2022 XXX 1st December

Hot & Cold QCD plans for 2023 Long Range Plan for Nuclear Science

The ALICE-USA Collaboration††thanks: See Appendix A for the list of
collaboration members ALICE-USA Collaboration

##  1 Executive Summary

The ALICE experiment was built to study many-body Quantum Chromo-Dynamics
(QCD) at high temperature and effectively zero baryon density, using
relativistic heavy-ion collisions at the Large Hadron Collider (LHC). These
collisions form the Quark Gluon Plasma (QGP), a state of matter where quarks
and gluons are no longer confined inside hadrons. The ALICE program centers
around the key questions related to QGP phenomena. These include the
macroscopic and microscopic properties of the QGP, and the details of the QGP
phase transition to hadrons, that is believed to have taken place in the early
Universe. At the same time, ALICE’s versatile setup allows for the study of pp
collisions, p–Pb collisions, and ultra-peripheral collisions. The associated
studies can provide some of the most stringent tests of QCD and Beyond
Standard Model searches. They serve as deep probes of the properties of cold
nuclear matter, and allow for investigations of stellar and interstellar
phenomena. The ALICE-USA collaboration consists of 13 institutes, representing
about 6% of the total ALICE authorship. It has, and continues to play, an
essential role in all areas of ALICE leadership: physics, instrumentation, and
management. ALICE-USA has been involved in $\sim$25% of ALICE’s $\sim$400
ALICE papers since 2009, including many of ALICE’s most cited and high profile
publications. We, the ALICE-USA collaboration, will provide two
recommendations for the U.S. 2023 Long Range Plan for Nuclear Science period,
and beyond. These recommendations are essential for maintaining the continued
success and development of the U.S. Nuclear Science community for both Hot and
Cold QCD. The recommendations will be accompanied by details of the vital
ALICE-USA scientific priorities. We will also describe the broader impact and
support needed for U.S. involvement in ALICE.

### Recommendation 1: Continue and broaden the contribution from U.S.
institutes for the ALICE program in Runs 3&4.

ALICE has just completed a number of major upgrades. ALICE-USA has made vital
contributions to the new Inner Tracking System (ITS) and Time Projection
Chamber (TPC) readout. ALICE-USA will now utilize these upgrades for a
comprehensive physics program. This ‘ALICE 2’ phase also provides a unique
opportunity for Hot and Cold QCD studies between the expected times when RHIC
discontinues taking data in 2025, and when the EIC begins taking data around
2035. The new detector setup began taking data in Run 3 (2022-2025), and will
continue to operate in Run 4 (2029-2032). During these periods, ALICE will
collect data from collisions of: Pb–Pb at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV,
pp at $\sqrt{s}=13.6$ TeV, pp at the same energy as Pb–Pb, p–Pb at
$\sqrt{s_{\mathrm{NN}}}=8$ TeV, p–O at $\sqrt{s_{\mathrm{NN}}}=9.9$ TeV, and
O–O at $\sqrt{s_{\mathrm{NN}}}=7$ TeV. The new data set of Pb–Pb collisions
($\sim$13 nb-1) represents an increase of two orders of magnitude over Runs
1&2\. These increased capabilities in Runs 3&4 will allow for:

  * •

The elucidation of the microscopic parton dynamics underlying QGP properties
using hard processes of lower cross section, such as heavy-flavor jets, over a
large range of transverse momenta;

  * •

The characterization of the macroscopic QGP properties with extended
precision, including the exploration of unknown dynamical QGP transport
parameters at LHC energies, such as the the baryon diffusion coefficient;

  * •

Deeper studies of the hadronization of heavy-flavor baryons and mesons
produced in high temperature QCD matter;

  * •

Multiple observables with an increased precision to contribute to the
development of a unified picture of particle production and QCD dynamics from
small (pp) to large (nucleus–nucleus);

  * •

Unique explorations of parton densities in protons and nuclei in a broad ($x$,
$Q^{2}$) kinematic range, reaching to $x\sim 10^{-6}$.

In relation to the last point, ALICE-USA is one of the key proponents of the
Forward Calorimeter (FoCal). This a new detector that will collect data in Run
4. The development, installation, and operation of the FoCal will occur during
the LRP duration, and prior to the start of the EIC data taking period. The
FoCal is designed to provide unique capabilities to probe the structure of
nucleons and nuclei in an unexplored regime of $x$ and $Q^{2}$. This kinematic
region, of $x\sim 10^{-6}$, will probe the regime where gluon saturated matter
is expected to be dominant. Such a small-$x$ region is not accessible at the
EIC. Global fits of this lower $x$ data, with new high precision EIC data,
will make an unprecedented step to understanding the full evolution of Parton
Distributions Functions (PDFs) in protons and nuclei. The R&D and design phase
of the EIC detectors have well defined synergies with the FoCal
instrumentation work. We also stress the additional benefits of U.S.
involvement in ALICE’s instrumentation R&D for the ITS3 sensors that will be
part of the Run 4 setup. These have various applications for the EIC detector
projects, as recognized in multiple forums. Support for both of these
endeavors is of vital importance.

Beyond the scientific and technological implications discussed, the active
participation of U.S. institutions in the ALICE collaboration in the coming
years would greatly benefit the training of the next generation of scientists.
It is vital that PhD students, postdoctoral researchers, and early career
faculty, have the opportunity to develop their data analysis, modeling, and
interpretation expertise using the largest and most complex datasets of QCD
phenomena ever garnered. These younger members of the community will also have
the opportunity to participate in the full cycle of detector operation and
development in the 2020s. Such experience is essential for maintaining the
U.S. as a leader in nuclear science. Therefore, continued support for ALICE
research is crucial. We urge a modest expansion of the ALICE efforts to allow
groups currently focused at RHIC to also participate prior to EIC operations.

### Recommendation 2: Begin participation in ALICE-3 R&D and construction.

ALICE-USA fully supports the ALICE 3 detector, a next generation detector that
is designed to operate in Runs 5&6 (2035 and beyond). It is currently in the
R&D phase, and construction of the ALICE 3 detector is expected to begin 2028.
It represents a unique direction to pursue Hot QCD studies at the highest
temperatures possible into and beyond the 2030s. ALICE 3 will have the largest
acceptance and most precise tracking ever achieved for a heavy-ion experiment.
It is expected to record $\sim$20 times more heavy-ion data compared to Runs
3&4\. The general purpose detector design of ALICE 3 will provide many physics
opportunities, which are expected to lead to new discoveries. It will also
provide a level of data quality required to transform our understanding of the
QGP, in terms of first principles QCD calculations. The key topics unique to
ALICE 3, and under the umbrella of current ALICE-USA interests, include:

  * •

Jet hadrochemistry over an unprecedented kinematic range for transverse
momentum and rapidity, to provide new ways to investigate QGP jet energy loss
and the microscopic structure of the QGP;

  * •

Measurements of multi-heavy-flavor and exotic charm states to provide unique
information on hadronization and hadronic interactions at high temperatures;

  * •

Beyond Standard Model physics with searches for axion-like particles in ultra-
peripheral collisions, which will leverage the high collision rates and large
acceptance.

The design of ALICE 3 will also enable measurements of high-precision, multi-
differential of electromagnetic radiation from the QGP to probe its early
evolution and the restoration of chiral symmetry through the coupling of
vector and axial-vector mesons. Measurements of net-quantum number
fluctuations over a wide rapidity range will constrain the susceptibilities of
the QGP, and test the realization of the cross-over phase transition expected
at LHC energies. Finally, for the first time, measurements of the production
of ultra-soft photons will be possible. These can quantitatively test the
infrared limits of quantum field theories such as QED and QCD. In order to
meet these physics goals, support for both the R&D and construction efforts is
critically required from the U.S. community. This needs to begin before the
end of the current 2023 Long Range Plan for Nuclear Science. Based on its
expertise, ALICE-USA is currently exploring contributions to the silicon and
calorimeter detectors. These parallel and independent efforts will also
benefit the EIC achieving its construction milestones.

##  2 Overview of ALICE upgrades and U.S. impact

ALICE is a large acceptance experiment, with world leading hadron
identification capabilities. Unique to the LHC experiments, ALICE’s primary
focus, design, and proposed upgrades, enable extremely accurate
characterizations of the QGP using a multitude of observables. It has
extensive capabilities in exploring few-body hadronic and nuclear
interactions. The first major upgrade for LHC Runs 3&4 enables an increase of
ALICE’s recorded luminosities by two orders of magnitude via the introduction
of a continuous readout system in the TPC. The second involves a greatly
improved tracking performance using new inner trackers - ITS2 in Run 3, and
ITS3 in Run 4. The ITS2 system has a reduction of a factor three in radiation
lengths, and factor two improvement in pointing resolution compared to the ITS
used in Runs 1&2\. Further factor of three improvements of the pointing
resolution for ITS3 (compared to ITS2) will occur via the introduction of
wafer-scale ultra-thin silicon strips, which are also being pursued by the
EPIC detector at the EIC. Details of these upgrades and others can be found
elsewhere [1]. For Run 4, the Forward Calorimeter detector (FoCal) upgrade,
with both electromagnetic and hadronic calorimeters, will provide unique
capabilities for both Cold and Hot QCD studies at forward rapidities of
$3.4<\eta<5.8$ [2].

For Runs 5&6, a completely new detector has been proposed, named ALICE 3 [3].
Designed by heavy-ion physicists for heavy-ion physics, it is a next
generation detector with a main tracking system that covers a large
pseudorapidity range ($-4<\eta<4$), and can reconstruct charged tracks down to
$p_{\rm T}\sim 100$ MeV/c. The light weight and small radiation length design
will greatly reduce the background and improve tracking resolution for
electromagnetic and heavy-flavor probes, compared to ALICE 2. The pointing
resolution at midrapidity is projected to be about three times better than
that of the ITS3, which is in part achieved by placing a highly novel vertex
detector 5mm from the beamline. The tracking system is placed in a
superconducting solenoid with a field of up to $B=2$ T, to obtain a momentum
resolution of 1–2% over a broad pseudorapidity and momentum range. This
tracking is complemented by multiple sub-detector systems for particle
identification; two TOF detectors and a RICH detector. These systems have the
ability to identify leptons and photons in the entire thermal emission range
of $p_{\rm T}\lesssim 3$ GeV/c, which is inaccessible for other LHC
experiments. Charged hadron identification on the $3\sigma$ level is possible
up to $p_{\rm T}\sim 14$ GeV/c, and decay hadrons can be reconstructed much
more efficiently and cleanly at low-$p_{\rm T}$ compared to ALICE 2. The fast
readout systems will be able to record all of the expected heavy-ion
luminosity provided by the LHC. The ALICE 3 program aims to collect an
integrated luminosity of about 35 nb-1 with Pb–Pb collisions and 18 fb-1 with
pp collisions at top LHC energy. The potential to further increase the
luminosity for ion running in the LHC by using smaller ions, e.g. 84Kr or
128Xe, as well as further runs with small collision systems, is being
explored.

Figure 1: Event display of one of the first LHC Run 3 Pb–Pb
$\sqrt{s_{\mathrm{NN}}}\leavevmode\nobreak\ =\leavevmode\nobreak\ 5.36$ TeV
collisions in November 18th 2022, using many detectors ALICE-USA has
contributed toward the construction and upgrade.

As of September 2022, the ALICE-USA groups consist of teams from two national
laboratories: Lawrence Berkeley (LBNL) and Oak Ridge (ORNL), and nine
universities: California (Berkeley), CalPoly, Chicago State, Creighton,
Houston, Kansas, Ohio State, Tennessee (Knoxville), Texas (Austin), Wayne
State, and Yale. While the majority of the program is supported by DOE, there
are two NSF supported university teams. ALICE-USA has been responsible for two
completed DOE funded projects - the EMCal/DCal in Runs 1&2 [4], and the most
recently completed Barrel Tracking Upgrade (BTU) for Runs 3&4 111See the BTU
website at https://sites.google.com/site/alicebtusite for further details. The
BTU project involved critical U.S. contributions to both the TPC and ITS
upgrades. A key example of the success of all these projects can be observed
in Fig. 1, which shows one of the first ALICE Run 3 event-displays of Pb–Pb
collisions using many detectors where ALICE-USA has made essential
contributions. Thanks to the long and successful experience building and
operating calorimeters, ALICE-USA is leading the FoCal project [5]. It also
oversees a major computing project providing the necessary U.S. contribution
to ALICE’s data processing infrastructure, under the umbrella of the Worldwide
LHC Computing GRID (WLCG).

In terms of the scientific output, the contributions from ALICE-USA span a
wide range of physics topics within the collaboration, and have been an
essential part of the ALICE physics program. Although ALICE-USA members are
present at an about 6% level in terms of authorship within the collaboration,
they have provided leadership in terms of paper committee contributions well
beyond the average contributions. ALICE-USA members were involved in 1 of 2
Nature articles published by ALICE [6], and 1 of 2 Nature Physics articles
[7]. ALICE-USA members were also involved in 5 of the top 7 most cited physics
publications from ALICE [8, 9, 10, 11, 7], and numerous other high-profile
publications. These papers span both the hard and soft physics using heavy-ion
and pp collisions. ALICE has published just over 400 papers since 2009, with
the overall U.S. involvement on the paper committee level being about 25%.

In addition to publications, members of the ALICE-USA have held numerous
leadership roles across all seniority levels of the collaboration regarding
physics output. These include: Deputy Physics Coordinator (two times),
multiple Physics Working Group (PWG) Conveners, and Physics Analysis Group
Coordinators. Of particular note, out of the 8 ALICE PWGs, currently ALICE-USA
members co-convene 4 (50%) of the those PWGs. Over the years members of ALICE-
USA were copiously represented in the governance of ALICE – currently with
members of the collaboration holding functions as members of the Conference
Committee and the Management Board, and as one of the two Deputy
Spokespersons. Moreover, in the past ALICE-USA members served as Chair of the
Collaboration Board and co-chair of the Editorial Board. Finally, for the
$\sim 100$ ALICE-USA members, roughly half are PhD students. At this rate,
ALICE-USA institutions have graduated $\sim 50$ PhD students over a 5 year
period. Many of our students have achieved leading roles in research positions
in our field and beyond, as well as in industry and teaching jobs.

##  3 A brief summary of ALICE accomplishments from Runs 1&2

The observations ALICE has made during Runs 1&2 (2009-2019) have profoundly
changed the landscape of QCD at high temperatures and collision energies [12].
They have been carried out in conjunction with other LHC experiments, and with
the continuation of the RHIC program - where huge advances in luminosities and
centre-of-mass energy coverage have been achieved. ALICE explorations of high
temperature QCD have continued to reveal emergent behavior in many-body
interactions at the highest possible temperatures in the laboratory. In heavy-
ion collisions, ALICE has measured significant yield suppression for a wide
range of hadrons and reconstructed jets in both inclusive and coincidence
channels, showing that in-medium energy loss occurs at the partonic level, and
quantifying its magnitude. ALICE measurements of heavy-flavor yield
suppression provide insight into details of this process, via the QCD “dead-
cone” effect. This manifests itself in a larger energy loss observed for charm
compared to bottom quarks. It has also, for the time, provided direct evidence
of the dead-cone effect for charm quarks in pp collisions, using state of the
art jet re-clustering algorithms. ALICE jet substructure measurements indicate
preferential suppression of wide-angle radiation in the jet shower within the
QGP, which is sensitive to color coherence and the space-time structure of
jets.

In the soft sector, ALICE measurements of identified hadron spectra and
anisotropic flow demonstrate that a QGP formed at LHC energies undergoes the
most rapid expansion ever observed for a many-body system in the laboratory.
The hydrodynamic description of a huge variety of such data has been tested in
heavy-ion collisions at the LHC, offering an environment far beyond the usual
application in fluid dynamics. It has shown a QGP is strongly coupled at
scales of the QGP’s temperature, on the order of a few hundred MeV. ALICE
measurements also imply thermalisation effects for charm quarks in a QGP, and
when coupled with transport model calculations, the ALICE data demonstrate
microscopically how equilibrium can occur on extremely small time scales. In
addition, ALICE results have demonstrated that a QGP transitions into
chemically equilibrated hadrons. ALICE has the most extensive set of
measurements ever achieved regarding identified particle production, including
for the first time a statistical description in the charm sector. ALICE
investigations into the hadron-gas phase, via resonance and femtoscopic
measurements, indicate this phase is prolonged, and that the decoupling of
particles from the expanding hadron gas is likely to be a continuous process.

Using RHIC and LHC Run 1&2 data, comprehensive efforts at global fitting for
precise determination of QGP properties and dynamics are currently underway,
utilizing the powerful approach of Bayesian Inference for rigorous comparisons
between theory and experiments. Such analyses are more advanced in the soft
sector, and are in their infancy in the hard probes sector. ALICE has played a
critical role in this endeavor by providing the experimental data, and
developing theory-experiment synergies. Along with other RHIC and LHC
experiments, ALICE has discovered QGP-like signatures in high multiplicity pp
and p-Pb collisions, which probe the thresholds of QGP formation. Such
findings have ignited a debate of whether pp and p-Pb collisions at LHC and
RHIC energies could create small QGP droplets.

ALICE has investigated few-body hadronic interactions on the soft and hard
scales to an unprecedented precision. In pp collisions, it has demonstrated
charmed hadron fragmentation functions are not universal with respect to e+e-
collisions, revealing unique hadronisation mechanisms that enhance charmed
baryons compared to mesons. It also has provided world leading constraints
regarding how rarely produced hadrons interact with stable nuclear matter,
whose behavior have broad implications for understanding various features in
the Universe, such as the equation of state of neutron stars and the
interstellar composition of dark matter. Finally, ALICE has been a leader in
the study of ultra-peripheral heavy-ion collisions since Run 1, exploring cold
QCD matter at the highest possible photon-induced energies. ALICE has observed
photoproduction of vector mesons on proton and ion targets, namely, the
$J/\psi$, $\psi(2S)$, and $\rho$ mesons. $J/\psi$ photoproduction is
moderately suppressed on Pb targets compared to free protons, in line with
nuclear shadowing effects for gluon parton distribution functions at $x\approx
10^{-3}$. Coherent $\rho$ photoproduction measurements also demonstrate
nuclear shadowing for both the Xe and Pb nuclei.

##  4 Key physics questions addressed for LHC Runs 3&4 and beyond from ALICE-
USA

Despite the many extraordinary scientific findings by the ALICE Collaboration,
many open questions remain, and will be addressed using data to be collected
in Runs 3&4 at the LHC (2022-2032). This section details such questions, with
a description of the leadership ALICE-USA groups intend to provide addressing
them. Opportunities using the ALICE 3 detector in the Run 5&6 period (2035 and
beyond) will also be discussed.

  1. 1.

What is the gluonic structure in protons & nucleons at small-$x$?

We propose to maximize the physics potential for small-$x$ physics in ALICE
with the installation a high-granularity forward calorimeter (FoCal) in the
LHC Run 4 (2029–2032). The FoCal will explore the small-$x$ parton structure
of nucleons and protons in a kinematic domain only accessible at the LHC [5].
It is designed to provide unique capabilities to investigate Parton
Distribution Functions (PDFs) in the unexplored regime of $x\sim 10^{-6}$ and
four momentum transfer $Q$ of a few GeV. In this regime, it is expected that
the hadronic structure requires non-linear dynamics due to the high gluon
densities, leading to gluon saturation. Gluon saturation is a necessary
consequence of the non-Abelian nature of QCD. Its discovery would represent a
landmark in our understanding of the strong interaction. The primary
objectives of the FoCal include: high-precision inclusive direct photon and
jets measurements in pp and p–Pb collisions, photon-jet and jet-jet
measurements in pp and p–Pb collisions, and measurements of vector mesons
photoproduction in ultra-peripheral p–Pb and Pb–Pb collisions. These
measurements by the FoCal would constitute an essential part of a
comprehensive small-$x$ program at the LHC, with a broad array of
complementary probes. While the FoCal has some complementarity to future
measurements by the EIC [13] and the LHCb experiment, it can reach
considerably lower $x$ values than the EIC.

Vector meson photoproduction and electroweak boson studies have provided
unique constraints on the gluon parton distribution function for nuclei using
Runs 1& 2 data [14, 15, 16]. They provide clear evidence that the partonic
structure of nuclei is different compared to free protons, with nuclear
shadowing effects increasing with decreasing longitudinal momentum fractions
$x$. In addition, no evidence of saturation of the gluon PDF in the proton is
observed between HERA and LHC energies, down to $x\sim 10^{-5}$ [17, 18]. For
a Pb target, the $J/\psi$ photoproduction cross-section is compatible with
moderate gluon shadowing, in line with the leading twist approximation and the
central values of the EPS09 and EPPS16 nuclear parameterizations [19, 20, 21,
22]. The ALICE data has smaller uncertainties than these parameterizations,
pointing to the utility of UPC data for constraining parton distributions
[23]. The comparison of the energy dependence of photoproduced vector mesons
measured in p–Pb and Pb–Pb collisions is expected to significantly reduce the
theoretical uncertainties [24], thus a systematic program of UPC vector meson
measurements is needed.

The FoCal will enable measurements of photoproduced J/$\rm\psi$ and $\psi$(2S)
mesons to reach a $x\sim 10^{-6}$ and $Q^{2}\approx 3-4$ GeV2. This is a
region that cannot be explored by the EIC. The energy dependence of the ratio
between $\psi(2S)$ and J/$\rm\psi$ is particularly sensitive to the difference
between linear and non-linear gluon evolution [25], hence saturation effects.
Photoproduction data in Runs 3&4 will allow ALICE to go beyond measurements of
vector meson cross-sections. Studies of open charm [26] and/or dijets [27] are
sensitive to gluon distributions with fewer theoretical assumptions than for
vector mesons. The study of angular correlations of diffractive dijets [28] is
also sensitive to the gluon nuclear density, particularly for low-momentum
track-based jets where ALICE would have a competitive advantage.

Measurements of $d\sigma/dt$ for coherent and incoherent photoproduction of
vector mesons are respectively sensitive to the transverse distribution of
gluons in a nucleus (similar to a GPD) [29, 30], and the event-by-event
fluctuations in the nuclear configuration, including gluonic hotspots [31, 32,
33]. ALICE will collect large enough data samples to make these measurements
for a number of different mesons [24], including the $J/\psi$ and the $\rho$.
The new ALICE streaming DAQ system will be important for increasing the vector
meson sample sizes, and reducing systematic uncertainties due to the trigger
requirements. The FoCal detector will allow ALICE to extend these measurements
to even lower $x$ values, providing enhanced sensitivity for gluon saturation.
The future UPC measurements using the FoCal will also benefit from having
other ALICE detectors. It is anticipated that the Zero Degree Calorimeters can
be utilized to distinguish the photon direction in ultra-peripheral Pb–Pb
collisions [34]. This will allow for the exploration of the lowest possible
$x$ values in Pb. ALICE measurements can also be carried out combining the
FoCal together with the central barrel or the forward muon detector. In this
way, a larger kinematic range can be explored, which is vital for gluon
saturation searches. More details of the physics prospects in ultra-peripheral
heavy-ion collisions using the FoCal detector can be found elsewhere [35].

Gluon saturation at low-$x$ has also been postulated to be the source of
quantum entanglement effects. These could lead to instant thermalization based
on the potential equivalence of entanglement and thermodynamic entropy, and a
determination of final state particle production in the context of minimal
decoherence effects. In order to study this type of parton-hadron duality,
ALICE can complement the EIC measurements in ep and eA systems via
measurements in pp, pA and AA. The survival of the coherent state as a
function of system size can be measured through particle multiplicities and
quantum tomographic correlation functions of particles from low-$x$ processes.
Quantum entanglement can also be studied by reconstructing the density state
operator using quantum tomography [36]. In this direction, the study of
exclusive four $\pi$ photoproduction will provide a search for exited $\rho$
states [37].

  2. 2.

What are the large scale nuclear structures and QGP transport parameters?

Large scale nuclear structures, such as the nuclear deformation and radius,
can be precisely investigated using measurements of anisotropic flow. This is
achieved courtesy of the QGP hydrodynamic response, where initial state
features are imprinted on final state anisotropic flow observables. These
nuclear structures are often accessible via low energy nuclear experiments,
but are limited to the electrically charged proton profiles. Measurements in
heavy-ion collisions can in principle access the entire nuclear matter
profile. These profiles are also inaccessible for DIS nuclear collisions by
the virtue of their design to probe sub nucleon scales. Such measurements have
already been used to determine 129Xe deformation, with values of the
quadrapole deformation $\beta_{2}=0.18\pm 0.02$ obtained in Run 2 from ALICE
data [38]. These have never been measured previously. Similar studies have
proved equally successful constraining the 238U and 197Au deformations from
RHIC data [39]. Firstly, we propose to carry out a suite of anisotropic flow
measurements in 16O–16O and p-16O collisions. These will occur in Run 3.
Hydrodynamic calculations have shown such measurements are sensitive to
$\alpha$ clustering in the 16O nucleus [40], a longstanding pursuit in the low
energy nuclear community. The clustering effects are calculable in Lattice
QCD. Clustering affects anisotropic flow measurements more strongly at LHC
energies than at RHIC.

Anisotropic flow and flow fluctuations measurements in central large nuclei
A-A collisions are particularly sensitive to the nuclear profile. In addition
to Xe–Xe collisions at the LHC, this has also been demonstrated with data from
the RHIC Isobar run coupled with hydrodynamic calculations, where these
profile parameters have been highly constrained [41]. Anisotropic flow
measurements of $v_{2}$ and $v_{3}$ in very central Pb–Pb collisions at the
LHC have also revealed the “ultra-central problem” - that is a deviation from
the hydrodynamic description where it is expected to be most applicable (for a
recent review see here [42]). Proposals to explore this further, such as the
introduction of an octupole deformation ($\beta_{3}$) for the Pb nucleus [43],
which has been predicted for the doubly magic Pb208 ground state, require
measurements of $v_{3}\\{4\\}/v_{3}\\{2\\}$ well beyond the accuracy of those
achieved in Runs 1&2 by ALICE [11]. There are also predictions for an
quadrapole deformation of $\beta_{2}=0.05$ for Pb208 [44], which would lead to
finite value of $v_{2}\\{4\\}$. This has yet to be observed in within
statistic precision for previous ALICE data in very central Pb-Pb collisions
[45]. Therefore, the greater statistics provided by Runs 3-6 will be crucial
in unraveling these nuclear properties. Their constraints are also critical
for the determination of QGP transport parameters obtained by the hydrodynamic
framework, in the regime where hydrodynamics is most applicable. These
transport parameters include the famous shear viscosity over entropy ratio
$\eta/s$. Such nuclear profile constraints could be particularly important for
ALICE 3, as non double magic nuclei such as 84Kr or 128Xe (with larger
deformations compared to the double magic 208Pb nucleus) are being explored
for heavy-ion running.

The hydrodynamic framework used to describe collective flow involves numerous
transport parameters, which characterize the coupling of the QGP. To date,
only two of those, the shear and bulk viscosity over entropy ratios ($\eta/s$
and $\zeta/s$), have benefited from extensive constraints using heavy-ion
data. Those constraints provide evidence the QGP is the most strongly coupled
system ever studied in the lab. The corresponding relaxation times, of the
order of 1 fm/c or less, demonstrate how thermalization can be achieved on the
most rapid timescales ever observed for a many body system. Comparisons of
these extracted transport parameters to fundamental descriptions offer a
unique test to Holographic models [46]. These include AdS/CFT, which predicts
$\eta/s=1/4\pi$ in the infinite coupling limit. Such approaches assume a
correspondence of 5 dimensional strong gravitational fields in a black hole
with a 4 dimensional high temperature QCD system. These transport parameters
are also predictable in Lattice QCD, but often have much larger uncertainties
[47].

Two-particle correlations with net baryons can be used to explore additional
transport parameters of the hydrodynamic evolution. The baryon diffusion
constant $D_{B}$ is an example of such a parameter beyond $\eta/s$ and
$\zeta/s$. It characterizes the mobility of baryon number, and is predicted to
be finite at the LHC, despite the fact that $\mu_{B}\sim 0$. A two-particle
correlation function has been proposed to constrain $D_{B}$ [48]. It explores
correlations of net-baryon fluctuations as a function of azimuthal and
rapidity separations. Such an analysis has yet to be carried out from Run 1 or
2 data, since it is statistically challenging. It will be greatly aided by the
increase of two orders of magnitude in the Pb–Pb integrated luminosity
foreseen for Runs 3&4\. The ALICE detector is particularly suited to this
task, given its world leading particle identification capabilities. Finally,
the recently published measurements of balance functions (BFs) of identified
charged hadrons $(\pi,\rm K,\rm p)$ in Pb–Pb collisions from Runs 1&2 play an
important role in constraining the charge diffusion coefficient $D_{e}$ for
quarks [49]. However, the central tracking acceptance of $-1<\eta<1$ for Runs
1&2 leads to large uncertainties in $D_{e}$ from ALICE BF data [50]. In turn,
this leads to values between $0.5D_{Latt}<D_{e}<4D_{Latt}$ being permissible,
where $D_{Latt}$ represents the Lattice QCD prediction. These uncertainties
will be significantly reduced when the same measurements are performed using
the ALICE 3 setup, as the acceptance where identified charged hadron
measurements can be made will increase to $-4<\eta<4$. The reduction in
$D_{e}$ uncertainties due to increases in the $\Delta\eta$ coverage are
expected to be at least a factor 4.

  3. 3.

How does the QGP affect hard probes?

The energetic partons produced in heavy-ion collisions from hard scatterings
in the initial collision undergo successive branching. This results in a
parton shower that can be modified in the presence of the QGP. While the
produced particles are highly collimated about the direction of the initial
parton, they also cover a range of different momentum scales. The properties
of these collimated sprays of particles, known as jets, and how they emerge
from QCD calculations, have been extensively studied in high-energy physics.
Jets are a primary tool for uncovering the details of interactions of partons
with the QGP medium. This is because jets lose energy and are modified as they
traverse the medium, a phenomenon called jet quenching. ALICE is pursuing a
multi-messenger approach that systematically studies different features of jet
quenching to explore the microscopic structure of the QGP and its governing
degrees of freedom.

The inclusive jet and hadron production, as well as their correlations from
low- to high-$p_{\rm T}$, allow for an assessment of one of the hallmarks of
jet quenching - jet energy loss to the medium. In practice, although energy is
conserved, the yields of jets with finite jet resolution parameter $R$ are
suppressed in heavy-ion collisions due to jet-medium interactions. The
suppression of the leading hadrons and fully reconstructed jets in Pb–Pb
collision at the LHC broadened the observations found at RHIC. That is the QGP
is opaque to jets over a large energy range [51, 52, 53]. Recent measurements
of hadron-jet correlations [54] show a potential explanation for the fate of
the lost energy in the jet quenching process - in both the momentum and
angular scale. The energy is distributed over large angles, and recovered in
low-$p_{\rm T}$ (below 20 GeV/c) jets. Additionally, new results on inclusive
jet suppression [55], using Machine Learning techniques [56], show that larger
$R$ jets (up to $R=0.6$) are suppressed relative to smaller $R$ jets at
$p_{\rm T}$ = 40 GeV/c, and the energy is not yet recovered. In Runs 3&4,
ALICE will continue measurements of jet energy loss with inclusive jet
suppression and semi-inclusive coincidences of hadron-jet [57], photon-hadron,
and photon-jet. The substantial increase in statistics and new experimental
techniques, such as mixed events and Machine Learning [56], will provide
increased precision and an extended kinematic range for jets, and high-$p_{\rm
T}$ hadrons and photons.

At the same time, the hadron-jet coincidences are sensitive to the _jet-
correlated_ response of the medium to the presence of energetic probes [57].
This flux of medium energy induced by the propagating jet is one of the most
sought after consequences of jet-medium interactions [58]. A medium composed
of scattering centers - quasi-particles - ought to, with some finite
probability, induce large angle Moliere scatterings of the propagating partons
[59, 60]. Such a jet deflection should manifest itself in the modified
acoplanarity of the di-jet pair in Pb–Pb compared to pp collisions. Utilizing
hadron-jet coincidences, ALICE has investigated this effect at the lowest jet
$p_{\rm T}$ possible and with a large jet $R$ [54]. Additionally, jet
deflection should manifest itself in large momentum kicks to the hard core of
the jet [61, 59]. ALICE also uses the groomed hardest $k_{T}$ to probe this by
looking at differences between pp and Pb–Pb collisions at larger groomed
$k_{\rm T}$ values [55]. Likewise, in this area, Runs 3&4 will provide further
precision to put improved quantitative constraints on the probability of this
process within the QGP.

Jet fragmentation patterns and modification of the jet structure are also key
to understanding of the interactions of the medium with the jet. ALICE has for
the first time measured fully corrected angular and momentum sub-jet structure
of jets [62, 63], and has made advances towards measurements of other
quantities such as jet angularities, jet-axes, the Lund Plane, and intrajet
hadron correlations [64, 54, 65, 66]. These measurements are compared to both
Monte Carlo models and analytical calculations. They offer a new and more
stringent view of the jet-medium interactions. In particular, new measurements
show the largest angle splittings within a groomed jet are suppressed as
compared to small-angle splittings, which can be connected to a characteristic
aperture scale at which the jet fragments/prongs interact with the medium
incoherently. Moreover, the new measurements of leading subjets suggest
sensitivity of the predicted flavor dependence of jet quenching that should be
quantified with more precise data. Concurrently, the new measurements of jet-
axes disfavor the intra-jet $p_{\rm T}$ broadening, as prescribed by the BDMPS
formalism as the main mechanism of energy loss in the QGP [67]. The Run 3&4
data will be instrumental in providing the necessary precision to further
investigate such observations. However, most importantly, the new high
statistics data will enable new differential measurements [24]. These will
study different regions of the parton shower phase-space related to rare/hard
vs. multiple-soft medium-induced radiation. This can be achieved via
simultaneous measurements of angular and momentum distributions of jet
structure such as the Jet Lund Plane [68].

The modification of jets in the medium is expected to depend on the path the
jet traveled in the medium. ALICE traditionally measures this in two ways.
They involve using correlations and measuring the jet yield suppression with
respect to the event-plane angle. Measurements by ALICE of hadron-jet
correlations show no dependence on the angle of the jet with respect to the
event plane angle within the uncertainties [69]. On the other hand,
measurements of the jet $v_{2}$ demonstrate a significant azimuthal anisotropy
[70]. ALICE has continued to explore these effects using higher statistics
data from Run 2. For example, a technique that utilizes Event Shape
Engineering [71] to select events based on their anisotropy within a
centrality class, demonstrates an increased suppression of out-of-plane yields
for more anisotropic events [72]. This technique can be further pursued using
the higher statistics Run 3&4 data to investigate the path length dependence
of jet substructure modification. Finally, a first look at photon-jet
correlation measurements with ALICE was investigated [73]. These studies
demonstrated the ability to measure photon-tagged jets at lower $p_{\rm
T}^{\gamma}$ (20 GeV/$c$). This measurement is statistically limited, thus
future ALICE photon-jet measurements will be crucial for measuring the
absolute energy loss, since the photon provides an estimate of the initial
$p_{\rm T}$ of the jet.

ALICE has also performed measurements of the heavy-flavor hadrons as an
additional probe of parton-medium interactions, with examples found in [74,
75, 76]. Heavy flavor hadrons offer a unique opportunity to study the medium
effects because of the well established probe - heavy quarks. These are
created early in the hard scatterings, and maintain their flavor identity
throughout the evolution of the medium. A new set of measurements focusing on
the leading heavy-flavor particles will take advantage of this theoretical
control. It will also profit from improved experimental control, such as
better discrimination of ”combinatorial jets” - the uncorrelated background to
hard scattering flux of energy, over the light flavor measurements. This
applies to both measurements of the heavy-flavor jet $p_{\rm T}$ spectra and
jet substructure observables. One of the striking findings by ALICE is the
observed mass dependence of energy loss. This manifests itself in smaller
nuclear modification factors for prompt $D$ mesons (formed from charm)
compared to non-prompt $D$ mesons (formed from bottom quarks) at $p_{\rm T}$
$>8$ GeV/$c$. This onset is interpreted as the dead cone effect [6]. In future
runs, ALICE will take advantage of its precision tracking to study this effect
in more detail, and contrast it with the impact of gluon splitting processes
within the parton shower [77].

In general, with the newly upgraded ALICE detector, the data from Runs3&4 are
expected to improve the quantification of how light and heavy quarks propagate
within the medium. The new high-statistics measurements will bring additional
constraints to the extraction of the transverse and longitudinal transport
coefficients. These include the jet transport parameter $\hat{q}$ [78, 79],
which quantifies the transverse diffusion. The longitudinal drag $\hat{e}$ and
diffusion $\hat{e_{2}}$ coefficients [80], as well as the heavy-flavor
diffusion coefficient $D_{s}$ [81], are also expected to be better
constrained. These coefficients characterize the coupling of such processes to
the medium, and the ultimate task is to perform a dedicated set of
measurements that will allow further connections to the emergence of the QGP
on a microscopic level. Runs 3&4 will also allow for a revisiting of the jet
substructure measurements for the heavy-flavor induced jets e.g. angular
structure of groomed and ungroomed jets. This can be achieved over a broad
kinematic regime, that is largely unique to ALICE (from $p_{\rm T}$
$\approx\leavevmode\nobreak\ 0$ of heavy-flavor hadrons to hundreds of GeV/$c$
jets). The new data will enable ALICE to fully explore the momentum and the
angular correlations within the jet substructure with a systematic study of
the Lund Plane (see considerations in [24]). With this new data, and new
techniques, the flavor dependence of the Lund Plane can be extended beyond the
charm sector [6] to the beauty sector [82].

For Runs 5&6, ALICE 3 will provide an environment for precision jet
measurements over much larger kinematic ranges than previously accessible at
the LHC. Specifically, jet hadrochemistry will become possible over such a
range with the excellent PID capabilities for identifying hadrons inside jets.
This is valuable to study in vacuum to provide inputs to hadronization models
[83], and also in the QGP since the interactions with the medium could change
the chemistry make-up of a jet [84]. Additionally, ALICE 3 will provide
increased resolution, purity, and statistics for heavy-flavor jets. The
increased psuedorapidity coverage will allow for measurements of heavy-flavor
jet correlations or photon-heavy-flavor jet correlations with unprecedented
purity at low transverse momentum scales, providing insight into the
microscopic properties of in-medium energy loss, including its path length
dependence. Finally, ALICE 3 allows for the possibility to resolve heavy-
flavor hadron pairs inside of jets to probe gluon splittings, which can be
used to study the space-time picture of jet quenching [85], and access a pure
gluon sample of jets.

Finally, while we are in the midst of fully understanding the measurements
that need to be done, the continued development of analysis techniques is
essential for jets in a complex environment. This is related to both the
careful selection of the most useful measurements [86], but also to the
development of the appropriate correction strategies allowing for meaningful
theoretical comparisons. These include fully corrected substructure
measurements [87, 88], and background mitigated hadron-jet correlations down
to lowest jet momenta with large jet resolution parameters. At the same time,
it is important to encourage the divulgence of uncertainty correlations for
the new generation of measurements. Such an activity will take advantage of
the newly developed Bayesian inference methods [78, 89]. This will also aid
Machine Learning projects that will drive the development of the modeling of
jet-medium interactions in the coming years [90, 91, 86, 92].

  4. 4.

How do rarely produced hadrons form and interact?

A detailed program by ALICE for Runs 3-6 will shed further light on more
exotic heavier flavor states in order to unambiguously determine the
hadronization mechanisms, and thus the creation of matter, as a function of
flavor. In this regard, the measurements of multi-heavy-flavour hadrons and
exotic states offer unprecedented sensitivity. To achieve high statistics for
multi-heavy-flavour particles, a novel experimental approach is needed to
track all their decay products, typically including hyperons, before they
decay. This calls very high tracking/vertexing precision very close to the
interaction point, particle identification capabilities over a wide transverse
momentum range, and high readout rates. Moreover, large acceptance is
required, not only for reasons of statistics, but also in order to investigate
the dependence of the production of multi-heavy-flavour hadrons on the
variation of the heavy quark density with rapidity.

Recent flavor dependent hadronization measurements have become a unifying
theme for the bulk and jet sector of QCD. The hadronization of a final state
parton cannot be described adopting a perturbative approach, and is usually
modelled through a phase of string breaking and/or cluster formation, which is
considered to be independent of the surrounding parton density [93]. However,
as also confirmed by measurements of e.g. baryon yields in pp collisions at
the LHC [7, 94], additional hadronization mechanisms may exist, whereby quarks
that are close in phase space can combine into colourless hadrons. Dynamic
modeling of these processes, in particular in the strange and charm sector,
has led to many novel approaches that were implemented in established event
generator models, such as PYTHIA. Color reconnection (CR) and multiple parton
interactions (MPI) were attempts to parameterize features of the initial state
and final state into the actual formation process of hadrons. Furthermore, it
was shown that initial state gluon saturation might lead to quantum
entanglement effects, which impact the final state hadron multiplicities [95].

Experimental advances in the strange sector include the detailed mapping of
strangeness enhancement in proton-proton collisions as a function of
centrality [7], and the potential flavor hierarchy in the light/strange hadron
formation during the QCD crossover [96]. These led to refinements of the
transport codes that were then followed up by more detailed measurements of
baryon to meson formation in the strange sector. Such advancements have been
studied in the charm sector, as a function of system size [94]. In heavy-ion
collisions, where partons may travel freely over distances much larger than
the typical hadron sizes, and a dense system of partons close to thermal
equilibrium is formed, recombination mechanisms become more dominant. These
make the production of baryons and other heavy hadrons more favourable than in
pp collisions [97]. First measurements of baryon/meson ratios in the charm
sector also indicate a low-$p_{\rm T}$ enhancement consistent with such a
picture that has dominated similar measurements in the light and strange
sector associated with the soft underlying event [98].

A comprehensive campaign of precision measurements of charm baryon production
yields across collision systems is planned for Runs 3&4\. This starts with
baryons containing only one charm quark, but several light and strange quark
combinations. The evolution of particle production as a function of increasing
strangeness or charm content is one of the main open questions regarding
hadron formation, which is a key feature of the non-perturbative sector of
QCD. For example, LHCb has found a series of tetra- and penta-quark states in
the charm sector. A detailed study of strange penta- and hexa-quark states in
ALICE found no evidence in the strange sector. The measured upper limits are
several orders of magnitude below estimates from statistical hadronization
models (SHM), which have done extremely well on postulating the yields of
composite objects in the light sector [99]. SHM predictions assume full
equilibration of all flavors up to charm. The comparison of the measured
yields to these predictions would provide a very sensitive measure of the
degree of equilibration of charm quarks in the medium, including its system-
size dependence.

Ultimately, it must be possible to relate the equilibration level to the
transport properties of the QGP via kinetic transport modeling in order to
provide a consistent description of the entire phenomenology. Complementing
these opportunities, relativistic heavy-ion collisions provide the only
available tool to study the formation and interaction of exotic hadronic
states inside a medium filled with deconfined color charges. This puts new
constraints on the properties of the states, the characterization of their
binding potential, and the details of their hadronization mechanisms. Nuclear
collisions at the LHC energies, in particular, offer a unique opportunity to
test these mechanisms in the presence of large charm-quark densities. In this
environment, while the screening of color charges may reduce the yield of
certain bound states, medium-induced or medium enhanced mechanisms may enhance
their production in the low-$p_{\rm T}$ region, as predicted by statistical
hadronization models. The interest of these observations is twofold. On one
hand, the rates of formation and dissociation of bound states depend on their
binding energy and size, and the measurement of the production rates and
anisotropic flow will allow one to distinguish compact multi-quark
configurations from molecular states. On the other hand, states whose nature
can be determined by other means, for example by measuring interaction
potentials through final state interactions, could be exploited as a
calibrated probe in nuclear collisions. This would shed light on the
underlying mechanisms and timescales driving the hadronization in a deconfined
medium, and test the properties of the deconfined medium itself.

Predictions for light flavor tetraquark states have existed for over 40 years
[100], but the existence of these states has up to now not been confirmed. To
begin to address this, ALICE has published three papers on tetraquark studies
of the $a_{0}(980)$ resonance, a tetraquark candidate, using kaon-kaon
correlations from Runs 1&2 [101, 102, 103] for pp and Pb–Pb collisions. In
Runs 3&4, ALICE will continue these two-meson correlation studies for other
low lying tetraquark candidates, the $K^{*}_{0}(700)$ and $f_{0}(500)$. The
high rate capabilities in Run 3&4, will also allow access to rare phenomena in
the hadronic phase formed after the phase transition. These include the
production of nuclei, hypernuclei and, combined with excellent heavy-flavour
capabilities, possibly the as-yet undiscovered supernuclei (nuclei in which a
nucleon is replaced by a charmed baryon). Wide acceptance, again combined with
outstanding heavy-flavour performance at low transverse momenta, will allow to
extend the studies of hadron-hadron potentials via two-particle correlations
to the charm sector. This will provide a powerful tool to investigate the
structure of the newly discovered charmed exotic states. In Runs 5&6, the
measurement of the production yields of multi-charm baryons, which can only be
produced by combination of uncorrelated charm quarks, would provide a
qualitatively new handle on the production of heavy-flavour hadrons.
Measurements of multi-charm hadrons, such as the $\Xi^{+}_{cc}$ (ccd),
$\Xi^{++}_{cc}$ (ccu), $\Omega^{+}_{cc}$ (ccs), $\Omega^{++}_{ccc}$ (ccc), and
exotic states such as the newly discovered T${}^{+}_{cc}$ (ccud) [104, 105],
would provide a direct window on hadron formation from the QGP. In fact, the
yields of multi-charm baryons relative to the number of produced charm quarks
are predicted to be significantly enhanced in AA relative to pp collisions
[106]. Enhancements are expected by as much as a factor 100 for the recently
observed $\Xi_{cc}$ baryon [107], and even by as much as a factor 1000 for the
as yet undiscovered $\Omega_{ccc}$ baryon. The observation and precise
quantification of such effects, would represent a major discovery for the
study of the properties of deconfined matter.

In addition, LHC heavy-ion collisions also have a discovery potential for more
exotic hadronic states. This is because of the long-lived deconfined medium
and the large cross sections for heavy quarks available at the highest
energies. These provide an ideal playground for the production of exotic
states with e.g. multiple b quarks. The most notable case is the
T${}^{-}_{bb}$ bb, whose experimental detection would profit from the long
predicted lifetime (c$\tau$ $\sim$ 2.3 mm) due to the stability of the state
with respect to strong decays [108] (contrary to the recently discovered,
shortly-lived T${}^{+}_{cc}$ state) . Discovery opportunities also exist for
compact bound hidden-charm hexaquarks, also predicted to be stable with
respect to strong interaction decays [109], and molecular states composed of
three D mesons. Studying exotic QCD states in nuclear collisions is therefore
of central importance for QCD physics. The feasibility of such studies has
been demonstrated by a recent measurement of $\chi_{c1}$(3872) production in
Pb–-Pb collisions by the CMS collaboration in the range $p_{\rm T}$ $>$ 10
GeV/c [110]. An immediate goal for ALICE 3 is to measure the production of
$\chi_{c1}$(3872) down to the $p_{\rm T}$ region $<$5-6 GeV/c, where a
significant enhancement of the yield was predicted [111]. This is not
accessible by other LHC experiments. For hadrons containing charm and beauty
quarks, scattering experiments are not feasible to determine the interaction
strength between hadrons, therefore the only way to access the information is
the femtoscopy technique. This technique consists of the measurement of
correlations in momentum space for hadron-hadron pairs, and can be used to
extract the corresponding scattering parameters [112]. The ALICE 3 upgrade
will allow the measurement of several hadron combinations including DD∗,
$\Lambda_{c}^{+}\Sigma_{c}^{0;+;++}$ and BB∗ in pp, p–Pb and Pb–Pb collisions
and thereby shed light on the nature of many exotic hadrons. In particular,
using the same method as mentioned above in studying tetraquark states with
meson-meson correlations, the ALICE 3 upgrade will allow the study of the
$f_{2}(2010)$ and $\psi(3770)$ tetraquark candidates with $\phi\phi$ and
$D^{+}D^{-}$ correlations, respectively.

  5. 5.

What are the mechanisms that lead to QGP-like signals in small systems?

The formation of the QGP in heavy-ions has been confirmed via numerous
measurements. A key example is the simultaneous observation of collective flow
phenomena traced to a strongly coupled liquid, and jet quenching. Measurements
in small collision systems, such as pp and p–Pb at the LHC, have shown
particle correlations and strangeness yields in high-multiplicity collisions
resemble observations associated with the creation of the QGP [113, 9, 114,
7]. On the other hand, jet quenching in small systems has not been observed.
The resolution of this enigma remains one of the most important questions for
our field. ALICE has contributed with a stringent limit on energy loss within
small collision systems [115], and plans to improve on those measurements with
the high-statistics data in the upcoming LHC runs [24]. ALICE can also use
machine learning methods to design and measure new jet observables that are
maximally modified by traversing small systems [86, 116].

Anisotropic flow measurements in high-multiplicity small system collisions at
RHIC and the LHC are consistent with hydrodynamic predictions [117, 118]. This
indicates these collisions may create the smallest possible QGP droplets.
However, these measurements from Runs 1&2 suffer from large uncertainties due
to ambiguities in the non-flow subtraction methods pursued at the LHC [119].
For Run 4, the introduction of the FoCal will enable $\Delta\eta$ separations
of up to 9 units for two-particle correlation anisotropic flow measurements of
$v_{n}$, with such separations being critical in reducing non-flow. The broad
increase from the ALICE 3 acceptance will also reduce non-flow contributions
in two-particle correlation $v_{n}$ measurements by a factor of 4 for Runs 5&6
[120]. Four particle $v_{n}$ measurements enjoy a reduction of a factor 64,
and this increases exponentially with the number of particles used to
determine $v_{n}$. Such reductions are vital in the transition of anisotropic
flow measurements in small systems from being qualitative to truly
quantitative. Measurements in Run 4 and ALICE 3 are therefore essential for
precise quantitative constraints of the studies of the applicability of
hydrodynamics at its limits in small systems [121]. The improvements in non-
flow suppression and increased statistics are also essential for identified
hadron $v_{n}$ measurements vs. $p_{\rm T}$ in small systems. These test
hydrodynamic mass ordering at low-$p_{\rm T}$, with arises due to a common
radial flow profile. Such measurements, using ALICE’s highly competitive
particle identification capacities, will be key for all future precision tests
of hydrodynamics in small systems. These tests can also be expanded for the
unmeasured $\Xi$ and $\Omega$ $v_{n}$ vs. $p_{\rm T}$, which have proved
statistically challenging in Runs 1&2.

Finally, the increased statistics for extremely high multiplicity pp
collisions in Runs 3&4 will allow for measurements of the $\Omega/\pi$ ratio
at $\rm{d}N_{ch}/\rm{d}\eta\sim 96$. This is six times larger than
corresponding measurements from Runs 1&2, and fully overlap with ALICE’s Pb–Pb
measurements [7]. This will in turn allow for a critical test regarding
particle production mechanisms and the interpretation of strangeness
enhancment in pp collisions [24]. If this ratio saturates and reaches the
thermal limit in Pb–Pb collisions, this would support the idea the QGP can be
created in pp collisions. On the other hand, if this ratio continues to
increase beyond the Pb–Pb values, this perhaps would favor non-QGP mechanisms,
such as color ropes implemented in the DIPSY model [122], that predict a
continuing increase.

  6. 6.

What are the connections and broad impacts of ALICE measurements to other
fields of physics?

One of the recent major discoveries are gravitational wave signals from
neutron star mergers. The tidal deformation during approach and the ring-down
of the frequency spectrum after the merger hold potentials clues on the core
composition of neutron stars. In particular, this applies for stars with a
large mass to radius ratio. Model predictions range from simple neutron
matter, to hyperon matter, and deconfined quark matter [123]. Theoretically,
the main required ingredient is an equation of state for the system as a
function of density and temperature. Experimentally, relativistic heavy-ion
experiments, and in particular ALICE with its high luminosity and data rate
beyond Run 3, will be able to map out the production of hyperons, hypermatter
and quark matter. Since deconfined matter and hyperon production have been
discussed in the previously, the focus here is on hyper nuclei production and
the measurement of the balance of attractive and repulsive forces in the
interaction between hyperons and protons/neutrons through femtoscopic
measurements. During Runs 1&2 these studies have shown great promise as
documented in a recent Nature article by ALICE [112]. Certain di-baryon
correlations in the light and strange sector show attractive potentials, which
should lead to bound hexa-quark states. These results should also be viewed in
the context of hyper nuclei production. In previous campaigns, ALICE has
successfully reconstructed the lightest hypernucleus, the hypertriton and its
anti-particle [124].

In Run 4, we hope to extend the statistical sample in Pb–Pb collisions to a
level that might make the reconstruction of A=4 hypernuclei, in particular the
${}^{4}_{\Lambda}$H. Their measurement is interesting for precision tests of
particle production models [125] and to constrain hyperon-nucleon potentials
[126], and thus the formation probability of hyper-matter in dense stellar
objects. Regarding Runs 5&6, anti-nuclei and anti-hyper-nuclei with A$>$4 such
as anti-${}^{5}_{\Lambda}$He or anti-6Li have yet to be discovered, and may
well be in reach of ALICE 3. The ALICE 3 apparatus is ideally suited for the
observation of the $A=4$ or $A=5$ hyper-nuclei like ${}^{4}_{\Lambda}$H or
${}^{5}_{\Lambda}$He. The measurement of $A=6$ nuclei would provide precision
tests for the formation of bound clusters thanks to the special nature of 6He
and 6Li. 6He is the lightest known (anti-)halo-nucleus. Its production is
therefore expected to be suppressed in coalescence models with respect to
thermal-statistical models, due to its much larger size. 6 Li is a stable
isotope with a spin of $J=1$. With respect to the helium isotopes 4He and 6He
with $J=0$, 6Li production is therefore expected to be enhanced by the
degeneracy factor g of its spin-substates; $g=2J+1=3$. The expected production
yields $d$N/$dy$ for A=5 hyper-nuclei and A=6 nuclei are in the 10-9 to 10-11
range in Pb–Pb collisions.

  7. 7.

What can ALICE achieve regarding Beyond Standard Model (BSM) Physics?

Although studying BSM physics is not ALICE’s primary purpose, there are areas
where it can make extremely useful contributions, with the benefit of the
upgrades and increased statistics in future runs. One such area is the study
of light-by-light scattering, $\gamma\gamma\rightarrow\gamma\gamma$, using
UPCs. Light-by-light scattering occurs via a box diagram, which includes
contributions from all (standard model or BSM) electrically charged particles.
In addition, it can be mediated by axion-like particles,
$\gamma\gamma\rightarrow A\rightarrow\gamma\gamma$, which leads to a diphoton
invariant mass spectrum that is peaked at the axion mass. By virtue of its
sensitivity to lower $p_{\rm T}$ particles, ALICE (especially ALICE 3) can
probe lower axion masses than ATLAS or CMS. ALICE is also sensitive to two-
photon production of $\tau^{+}\tau^{-}$. Such studies of this reaction can be
used to put limits on BSM phenomena such as a $\tau$ anomalous magnetic
moment.

The discovery of stable massive particles (SMPs) beyond the Standard Model
(BSM), such as monopoles, gluinos, heavy leptons, etc., would address a number
of important questions in modern physics, including the nature of dark matter,
and the unification of fundamental forces. The majority of predictions suggest
that SMPs are far too massive to be produced in a foreseeable accelerator
[127]. However, there are suggestions some of them, e.g. magnetic monopoles,
could appear in a mass range accessible at the LHC [128], in particular in
Pb–Pb collisions. Detection of such particles is a very challenging due to
their highly ionizing nature, which leads to saturation and even malfunction
of electronics of e.g. silicon-based detectors [129]. The newly upgraded GEM
(Gas Electron Multiplier) based Time Projection Chamber (TPC) of ALICE is free
from this challenge, and is capable of measuring very large energy deposits
anticipated from monopoles.

An exciting avenue to study dark matter signals lies with cosmic-ray
antinuclei, such as antihelium. This is considered another promising signature
of the existence of weakly-interactive mass particles (WIMP), which represent
an important candidate for dark matter [130]. Since the background from
hadronic interactions of primary rays are negligible [131], the preliminary
evidence of a handful of anti-3He events collected by the AMS collaboration,
could be due to a previously neglected process. Namely, these could be the
production of $\Lambda_{b}^{0}$ baryons in dark-matter annihilation, and their
subsequent decay into anti-3He [132]. However, the decay rates of
$\Lambda_{b}^{0}$ to anti-nuclei are not experimentally measured, and serve as
crucial inputs to understand the AMS data. The large beauty-quark production
cross section at LHC energies, and the identification capabilities for nuclei
over practically the full phase space, make ALICE 3 the ideal experiment for
such studies.

## References

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##  Appendix A The ALICE-USA Collaboration

N. Alizadehvandchali9, N. Apadula5, M. Arslandok13, C. Beattie13, R.
Bellwied9, J.T. Blair8, F. Bock6, H. Bossi13, A. Bylinkin10, H. Caines13, I.
Chakaberia5, M. Cherney3, T.M. CormierI,6, R. Cruz-Torres5, P. Dhankher4, D.U.
Dixit4, R.J. Ehlers5,6, W. Fan5, M. Fasel6, F. Flor9, A.N. Flores8, D.R.
Gangadharan9, E. Garcia-Solis2, A. Gautam10, E. Glimos11, V. Gonzalez12, A.
Hamdi5, R. Hannigan8, J.W. Harris13, A. Harton2, H. Hassan6, L.B. Havener13,
C. Hughes11, T.J. Humanic7, A. Hutson9, T. Isidori10, B. Jacak4,5, P.M.
Jacobs5, F. Jonas6, A. Khatun10, M. Kim4, J.L. Klay1, S. Klein5, A.G. Knospe9,
Y.S. Lai5, E.D. Lesser4, I. Likmeta9, A. Liu4, C. Loizides6, C. Markert8, J.L.
Martinez9, A.S. Menon9, J.D. Mulligan5, A.I. Nambrath4, C. Nattrass11, N.
Novitzky6, A.C. Oliveira Da Silva11, M.H. Oliver13, L. Pinsky9, M. Płoskoń5,
M.G. Poghosyan6, C.A. Pruneau12, R.E. Quishpe9, S. Ragoni3, K.F. Read6,11,
O.V. Rueda9, D. Sarkar12, M.H.P. Sas13, J. Schambach6, N.V. Schmidt6, A.R.
Schmier11, J.E. Seger3, O. Sheibani9, N. Smirnov13, J. Song9, P.J.
Steffanic11, J.D. Tapia Takaki10, C. Terrevoli9, D. Thomas8, A.R. Timmins9,
S.A. Voloshin12, S.L. Weyhmiller13, J.R. Wright8

## Affiliation Notes

I Deceased

## Collaboration Institutes

1 California Polytechnic State University, San Luis Obispo, California
2 Chicago State University, Chicago, Illinois
3 Creighton University, Omaha, Nebraska
4 Department of Physics, University of California, Berkeley, California
5 Lawrence Berkeley National Laboratory, Berkeley, California
6 Oak Ridge National Laboratory, Oak Ridge, Tennessee
7 Ohio State University, Columbus, Ohio
8 The University of Texas at Austin, Austin, Texas
9 University of Houston, Houston, Texas
10 The University of Kansas, Lawrence, Kansas
11 University of Tennessee, Knoxville
12 Wayne State University, Detroit, Michigan
13 Yale University, New Haven, Connecticut