the stability
trajectory. Also, it suggests that nuclides become unstable for some high
values of $Z$. To address the last hypothesis, it is necessary to parametrize
the inner nuclide configuration since labeling with particle constituents
number only introduces degeneracy, i.e., two different nuclides could have the
same mass number.

In the case of the Woods-Saxon-like model, plotted in the right panel of
figure 1, DCE grows with the atomic number, as was expected. We observed a
small local jump where spin-1 ${}_{9}^{18}$F has a bigger CE than spin-0
${}_{10}^{20}$Ne. We do not observe another local jump when running
calculations up to ${}_{28}^{56}$Ni. Neither when we have spin shiftings, as
it happens in the neighborhood of ${}_{19}^{38}$K. Also, as in the softwall
model case, the nuclide organization defines a trajectory in terms of
stability. However, we can explore this fact further since we do not have a
holographic mechanism to describe nucleon configuration inside nuclides.

##  IV Conclusions

In the present work, we explored three different holographic models for
nuclear mass spectroscopy. Motivated by the fact that nuclear force emerges
from the strong interaction, we consider the hardwall and softwall models as
the first approximation to model light nuclide masses. In both situations, for
a fixed nuclide, radial excitations could be interpreted as nuclides different
from the ground state since their mass difference is bigger than the proton
mass. We propose a different dilaton associated with a holographic Woods-
Saxon-like potential to improve this situation. This sort of potential has a
bounded above spectrum, consistent with the experimental light nuclide
spectrum.

The outcomes we present in table 1 give reasonable results in terms of
precision and accuracy. However, if we explore the connection between higher
excited states and mass shifts, as table 2 summarizes, we realize that the
Woods-Saxon-like holographic reconstruction is the more accurate in
reproducing the nuclear mass spectra.

In our case, the _light nuclide spectrum_ is defined as the collection of
ground states calculated from holographic potentials, eqn. (13), depending on
the constituent number (see table 1), i.e., the mass number $A$. This
situation is different from hadronic physics, where a spectrum is defined as
the ground state and their excitations calculated from a single potential with
the constituent number fixed.

We expect that radial excitations do not grow in energy, implying a different
nuclide from the ground state (see table 2). The bulk dilaton should give rise
to a holographic potential whose spectrum is bounded from above. In our case,
we use a holographic modification of the Woods-Saxon potential.

Even though the hardwall model does not reproduce the _light nuclide mass
spectrum_ with high precision, it provides qualitatively correct results.
Also, the deviation from the observed spectrum is low compared with
experimental data. We have an RMS near 11$\%$. The softwall model also
involves only one parameter and has an RMS near 4 $\%$. However, a good
description of the nuclear mass spectrum requires a non-trivial dependence of
the dilaton coupling on the atomic number, which we implement _ad-hoc_.
Indeed, since the dilaton coupling changes with an atomic number, the softwall
model considers a one-parameter family of softwall dilatons to describe the
nuclear spectra. We also remark that, although hardwall and softwall are
models to describe hadrons originally, these models could work, as a first
approximation, when describing light nuclei.

It is interesting to comment about the role played by the dilaton field.
Following the softwall original motivation, the dilaton field carries
information about the strong interaction nature, translating into the
emergence of Regge Trajectories. Thus, our first hypothesis is that the
dilaton field can be promoted to the dual object that carries information
about confining forces at the boundary. The real question behind this
hypothesis relies upon how to build up a proper dilaton to mimic such a
confining interaction at the boundary. The answer is in the holographic
bottom-up potential depicted in eqn. (13). We followed this path to construct
the holographic version of the Woods-Saxon potential, giving the most accurate
bottom-up description for nuclide spectroscopy (RMS error near 1 $\%$).

In the first case, the excited states make a remarkable difference between
hadronic and nuclear spectroscopy since the nature of such states. In the
hadronic case, constituents remain the same while the difference between
ground and excited states lies in their inner configuration. These
configuration patterns define new metastable states different from the ground
one, i.e., the mass difference between the excited state and the ground state
is not negligible compared to the latter. In the nuclear case, unless we
consider SU$(2)$ isospin symmetry, constituents would change when the ground
state moves to excited metastable states. This fact implies that the excited
nuclei and the ground state mass difference should not be bigger than one
nucleon mass, where transitions are energetically forbidden with more than one
nucleon transmuting into another. Thus, a good holographic approach to nuclear
spectroscopy should have mass differences between excited and ground states
negligible compared to nucleon mass.

In addition to the spectroscopic analysis, we also perform a stability
analysis by considering the configurational entropy.

In the configurational entropy case, since these sorts of bottom-up models do
not include inner nuclear structure, this observable brings clues about
stability regarding the number of constituents. Recall that this constituent
number information is enclosed in the bulk mass through the conformal
dimension of the operator that creates nuclides at the boundary. This operator
is written in terms of the twist, carrying constituent information. On the
other hand, it is expected that the configurational entropy behavior should
characterize the stability in terms of the number of constituents or the
atomic number for nuclear systems. Regarding hadrons, configurational entropy
analysis depends on the spin and hadronic mass since it resembles the Regge
Trajectory in terms of stability, i.e., higher radial excitations of a given
family have bigger configurational entropy than the hadronic ground state.
Similar behavior comes when decay constants are introduced in the analysis:
higher excitations have lesser decay constant than the ground state. In the
nuclear case, configurational entropy analysis shows that the hardwall is not
a good holographic model since light nuclides are organized in spin-dependent
structures in the DCE plot (see figure 1). Softwall and Wood-Saxon models have
better results since the entropy grows with the constituent number
independently from spin.

After the spectroscopic and configurational entropic analysis, we conclude
that the best among the three models depicted here to describe the light
nuclide spectrum is the Woods-Saxon-like bottom-up approach.

###### Acknowledgements.

We acknowledge the financial support of FONDECYT (Chile) under the grant No.
1180753 (A. V. and M. A. M. C.). Saulo Diles thanks to Fundação Amazônia de
Amparo a Estudos e Pesquisas (FAPESPA) for partial financial support.

## References

  * Gutsche  _et al._ [2012] T. Gutsche, V. E. Lyubovitskij, I. Schmidt, and A. Vega, Dilaton in a soft-wall holographic approach to mesons and baryons, Phys. Rev. D 85, 076003 (2012), arXiv:1108.0346 [hep-ph] .
  * Gutsche  _et al._ [2015] T. Gutsche, V. E. Lyubovitskij, I. Schmidt, and A. Vega, Nuclear physics in soft-wall AdS/QCD: Deuteron electromagnetic form factors, Phys. Rev. D 91, 114001 (2015), arXiv:1501.02738 [hep-ph] .
  * Braga  _et al._ [2016a] N. R. F. Braga, M. A. Martin Contreras, and S. Diles, Decay constants in soft wall AdS/QCD revisited, Phys. Lett. B 763, 203 (2016a), arXiv:1507.04708 [hep-th] .
  * Braga and da Rocha [2017] N. R. F. Braga and R. da Rocha, Configurational entropy of anti-de Sitter black holes, Phys. Lett. B 767, 386 (2017), arXiv:1612.03289 [hep-th] .
  * Braga  _et al._ [2016b] N. R. F. Braga, M. A. Martin Contreras, and S. Diles, Holographic model for heavy-vector-meson masses, EPL 115, 31002 (2016b), arXiv:1511.06373 [hep-th] .
  * Folco Capossoli and Boschi-Filho [2016] E. Folco Capossoli and H. Boschi-Filho, Glueball spectra and Regge trajectories from a modified holographic softwall model, Phys. Lett. B 753, 419 (2016), arXiv:1510.03372 [hep-ph] .
  * Martin Contreras and Vega [2020] M. A. Martin Contreras and A. Vega, Different approach to decay constants in AdS/QCD models, Phys. Rev. D 101, 046009 (2020), arXiv:1910.10922 [hep-th] .
  * Mateos [2011] D. Mateos, Gauge/string duality applied to heavy ion collisions: Limitations, insights and prospects, J. Phys. G 38, 124030 (2011), arXiv:1106.3295 [hep-th] .
  * Casalderrey-Solana  _et al._ [2016] J. Casalderrey-Solana, D. Mateos, W. van der Schee, and M. Triana, Holographic heavy ion collisions with baryon charge, JHEP 09, 108, arXiv:1607.05273 [hep-th] .
  * Sachdev [2011] S. Sachdev, Condensed Matter and AdS/CFT, Lect. Notes Phys. 828, 273 (2011), arXiv:1002.2947 [hep-th] .
  * de Boer  _et al._ [2010] J. de Boer, K. Papadodimas, and E. Verlinde, Holographic Neutron Stars, JHEP 10, 020, arXiv:0907.2695 [hep-th] .
  * Argüelles  _et al._ [2020] C. R. Argüelles, T. Canavesi, M. Díaz, and N. Grandi, Thermodynamic instabilities in holographic neutron stars at finite temperature, Class. Quant. Grav. 37, 205002 (2020), arXiv:1911.02554 [hep-th] .
  * Policastro  _et al._ [2002] G. Policastro, D. T. Son, and A. O. Starinets, From AdS / CFT correspondence to hydrodynamics, JHEP 09, 043, arXiv:hep-th/0205052 .
  * Kovtun  _et al._ [2005] P. Kovtun, D. T. Son, and A. O. Starinets, Viscosity in strongly interacting quantum field theories from black hole physics, Phys. Rev. Lett. 94, 111601 (2005), arXiv:hep-th/0405231 .
  * Kovtun [2012] P. Kovtun, Lectures on hydrodynamic fluctuations in relativistic theories, J. Phys. A 45, 473001 (2012), arXiv:1205.5040 [hep-th] .
  * Diles  _et al._ [2020] S. M. Diles, L. A. H. Mamani, A. S. Miranda, and V. T. Zanchin, Third-order relativistic hydrodynamics: dispersion relations and transport coefficients of a dual plasma, JHEP 2020 (5), 1, arXiv:1909.05199 [hep-th] .
  * Maldacena [1998] J. M. Maldacena, The Large N limit of superconformal field theories and supergravity, Adv. Theor. Math. Phys. 2, 231 (1998), arXiv:hep-th/9711200 .
  * Boschi-Filho and Braga [2003] H. Boschi-Filho and N. R. F. Braga, Gauge / string duality and scalar glueball mass ratios, JHEP 05, 009, arXiv:hep-th/0212207 .
  * Colangelo  _et al._ [2008] P. Colangelo, F. De Fazio, F. Giannuzzi, F. Jugeau, and S. Nicotri, Light scalar mesons in the soft-wall model of AdS/QCD, Phys. Rev. D 78, 055009 (2008), arXiv:0807.1054 [hep-ph] .
  * Brodsky and de Teramond [2006] S. J. Brodsky and G. F. de Teramond, Hadronic spectra and light-front wavefunctions in holographic QCD, Phys. Rev. Lett. 96, 201601 (2006), arXiv:hep-ph/0602252 .
  * de Teramond and Brodsky [2010] G. F. de Teramond and S. J. Brodsky, Gauge/Gravity Duality and Hadron Physics at the Light-Front, AIP Conf. Proc. 1296, 128 (2010), arXiv:1006.2431 [hep-ph] .
  * Ballon-Bayona  _et al._ [2021] A. Ballon-Bayona, L. A. H. Mamani, and D. M. Rodrigues, Spontaneous chiral symmetry breaking in holographic soft wall models, Phys. Rev. D 104, 126029 (2021), arXiv:2107.10983 [hep-ph] .
  * Gleiser and Stamatopoulos [2012a] M. Gleiser and N. Stamatopoulos, Entropic Measure for Localized Energy Configurations: Kinks, Bounces, and Bubbles, Phys. Lett. B 713, 304 (2012a), arXiv:1111.5597 [hep-th] .
  * Gleiser and Sowinski [2013] M. Gleiser and D. Sowinski, Information-Entropic Stability Bound for Compact Objects: Application to Q-Balls and the Chandrasekhar Limit of Polytropes, Phys. Lett. B 727, 272 (2013), arXiv:1307.0530 [hep-th] .
  * Braga and da Rocha [2018] N. R. F. Braga and R. a. da Rocha, AdS/QCD duality and the quarkonia holographic information entropy, Phys. Lett. B 776, 78 (2018), arXiv:1710.07383 [hep-th] .
  * Braga  _et al._ [2018] N. R. F. Braga, L. F. Ferreira, and R. a. Da Rocha, Thermal dissociation of heavy mesons and configurational entropy, Phys. Lett. B 787, 16 (2018), arXiv:1808.10499 [hep-ph] .
  * Bernardini and Da Rocha [2018] A. E. Bernardini and R. Da Rocha, Informational entropic Regge trajectories of meson families in AdS/QCD, Phys. Rev. D 98, 126011 (2018), arXiv:1809.10055 [hep-th] .
  * Ferreira and Da Rocha [2019] L. F. Ferreira and R. Da Rocha, Pion family in AdS/QCD: the next generation from configurational entropy, Phys. Rev. D 99, 086001 (2019), arXiv:1902.04534 [hep-th] .
  * Ferreira and da Rocha [2020a] L. F. Ferreira and R. da Rocha, Tensor mesons, AdS/QCD and information, Eur. Phys. J. C 80, 375 (2020a), arXiv:1907.11809 [hep-th] .
  * Huang  _et al._ [2021] W. J. Huang, M. Wang, F. G. Kondev, G. Audi, and S. Naimi, The AME 2020 atomic mass evaluation (I). Evaluation of input data, and adjustment procedures, Chin. Phys. C 45, 030002 (2021).
  * Boschi-Filho and Braga [2004] H. Boschi-Filho and N. R. F. Braga, QCD / string holographic mapping and glueball mass spectrum, Eur. Phys. J. C 32, 529 (2004), arXiv:hep-th/0209080 .
  * Erlich  _et al._ [2005] J. Erlich, E. Katz, D. T. Son, and M. A. Stephanov, QCD and a holographic model of hadrons, Phys. Rev. Lett. 95, 261602 (2005), arXiv:hep-ph/0501128 .
  * Karch  _et al._ [2006] A. Karch, E. Katz, D. T. Son, and M. A. Stephanov, Linear confinement and AdS/QCD, Phys. Rev. D 74, 015005 (2006), arXiv:hep-ph/0602229 .
  * Woods and Saxon [1954] R. D. Woods and D. S. Saxon, Diffuse surface optical model for nucleon-nuclei scattering, Phys. Rev. 95, 577 (1954).
  * Rotenberg [1955] M. Rotenberg, Nuclear hartree-fock calculations, Phys. Rev. 100, 439 (1955).
  * Negele [1982] J. W. Negele, The mean-field theory of nuclear structure and dynamics, Rev. Mod. Phys. 54, 913 (1982).
  * Costa  _et al._ [1999] L. S. Costa, F. V. Prudente, P. H. Acioli, J. J. S. Neto, and J. D. M. Vianna, A study of confined quantum systems using the woods-saxon potential, Journal of Physics B: Atomic, Molecular and Optical Physics 32, 2461 (1999).
  * Bernardini and da Rocha [2016] A. E. Bernardini and R. da Rocha, Entropic information of dynamical AdS/QCD holographic models, Phys. Lett. B 762, 107 (2016), arXiv:1605.00294 [hep-th] .
  * Bernardini  _et al._ [2017] A. E. Bernardini, N. R. F. Braga, and R. da Rocha, Configurational entropy of glueball states, Phys. Lett. B 765, 81 (2017), arXiv:1609.01258 [hep-th] .
  * Lee [2017] C. O. Lee, Configurational entropy of charged AdS black holes, Phys. Lett. B 772, 471 (2017), arXiv:1705.09047 [gr-qc] .
  * Karapetyan [2022] G. Karapetyan, Nuclear configurational entropy and high-energy hadron-hadron scattering reactions, Eur. Phys. J. Plus 137, 590 (2022), arXiv:2112.11359 [nucl-th] .
  * Soto and Golestanian [2013] R. Soto and R. Golestanian, Run-and-tumble in a crowded environment: persistent exclusion process for swimmers, arXiv preprint arXiv:1306.0481 (2013).
  * de Castro  _et al._ [2021] P. de Castro, S. Diles, R. Soto, and P. Sollich, Active mixtures in a narrow channel: Motility diversity changes cluster sizes, Soft Matter 17, 2050 (2021).
  * Gleiser and Stamatopoulos [2012b] M. Gleiser and N. Stamatopoulos, Information Content of Spontaneous Symmetry Breaking, Phys. Rev. D 86, 045004 (2012b), arXiv:1205.3061 [hep-th] .
  * Colangelo and Loparco [2019] P. Colangelo and F. Loparco, Configurational Entropy can disentangle conventional hadrons from exotica, Phys. Lett. B 788, 500 (2019), arXiv:1811.05272 [hep-ph] .
  * Ferreira and da Rocha [2020b] L. F. Ferreira and R. da Rocha, Nucleons and higher spin baryon resonances: An AdS/QCD configurational entropic incursion, Phys. Rev. D 101, 106002 (2020b), arXiv:2004.04551 [hep-th] .
  * da Rocha [2021] R. da Rocha, Information entropy in AdS/QCD: Mass spectroscopy of isovector mesons, Phys. Rev. D 103, 106027 (2021), arXiv:2103.03924 [hep-ph] .
  * Braga and da Mata [2020a] N. R. F. Braga and R. da Mata, Configuration entropy description of charmonium dissociation under the influence of magnetic fields, Phys. Lett. B 811, 135918 (2020a), arXiv:2008.10457 [hep-th] .
  * Braga and da Mata [2020b] N. R. F. Braga and R. da Mata, Configuration entropy for quarkonium in a finite density plasma, Phys. Rev. D 101, 105016 (2020b), arXiv:2002.09413 [hep-th] .
  * Braga and Junqueira [2021] N. R. F. Braga and O. C. Junqueira, Configuration entropy and confinement/deconfinement transiton in holographic QCD, Phys. Lett. B 814, 136082 (2021), arXiv:2010.00714 [hep-th] .


2022 Prospects for low-frequency radio astronomy in S. A.

# The Instituto Argentino de Radioastronomía (IAR): Past, present, and future

G. E. Romero11affiliation: Instituto Argentino de Radioastronomía (IAR),
Argentina (romero@iar.unlp.edu.ar). 22affiliation:  Facultad de Ciencias
Astronómicas y Geofísicas, Universidad Nacional de La Plata, Argentina
(romero@fcaglp.unlp.edu.ar).

###### Abstract

I present a brief review of the history of the Instituto Argentino de
Radioastronomía, a description of its current facilities and projects, and a
view of his prospects for the future.

Presento un breve relato de la historia del Instituto Argentino de
Radioastronomía, junto con una descripción de sus instalaciones y proyectos
actuales, así como una visión de sus perspectivas futuras.

History of astronomy Instrumentation: detectors Instrumentation:
interferometers Radio continuum: general Radio lines: general

##  0.1 Origins

The origins of radio astronomy in South America and of the Instituto Argentino
de Radioastronomía (IAR), the first observatory devoted to this branch of
astronomy in this part of the world, are associated with the name of Merle
Anthony Tuve (June 27, 1901 - May 20, 1982, Figure 1). He was an American
geophysicist who pioneered the use of pulsed radio waves. His discoveries
paved the way for the development of radar and nuclear physics. Merle Tuve was
director of the Department of Terrestrial Magnetism (DTM) at the Carnegie
Institution for Science (1946-66), and from that position he played a key role
in the creation of the IAR.

Tuve became interested in the nascent field of radio astronomy when he learned
in 1952 of the first detection of the HI line from the galactic plane by Edwin
and Purcell of Harvard University. He soon built a radiometer and began his
research in radio astronomy. He soon realized the importance of having radio
observatories in the southern hemisphere, since most of the Galaxy is visible
only from that part of the world. At the time, the southern sky was virtually
unexplored in the radio spectrum. Since Australia was already working on the
construction of what would become the Parkes Observatory, Tuve thought that
South America would provide an excellent platform for the installation of a
second southern radio observatory. It would be more easily accessible from the
United States and would be an important complement to observational studies to
be conducted from Australia. In 1957 he visited several countries in the
region: Brazil, Argentina, Chile, and Peru. In Argentina, astronomy had been
systematically developed since President Domingo Faustino Sarmiento founded
the Córdoba Observatory in 1871, and Tuve’s proposal was enthusiastically
received. As a direct consequence of Tuve’s visit and his promise to send the
country the necessary components to build a solar interferometer, the
University of Buenos Aires (UBA) created, on November 13, 1958, the Commission
of Astrophysics and Radio Astronomy (CAR), integrated by Dr. Félix Cernuschi,
Dr. Enrique Gaviola and Eng. Humberto Ciancaglini, with Dr. Gaviola as its
president.

Gaviola (1900-1989) was one of Argentina’s most distinguished scientists. He
received his doctorate under the supervision of Max von Laue in Berlin.
Recommended by Einstein, he received a fellowship from the Rockefeller
Foundation to continue his research in the United States. Between 1928 and
1929 he worked at the DTM, where he met Tuve.

Tuve sent the parts of the interferometer to the CAR, so that the construction
of the different parts was done in the offices of the Commission, and then the
instrument was installed on the land of the Faculty of Agronomy of the UBA. It
consisted of 16 antennas operating at 86 MHz. It did not produce any
scientific results, but it was important for the training of students and
engineers involved in the project. At the same time, other efforts were made
to train some engineers in the new technologies: Jorge Sahade, who was in the
USA at the time, and the then director of the National Radio Astronomy
Observatory (NRAO), Dr. Otto Struve, organized a trip to the DTM and NRAO for
two young engineers: Emilio Filloy and Rubén Dugatkin. They were supported by
grants from the Scientific Research Commission (CIC) of the Province of Buenos
Aires, Argentina, and finally traveled to the United States in September 1961.

Figure 1: Merle Anthony Tuve. Director of the Department of Terrestrial
Magnetism (DTM) at the Carnegie Institution for Science (1946–66) and key
person in the development of radio astronomy in Argentina.

On December 7, 1961, Tuve sent a letter to Dr. Bernardo Houssay, President of
the National Research Council of Argentina (CONICET), proposing the
establishment of a radio astronomy station in Argentina, along with a
memorandum describing the main instrument of the facility: a 30-m single-dish
equatorial mount radio telescope. The instrument would be able to cover from
the south celestial pole to $-10\deg$ in declination and from $-2$ to $+2$
hours in right ascension. The DTM would provide all materials for the
construction of the telescope. This letter from Tuve initiated a series of
exchanges with Houssay that would eventually lead to the creation of the IAR.

##  0.2 History

On April 27, 1962, CONICET created the National Institute of Radio Astronomy
(INRA). Soon after, an agreement was signed with the UBA, the University of La
Plata (UNLP), and the CIC, stating that all these institutions would work
together to support the new institute. Finally, the name was changed to
Instituto Argentino de Radioastronomía (Argentine Institute of Radio
Astronomy) to avoid confusion with another institute dedicated to agricultural
technologies, but with the same acronym.

The history of the IAR has been presented in detail by Bajaja (2009). What is
offered in this section is a brief account along with some personal views.

Dr. Carlos M. Varsavsky (1933-1983, Figure 2), from UBA, was chosen as
director of the Institute and Dr. Carlos Jaschek of the La Plata Observatory
was appointed as deputy director. Eng. Juan del Giorgio was appointed
technical advisor. Varsavksy received his Ph.D. in astronomy from Harvard in
1959. He was known as the translator into English of the classic book Cosmic
Radio Waves by I.S. Shklovsky. In 1960 he returned to Argentina and became
Professor of Physics at the University of Buenos Aires, where he lectured on
radio astronomy and joined the CAR group. The original staff of the IAR
consisted of the aforementioned authorities, the engineers Rubén Dugatkin,
Emilio Filloy (both recently returned from the USA), Omar González Ferro, and
the students Valentín Boriakoff (electronics), Fernando Raúl Colomb, and
Esteban Bajaja (both students of physics).

Figure 2: Carlos Varsavsky, first director of IAR.

The first task of the group was to find a suitable site for the installation
of the radio telescope. The site should be accessible from both La Plata and
Buenos Aires, but relatively isolated to minimize radio interference from
human sources. The selected location was within the Pereyra Iraola Park, a
protected reserve located near Villa Elisa, 40 km from the city of Buenos
Aires. Ten hectares were ceded by the Province of Buenos Aires through an
agreement signed on October 30, 1962. Almost immediately, work began on the
site. A 1944 International truck and a 50 kVA ex-military diesel generator
were used to build the first workshop and other facilities (see Figure 3).
Amazingly, both are still in use in 2023.

Figure 3: Construction of the first workshop at IAR’s estate. The
International truck and the original Diesel generator can be seen, along with
the first water tank.

To supervise the work of assembling the radio telescope, at the beginning of
November 1963, Eng. Everett Ecklund was sent from the Carnegie Institution of
Washington (CIW). The construction started on November 14th. The construction
process is described in detail by Bajaja (2009) and will not be repeated here.
Figure 4 shows the construction of the dish, still resting on the ground.

Figure 4: Construction of the dish of the first radio telescope at the IAR.
Early 1964.

Once the dish was completed, a first receiver with 10 channels was installed
and on April 1, 1965, the first detection of HI was achieved. The dish
(central ring + platform + ribs + surface) was supported on pillars on the
ground, facing the zenith. This historic landmark occurred only 12 years after
the original detection of the HI line at Harvard. It was a milestone in the
history of the IAR and radio astronomy in the southern hemisphere. At the end
of that same year, the dish was mounted on the pedestal (Figure 5).

Figure 5: Mounting the radio telescope disk on its pedestal. 1965.

As the construction of the radio telescope progressed, other buildings were
added to the complex: a main building for offices, library, and control of the
instrument (see Figure 6); and a building with laboratories and workshops for
electronics. Finally, the inauguration of the radio telescope took place on
March 26, 1966, in the presence of personalities from national and foreign
institutions, including H. Van de Hulst, the first proponent of the HI 21-cm
line as a diagnostic tool of the interstellar medium (Figure 7). During the
inauguration ceremony, speeches were given by Dr. Tuve, in his capacity as
Director of the DTM and representative of the CIW, and by Dr. Varsavsky, as
Director of the IAR. Soon after, the scientific observations began: IAR as a
radio astronomy observatory was born.

Figure 6: Main building of IAR, finished in early 1966. Figure 7: March 26,
1966: inaugurating ceremony of the first radio telescope of IAR.

It did not take long for politics to interfere with the normal development of
activities. On June 28, 1966, the Argentine Armed Forces carried out a coup
against the constitutional government of Arturo Umberto Illia and established
the self-proclaimed “Argentine Revolution”. On July 29, 1966, five academic
departments of the University of Buenos Aires (UBA), occupied by students,
professors, and graduates, were brutally repressed and evacuated in opposition
to the decision of the de facto government to intervene in the universities
and overthrow the government regime. The episode in which professors were
beaten and expelled was known as “The Night of the Long Batons.” Subsequently,
the military government of General Juan Carlos Onganía decided to unilaterally
revoke the academic freedom established in the 1918 university reform. Most
professors resigned and many left the country.

Varsavsky, who was wounded in the episode (Fig. 8), resigned from the
university and moved his office to the IAR. Many students came with him. Some
of them later became famous scientists: Diego and Catherine Cesarsky, Jorge
Vernazza, Peter Mészáros, Federico Strauss, Zulema Abraham, Silvia Garzoli,
Esteban Bajaja, Raul Colomb, and Wolfgang Poeppel, among others (see Fig. 9).

Figure 8: July 29, 1966: “Night of the Long Batons”, at the Deparment of
Physics, UBA. Varsavsky, wounded in the head, can be seen first in the row,
with an umbrella.

Despite the political turmoil, research with the new radio telescope
continued. Then, in 1969, during the month of May, some serious incidents
occurred in the cities of Rosario and Corrientes, with the death of two
students. In the large city of Córdoba, some unions and groups of students
called for a strike to be held on May 30. Then, on May 28, a note signed by
the staff of the IAR, criticizing the regime of the de facto president
Onganía, was addressed to CONICET, whose authorities were invited to join the
strike of May 30. The note was also sent to the newspapers. On May 29, 1969,
very serious disturbances occurred in Córdoba, riots that were later called
“El Cordobazo” and that marked the beginning of the end of the Onganía
government. First the police and then the army intervened to suppress the
uprising. Four people were killed, more than 100 were wounded, and many were
imprisoned. Meanwhile, the note sent by IAR personnel led to serious
disciplinary action by CONICET: Varsavsky was fired as director, and the
closure of the IAR was considered. Finally, bureaucratic sanctions were
imposed on the staff. Disillusioned by the lack of support from the CIW,
Varsavsky resigned from CONICET and gave up scientific research. In 1977,
after his nephew Varsavsky was kidnapped and murdered by state terrorism, he
left the country. He immigrated with his family to the United States, where he
became deputy director of the Institute for Economic Analysis at New York
University. He would die in 1983 at the age of 49.

Figure 9: IAR staff toward 1966.

Emilio Filloy replaced Varsavsky as interim director until Dr. Kent Turner of
DTM was appointed as the new director. During his tenure, there were two
important milestones in the history of the IAR: a second, twin radio telescope
was built (Fig. 10) and a new building housing the telescope control room was
inaugurated (Fig. 11). Turner was also involved in the supervision of
students, including Félix Mirabel, who later became one of the most renowned
Argentine astronomers. In 1973, elections were held and Héctor Cámpora was
elected as the new president. The climate was not favorable for US citizens at
that time, so Turner resigned and returned to the US. The new interim director
was Raúl Colomb. In 1975, after returning to the country from a long stay in
the Netherlands, Esteban Bajaja was appointed director by CONICET.

Figure 10: Assembly of the second IAR radio telescope, 1973. Figure 11: New
control room in the building constructed to house it, toward 1976.

During Bajaja’s tenure as director, a new receiver for the first telescope was
built through a renewal agreement with the DTM. This receiver was a
significant improvement over the previous one. It was designed for operation
at 21 and 18 cm, with noise temperatures of 83K and 90K, respectively. A
PDP-11/20 computer was used for data acquisition. A new front end was also
installed at Antenna 1 on July 21, 1979. It was equipped with a corrugated
feeder, carefully designed to attenuate spurious radiation from directions
outside the surface of the antenna (the so-called “spillover” effect). The
filters for the spectral observations consisted of a set of 24 crystal
filters, 2.2 kHz wide, 3 kHz apart, for observing very narrow lines of HI or
for observing lines of OH. Another set of 112 channels of 10 kHz width and 10
kHz spacing was used for general observations of Galactic HI. Finally, a set
of 84 channels of 75.8 kHz (16 kms-1), separated by 75.8 kHz, was devoted to
the observation of galaxies. All of these filters occupied a large part of the
control room.

With the new equipment, the observatory began a number of important scientific
programs, including surveys of HI in galaxies, high-speed clouds, HI in the
Magellanic Clouds, a survey of galactic HI at low and intermediate velocities,
studies of supernova remnants and HII regions, and observations of
recombination lines and OH molecules. The first telescope operated at nearly
100 % of its capacity.

After a very successful administration, Bajaja asked for a leave of absence to
take a sabbatical year devoted to research. He was replaced as director by
Raúl Colomb during the period 1982-1984. Colomb focused on the newest radio
telescope, in which a continuum receiver was installed. The idea was to
perform a survey of the entire sky in collaboration with the Max Planck
Institute for Radio Astronomy (MPIfR) in Bonn. In the northern hemisphere, the
observations were made with a 25-meter radio telescope located on Stockert
hill in Bad Münstereifel, North Rhine-Westphalia, Germany. A receiver with
polarimetric capabilities was developed at the IAR for the project.

Bajaja resumed the directorship in 1984, but soon resigned in discontent
because CONICET introduced major changes in the management of scientific
budgets. Colomb returned as director for the next 10 years. During these
years, most of the efforts were devoted to the surveys. In 1989, an agreement
was signed with the Planetary Society to install an 8.4 million channel
spectrum analyzer to search for radio signals from extraterrestrial
intelligent beings. This instrument, called META II (Mega Channel
Extraterrestrial Assay), was built at Harvard by two IAR engineers: Juan
Carlos Olalde and Eduardo Hurrell. The instrument became operational on
October 12, 1990, and was active for more than two years, accumulating about
9,000 hours of observations. During these observations, 1,600,000 independent
spectra of 8.4 million channels each were obtained, more than $10^{13}$ bytes
of data. Of the spectra obtained, only those containing peculiar signals were
recorded. Finally, 10 suspected cases were selected, but none were confirmed.

In 1989, the author of this article and Jorge Combi started working at the IAR
as telescope operators. In total, they spent more than 300 nights observing
with the telescopes during the next two years. This gives an idea of the
intensive use of the radio telescopes at that time. In 1992, they both
received grants from CONICET to do their Ph.D. research at the IAR, and began
to use the instrumental knowledge they had acquired during their years as
operators to develop their own research projects on blazars and high-energy
particles in the galaxy. Without knowing it, they started relativistic
astrophysics in Argentina.

In 1995, Colomb asked for a leave of absence to take a leadership position in
the newly created National Commission for Space Activities (CONAE), and was
again replaced by Bajaja. Bajaja remained director until September 1997, when
he asked for a new sabbatical year and submitted his resignation as director.
Ricardo Morras replaced him as interim director. In November 1997, CONICET
appointed Marcelo Arnal as the new director, on the recommendation of Bajaja.
After only 2 years, Arnal’s directorship was terminated by CONICET (see Bajaja
2009), and Ricardo Morras was reappointed as interim director. However, he
would remain in charge of the institute for 7 years. This was the time it took
for CONICET to call for an opposition to cover the position of director.

Morras had to face serious difficulties due to the economic crisis in the
country during the years 2001 and 2002 and the consequences of the inherited
problems with CONICET. The budget was cut and even the possibility of closing
the institute was considered by the authorities; the telescopes had been out
of operation during the previous administration and would remain out of
service for almost two decades. However, Morras seized the opportunity to
begin a fruitful collaboration with CONAE to develop technology for the space
program, taking advantage of the strength of the technical staff. As work
intensified with new projects related to different satellites, new facilities
were built at the Institute. These included a new building for laboratories
and a full anechoic chamber (one of only two such facilities in the country,
Figure 12), and a far-field test facility (Figure 13). With these new
capabilities, the IAR also began to offer technological services. Slowly,
under the administration of Morras and the technological supervision of Engs.
Juan Sanz and Juan J. Larrarte, the Institute became a recognized
technological research and development center within the Argentine system. The
number of technical personnel increased dramatically during these years.

Figure 12: Interior of the anechoic chamber.

.

Figure 13: Tower of the far field testing facility.

.

From a scientific point of view, most of the research has been done in the
field of relativistic astrophysics. In 2000, Romero and Combi, together with
Paula Benaglia and Diego Torres, founded the Group of Relativistic
Astrophysics and Radio Astronomy (GARRA). This group, led by Romero, was the
first to exploit the recent results of gamma-ray instruments, adopting a
multi-wavelength approach and interweaving theory and observations. Such an
approach, applied systematically for the first time in South America, was
extremely successful. Soon the group grew to include up to 20 researchers and
students, publishing hundreds of papers and gaining a strong international
reputation. Eventually, Romero became professor of relativistic astrophysics
at the University of La Plata, Combi created the chair of X-ray astronomy at
the same university, Benaglia, who became a leading expert in radio
interferometry, founded the group FRINGE, and Torres emigrated first to the
USA and then to Spain, where he became director of the Institute of Space
Science of Catalonia.

At the end of 2007, CONICET finally opened the opposition for the directorship
and E.M. Arnal returned as director. In a first term he continued the policies
of Morras, but after a while he devoted most of his efforts to a new project:
a submillimeter radio telescope to be installed in the province of Salta, in
the northwestern part of the country, at an altitude of 5000 meters. This
project, whose original idea had been conceived long before by Félix Mirabel,
was finally approved by the new Ministry of Science (MinCyT) and a substantial
budget of several million dollars was allocated. The radio telescope would be
purchased by Brazil from the German company Vertex. It was the same design
used by ALMA Array in its telescopes. The site and infrastructure would be the
responsibility of Argentina, as would the integration of the instrument. The
final instrument would be operated as an Argentine-Brazilian facility (see
Romero 2020 for a description of the project, called LLAMA). The scale of such
a project was underestimated and management problems soon emerged. The
consequences for IAR were serious. The director devoted most resources to
LLAMA and other projects were affected. Technological activities decreased
dramatically and most of the contracted staff moved to other institutions. At
the same time, work on the site made little progress. When the telescope
finally arrived in Argentina in 2017, Arnal resigned from the project. At the
end of the year, he retired as director of the IAR and left the institute.
Since he had not appointed a deputy director, the institute was left headless.
The situation was very delicate, as the instrument had been left at the
international customs, where high fees had to be paid for its storage. While a
new advertisement for the position of Director was organized, two successive
interim Directors were appointed by CONICET: Eng. Leandro García and Dr.
Leonardo Pellizza. García managed to get the telescope out of customs, but the
budget for the project was cut by MinCyT.

2  Director Period Carlos M. Varsavsky 1962 – 1969 Emilio Fillioy 1969 – 1971
Kent Turner 1971 – 1973 Fernando Raúl Colomb 1973 – 1975 Esteban Bajaja 1975 –
1982 Fernando Raúl Colomb 1982 – 1984 Esteban Bajaja 1984 – 1985 Fernando Raúl
Colomb 1985 – 1995 Esteban Bajaja 1995 - 1997 Edmundo M. Arnal 1997 – 1999
Ricardo Morras 1999 – 2007 Edmundo M. Arnal 2007 – 2017 Leandro García 2018 –
2018 Leonardo Pellizza 2018 – 2018 Gustavo E. Romero 2018 – 20…

Table 1: Directors of IAR (1962-2023)

In August 2018, Gustavo E. Romero took over as the new director of the IAR.
The situation was difficult: the LLAMA project was stopped by MinCyT, there
was no other institutional project, the deterioration of the Institute’s
facilities was unprecedented, the IAR’s bank accounts were frozen by court
order, the Institute was completely isolated within the Argentine science and
technology system (there was no communication even with the nearby AGGO
Observatory), the radio telescopes had been out of service for about 20 years,
technological development and the associated technology transfer had stopped,
the technical staff was decimated: about 40 professionals left the IAR in the
previous 10 years (see Fig. 14).

MinCyT soon decided to transfer the management of LLAMA to ITEDA, an institute
dedicated to astroparticle detection and related technologies. After a year
and a half, when the government changed in 2019, the responsibility for
integrating the instrument was transferred again, this time to INVAP, a
government technology company. IAR remained as a consultant and contractor for
some specific tasks. Romero was then able to devote his full attention to the
reorganization of the Institute. After resolving the legal problems left by
LLAMA’s management, he implemented a new structure to allow for better
coordination and cooperation in achieving institutional goals. Two large areas
were created: scientific and technological, each with a person in charge. The
technological area has two large sectors, the Observatory and the Technology
Development and Transfer Sector, again each with a head appointed by the
director. Under these structures, several departments have been created, each
with its own supervisor: the Electronic Department, the Mechanical Department,
the Systems Department and the Outreach Department. All these departments have
their own structure and must provide the services necessary to achieve the
institutional objectives. This system proved to be very efficient.

Figure 14: Number of contracted technical personnel through the years.

.

The next step was to obtain funding for a new institutional project. To
achieve this, a new security plan was proposed to CONICET: the six hectares of
the IAR property were fenced off, access to the institute was automated, and
the entire property was monitored with more than 40 cameras. Security was
unified with that of the neighboring AGGO observatory, with which an agreement
was signed. With half of the budget spent on security guards in the past, this
new system, fully developed and implemented by IAR staff in record time, freed
up significant funds for other purposes.

These purposes were dictated by the new institutional project: the recovery of
the radio telescopes, the complete upgrade of their technology, and the
development of a new astronomical facility: the so-called Multipurpose
Interferometric Array (MIA), which is now progressing with the construction of
a demonstrator. Technology transfer has also been strongly promoted and new
projects have emerged: from nanosatellites to medical equipment to combat the
COVID-19 pandemic. Part of the funds that had been obtained from the services
were used for the improvement of the working conditions. The progress
stimulated renewed support from CONICET, CIC and UNLP. New facilities were
built, including a clean laboratory (Fig. 15). International support was
strong, with generous donations and cooperation from the Rochester Institute
of Technology (RIT), the Harvard-Smithsonian Observatory, the Collaboration
for Astronomy Signal Processing and Electronics Research (CASPER), the Parkes
Observatory, and others. More than 15 different types of agreements have been
signed in 4 years. Also, the association with CONAE was strengthened with a
new agreement and digital receivers for radio astronomy were developed for the
two deep space antennas operating in Argentina. The first one was installed in
December 2022.

Figure 15: New clear laboratory at IAR (inaugurated in 2021).

In 2019, both radio telescopes are IAR were baptized and the observatory was
reopened (Figs. 16 and 17). The telescopes, which had been called “Antenna 1”
and “Antenna 2” for almost 50 years, were now named after Carlos M. Varsavsky
and Esteban Bajaja, respectively. By the time of its 60th anniversary, the IAR
was once again a leading research institute in Argentina.

Figure 16: Gustavo E. Romero, with authorities from CONICET, CIC, and MinCyT,
during the baptism of the fully restored and upgraded telescopes. Figure 17:
IAR in 2019, the day the radio telescopes were baptise.

##  0.3 Some highlights

The first scientific paper published with affiliation at “IAR” was a review by
Varsavsky (1966) on the detectability of molecular hydrogen in interstellar
space. This was followed by several papers based on observations made with
other observatories. The first paper based entirely on observations made with
the IAR radio telescope was Mészáros (1968). This paper presented the first HI
map constructed from data obtained with the new instrument. It was an
observation of a dark cloud near $\rho$ Ophiuchi (see Fig. 18). Towards the
end of the 1960s, a steady stream of publications from the IAR began. So far,
more than 1300 papers have been published with the affiliation “Instituto
Argentino de Radioastronomía”. Behind these papers is a huge amount of
research. There are three periods in which the activity increased
significantly: first, between 1980 and 1990, with the installation of the new
receiver in the first radio telescope and the corresponding correlator. Then,
between 2003 and 2013, by a burst of activity in theory by the GARRA group.
And finally, the current period with many contributions from all groups,
FRINGE, GARRA and PuMA.

In Fig. 19 I show the evolution of the number of papers published per year by
IAR. The absolute maximum, regarding articles in refereed journals, occurred
in 2022, with 51 publications. Figure 20 shows the number of citations per
year. The citations have been growing steadily, exceeding 1000 per year in the
last 5 years. In the remainder of this section, I will mention a few
highlights among the published research.

Figure 18: First HI map made with data obtained with IAR first radio telescope
(Mészáros 1968).  Figure 19: Papers published by authors from IAR through the
history of the institute. Altogether, more than 1300 papers have been
published so far. Source: NASA’s ADS.  Figure 20: Number of citations of IAR’s
publications through the years. Source: NASA’s ADS.

In 1976 the first results of a survey of HI were published by Colomb et al.
(1976). It was not until 2005, 30 years later, that the final results of the
Leiden/Argentine/Bonn (LAB) Survey of Galactic HI were published (Kalberla et
al. 2005). This database has become extremely useful to the astrophysical
community, as evidenced by the large number of citations (3200+). Figure 21
shows the HI map of the whole sky in the velocity range $-400<v<+400$ km s-1.

Figure 21: HI emission integrated over the velocity range $-400<v<+400$ km s-1
in the LAB dataset, shown in an Aitoff projection (Kalberla et al. 2005).

The continuum survey was published in 2001 (Testori et al. 2001, Reich et al.
2001) and the associated linear polarization survey in 2008 (Testori et al.
2008). The radio telescopes have also been used to study individual sources,
both in the HI line and in the continuum. Mirabel and Turner (1975) made
observations of high-speed clouds. This line of research was continued for
many years by Mirabel and Morras. Colomb & Dubner (1980) published a first
paper on supernova remnants (SNR) based on an analysis of the atomic gas.
Combi et al. (1999) detected the Vela Jr. SNR in radio, using the second IAR
radio telescope, shortly after its discovery in X-rays. Romero et al. (1994)
reported the detection of what would later be called an “extreme scattering
event” in the direction of a compact active galactic nucleus. Combi & Romero
(1995) carried out an investigation of the possible radio counterparts of an
unidentified gamma-ray source. This paper also contained the first theoretical
study of high-energy astrophysics made in the country. It would initiate a
far-reaching research program that is still ongoing.

The first paper entirely devoted to theory was published by Olano (1982), with
a simple but very successful model of the local gas associated with Gould’s
belt. Romero et al. (1995a) presented the first of many papers on the physics
of relativistic jets, while Romero et al. (1995b) started research on
gravitational lensing at the IAR. Such a research program continues to this
day, now including studies using numerical relativity. The theoretical study
of microquasars, originally discovered by Mirabel working in France in the
early 1990s, started with Kaufman Bernadó et al. (2002) and Romero et al.
(2002). This research program, which would lead to several Ph.D. theses,
continues, as do numerical studies related to this topic (Romero et al. 2007).
Benaglia & Romero (2003) predicted that colliding wind binaries should produce
gamma rays and presented some early estimates. This work was followed by many
others, both theoretical and observational, on massive stars, their winds, and
their interactions.

Important discoveries have also been made by observations with other
instruments. Brinks & Bajaja (1986) found a large number of holes in the HI
distribution of the Andromeda galaxy using the Westerbork radio telescope.
Benaglia et al. (2010) detected the first non-thermal bowshock around a
runaway star. This was followed by a catalog of bowshocks (Peri et al. 2012,
2015) and many theoretical developments (e.g. del Valle & Romero 2012).

##  0.4 Present

Today, both radio telescopes have the capability to operate remotely and
continuously to study different astronomical objects. After various upgrades
(see Gancio et al. 2020), they are mainly dedicated to the observation of fast
variability in different types of sources such as neutron stars, magnetars,
blazars and pulsar timing, which will allow the detection and study of
gravitational waves (see https://nanograv.org/). The telescopes have digital
receivers using state-of-the-art hardware platforms based on high-performance
FPGAs developed by the CASPER (Collaboration for Astronomy Signal Processing
and Electronics Research) group. With these instruments and new front ends,
the telescopes have a bandwidth of 400 MHz each, with two polarizations. The
Varsavsky telescope has a center frequency of 1420 MHz, while the Bajaja
telescope is centered at 1300 MHz. The temperature of both receivers is less
than 80 K. The pulsar observations are performed with 512 channels and a band
of 400 MHz, a typical bandwidth of each channel of 781 kHz, and an integration
time of 73 microseconds. The back-end is shown in Fig. 22. Fig. 23 shows a
detail of a ROACH board.

Figure 22: Digital back-end based on ROACH boards developed by CASPER
collaboration and storage in the control room of IAR’s radio telescopes.
Figure 23: View of a ROACH board integrated to the back-end.

The time reference is provided by GPS, GNSS, and an atomic clock. All
connections are made with optical fibre. Operation is fully automatized.

In addition to hardware of its own observatory, the IAR contributes with
developments for other institutions such as CONAE and the Next Generation
Event Horizon Telescope (Fig 24).

Figure 24: View of the prototype digital receiver for the Next Generation
Event Horizon Telescope, being developed at IAR in collaboration with the
Harvard-Smithsonian Astronomical Observatory.

In addition to radio astronomical work and theoretical research, the IAR today
is also dedicated to the development of technological innovations and
applications for general use. These range from ozone reactors (Fig. 25) to
nanosatellites (Fig. 26). New facilities have been added for such tasks (Figs.
27 and 28). The new ISO 8 cleanroom is class 100,000 with a class 100
horizontal laminar flow cabinet. The main cleanroom is 32 m2 and there is an
auxiliary storage room. A view of most of the facilities available at IAR
today is shown in Fig. 29.

Figure 25: Assembly line of ozone reactors for sterilization close spaces.
Figure 26: PocketQube Helmholtz cage for testing satellite magnetotorquers.
Figure 27: New building with clean labs. Figure 28: Cryogenic laboratory.
Figure 29: Aerial view of the central part of the IAR, 2022.

##  0.5 A view of the future

The two 30-meter radio telescopes at the IAR are now equipped with state-of-
the-art electronics. However, a challenge for the immediate future is to
dramatically increase the storage capacity. Both pulsar timing observations
and searches for fast radio bursts generate huge amounts of data. The current
administration of the IAR is well aware of this problem and it is expected
that a new acquisition system with up to 1 PB of storage will be implemented
soon.

Figure 30: Design of MIA individual telescopes. Figure 31: First dish of MIA
pathfinder, 2022.

In addition to these and other upgrades, it is clear that the IAR needs a new
observing facility if it is to remain competitive. Because of the severe radio
interference at cm wavelength in the province of Buenos Aires, such a facility
should be installed in an isolated area and operated remotely. The project
designed to fulfill this need was initiated in 2019 and is called the
Multipurpose Interferometric Array (MIA). MIA will be a low frequency (between
50 MHz and 2 GHz) and high bandwidth (250 MHz in phase I) interferometer
formed by a core of 16 antennas of 5 m diameter (Fig. 30). The maximum
baseline of the instrument will be 55 km. It will be located in a site with
low radio interference, in the western part of the Argentine Republic. In a
second phase (Phase II), the antenna array can be extended up to 64 antennas,
including longer baselines. This instrument will allow the achievement of
several scientific and technological objectives. MIA will enhance the
scientific and technological capabilities of the IAR and will keep the
Institute scientifically competitive from an observational point of view until
the middle of the 21st century.

The instrument will be able to operate in three main modes: continuum
observations, line observations, and high temporal resolution timing
measurements. Its main contributions will be in three areas: Galactic variable
objects, HI at different redshifts, and fast radio transients.

A prototype called “MIA Pathfinder” is now under construction at IAR. It
consists of 1 NODE with 3 antennas (the Phase 1 instrument will have 3 NODES).
It will operate in the L-band (1 - 2.3 GHz) with two orthogonal linear
polarizations. The instantaneous bandwidth will be 250 MHz and the receiver
temperature will be about 45 K. As of November 2022, the first dish has been
completed (Fig. 31).

In addition to the new interferometer, the Institute is working on the concept
of a lunar radio telescope: the Lunar Antenna for Radio Astronomy (LARA). This
instrument is being developed for the Argentine Space Agency CONAE and is a
small payload to be transported on an L2 lunar orbiter based on the
Netherlands-China Low-Frequency Explorer (NCLE) and CubeRRT. It consists of a
deployable antenna using elastic energy and a digital receiver operating in
the range of 30 MHz to 300 MHz (Fig. 32). The scientific objective is to
provide CONAE with a technological demonstration for a lunar mission that will
be able to measure synchrotron radiation from Jupiter, solar bursts, fast
radio transients from outside the solar system, and the level of radio
interference in the Moon.

Figure 32: Concept of the Lunar Antenna for Radio Astronomy (LARA). Figure 33:
The staff of IAR, 2022.

##  0.6 Conclusions

For 60 years, the IAR has been the leading institution in radio astronomy in
Argentina. In both good and difficult times, the IAR and its staff have
contributed to the development of science and technology in the country and
the region. The Institute has evolved from a simple station around a radio
telescope in the early 1960s to a center for advanced research and an example
of technology development and transfer today: the knowledge gained from the
development of astronomical facilities has been applied to the production of a
wide variety of technological systems with applications ranging from space
research to medical devices. At the same time, the IAR has become a benchmark
in the training of human resources, both in astrophysics and engineering.
Undoubtedly, it will continue to lead the advancement of knowledge and
technology in Argentina for decades to come.

Acknowledgements: I am grateful to Daniela Pérez for a reading the manuscript
and making valuable suggestions. I have been supported by grant PIP
11220200100554CO (CONICET) and the State Agency for Research of the Spanish
Ministry of Science and Innovation under grant
PID2019-105510GB-C31/AEI/10.13039/501100011033 and through the “Unit of
Excellence María de Maeztu 2020-2023” award to the Institute of Cosmos
Sciences (CEX2019-000918-M). I thank Paula Benaglia, Ileana Andruchow, and the
LOC of the meeting for an excellent job. My deepest gratitude to the staff of
IAR for their continuous work and efforts that gives life to such a great
institution.

## References

  * Bajaja (2009) Bajaja, E. 2009, In: Historia de la Astronomía Argentina, 217. Eds. G. E. Romero, S. A. Cellone & S. A. Cora (La Plata: AAA Book Series) https://www.iar.unlp.edu.ar/biblio/htdocs/artic/ contri/Bajaja-hist-IAR.pdf
  * BenagliaRomero (2003) Benaglia, P. & Romero, G. E. 2003, A&A, 399, 1121
  * Benagliaetal (2010) Benaglia, P., Romero, G. E., Martí, J., Peri, C. S., Araudo, A. T. 2010, A&A, 517, L10
  * Brinks-Bajaja (1986) Brinks, E. & Bajaja, E. 1986, A&A, 169, 1
  * Colombetal (1976) Colomb, F. R., Gil, M., Morras, R. 1976, A&AS, 26, 195
  * Colombetal (1993) Colomb, F. R., Hurrell, E., Olalde, J., Lemarchand, G. 1993, Third Decennial US-USSR Conference on SETI, ASP Conference Series, 47, 279
  * Colombetal (1995) Colomb, F. R., Hurrell, E., Lemarchand, G., Olalde, J. 1995, Progress in the Search for Extraterrestrial Life, ASP Conference Series, 74, 345
  * ColombDubner (1980) Colomb, F. R. & Dubner, G. M. 1980, A&A, 82, 244
  * CombiRomero (1995) Combi, J. A. & Romero, G. E. 1995, A&A, 303, 872
  * Combietal (1999) Combi, J. A., Romero, G. E., Benaglia, P. 1999, ApJ, 519, L177
  * delValle-Romero (2012) del Valle, M. V. & Romero, G. E. 2012, A&A, 543, 56
  * Gancioetl (2020) Gancio, G., et al. 2020, A&A, 633, 84
  * Kalberlaetal (2005) Kalberla, P. M. W, et al. 2005, A&A, 440, 775
  * Kaufmanetal (2002) Kaufman Bernadó, M. M., Romero, G. E., Mirabel, I. F. 2002, A&A, 385, L10
  * Meszaros (1968) Mészáros, P. 1968, Astrophysics and Space Science, 2, 510
  * MirabelTurner (1975) Mirabel, I. F. & Turner, K. 1975, Astrophysics and Space Science, 38, 381
  * Olano (1982) Olano, C.A. 1982, A&A, 112, 195
  * Perietal (2012) Peri, C. S., Benaglia, P., Brookes, D. P., Stevens, I. R., Isequilla, N. L. 2012, A&A, 538, 108
  * Perietal (2012) Peri, C. S., Benaglia, P., Isequilla, N. L. 2015, A&A, 578, 45
  * Reichetal (2001) Reich, P., Testori, J. C., Reich, W. 2001, A&A, 376, 861
  * Romeroetal (1994) Romero, G. E., Combi, J. A., Colomb, F. R. 1994, A&A, 288, 731
  * Romeroetal (1995a) Romero, G. E., Combi, J. A., Vucetich, H. 1995a, Astrophysics and Space Science, 225, 183
  * Romeroetal (1995b) Romero, G. E., Surpi, G., Vucetich, H. 1995b, A&A, 301, 641
  * Romeroetal (2002) Romero, G. E., Kaufman Bernadó, M. M., Mirabel, I. F. 2002, A&A, 393, L61
  * Romeroetal (2007) Romero, G. E., Okazaki, A. T., Orellana, M., Owocki, S. P. 2007, A&A, 474, 15
  * Romero (2020) Romero, G. E. 2020, Science Reviews, 1, 34
  * Testorietal (2001) Testori, J. C., et al. 2001, A&A, 368, 1123
  * Testorietal (2008) Testori, J. C., Reich, P., Reich, W. 2008, A&A 484, 733
  * Varsavsky (1966) Varsavsky, C. M. 1966, Space Science Reviews, 5, 419