2021 252 4 January

Coherent $\mathbf{\rho^{0}}$ photoproduction in ultra-peripheral Xe–-Xe
collisions

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

The first measurement of the coherent photoproduction of $\rho^{0}$ vector
mesons in ultra-peripheral Xe–Xe collisions at $\sqrt{s_{\mathrm{\rm
NN}}}=5.44$ TeV is presented. This result, together with previous $\gamma$p
and $\gamma$–Pb measurements, describes the atomic number ($A$) dependence of
this process, which is particularly sensitive to nuclear shadowing effects and
to the approach to the black-disc limit of QCD at a semi-hard scale. The cross
section of the ${\rm Xe}+{\rm Xe}\to\rho^{0}+{\rm Xe}+{\rm Xe}$ process,
measured at midrapidity through the decay channel $\rho^{0}\to\pi^{+}\pi^{-}$,
is found to be ${\rm d}\sigma/{\rm d}y=131.5\pm
5.6{\rm(stat.)}^{+17.5}_{-16.9}{\rm(syst.)}$ mb. The ratio of the continuum to
resonant contributions for the production of pion pairs is also measured. In
addition, the fraction of events accompanied by electromagnetic dissociation
of either one or both colliding nuclei is reported. The dependence on $A$ of
cross section for the coherent $\rho^{0}$ photoproduction at a centre-of-mass
energy per nucleon of the $\gamma A$ system of $W_{\gamma A,n}=65$ GeV is
found to be consistent with a power-law behaviour $\sigma(\gamma
A\rightarrow\rho^{0}A)\propto A^{\alpha}$ with a slope $\alpha=0.96\pm
0.02{\rm(syst.)}$. This slope signals important shadowing effects, but it is
still far from the behaviour expected in the black-disc limit.

##  1 Introduction

The Large Hadron Collider (LHC) is a source of photon-induced processes. The
electromagnetic fields of the relativistic particles are strongly contracted
allowing for their interpretation as a flux of quasi-real photons, which
interact with the particles travelling in the opposite direction. When the
impact parameter of the collision is larger than the sum of the radii of the
incoming particles, purely strong interactions are suppressed due to the short
range of this force and photon-induced processes dominate. These are ultra-
peripheral collisions (UPCs) [1, 2, 3].

Among all the possible processes in UPC, the coherent production of vector
mesons stands out due to the large associated cross sections and the
cleanliness of its experimental signature: the quasi-real photon interacts
with the coherent QCD field of the other incoming particle to produce only a
vector meson. Due to the coherence condition, the average transverse momentum
of the vector meson, $\left<p_{T}\right>$, is related to the transverse size
of the nucleus $R_{A}$ as $\left<p_{T}\right>\sim\hslash/R_{A}$ [1], yielding
$\left<p_{T}\right>\sim 37$ (30) MeV/$c$ for a Xe (Pb) nucleus. A related
process is the incoherent production where the photon interacts with a nucleon
in the nucleus, which implies a larger average transverse momentum of the
produced vector meson. In addition, secondary electromagnetic interactions of
the colliding nuclei may excite one or both of them and upon de-excitation
produce neutrons at beam rapidities [4]. This effect depends on the square of
the electric charge of the nucleus, so it is expected to be substantially
weaker for Xe than for Pb.

One of the photoproduction processes with the largest cross section is the
production of a $\rho^{0}$ vector meson, which offers the opportunity to study
the approach to the black-disc limit of QCD with a semi-hard scale [5]. This
process has been extensively studied at the Relativistic Heavy Ion Collider
(RHIC) in Au–Au, and at the LHC in Pb–Pb UPC. Measurements at RHIC were
performed by the STAR Collaboration at centre-of-mass energies per nucleon
pair ($\sqrt{s_{\mathrm{\rm NN}}}$) of 62.4 GeV [6], 130 GeV [7], and 200 GeV
[8], while the studies at the LHC by the ALICE Collaboration were carried out
at 2.76 TeV [9] and 5.02 TeV [10]. All measurements were performed at
midrapidity. At the time of the first experimental results, the model
predictions of the cross section varied by a factor of around two. The
availability of new and more precise data motivated an improvement of the
different theoretical approaches, which in general are now closer to data
[10]. The situation, although better than a few years ago, still calls for
more data to improve our understanding of this process. Furthermore, the
coherent production of a $\rho^{0}$ vector meson off a nucleus allows for the
study of shadowing, the experimental fact that the nuclear structure functions
are suppressed compared to the superposition of those of their constituent
nucleons [11]. This phenomenon is expected to depend on the atomic number $A$
of the nucleus so measurements for different values of $A$ offer another tool
to test our understanding of shadowing at high energies and semi-hard scales.

In collisions of two heavy ions with atomic number $A$, either nucleus can be
a source of photons, which results in two contributions to the cross section.
At midrapidity both contributions are the same, but at forward rapidities one
corresponds to a high-energy photon while the other to a low-energy photon. If
one could disentangle both contributions it would be possible to study in the
same experiment the energy dependence of the process, allowing for the study
of the energy dependence of the underlying QCD dynamics. Two techniques have
been put forward to this end [12, 13]. The first one make use of UPCs and
peripheral collisions. The one described in Ref. [13] proposes to classify the
measured events depending on the presence of the beam-rapidity neutrons
mentioned earlier, and use these different cross sections to disentangle the
low and high energy contributions. Measurements at midrapidity are ideal to
test this proposal because both contributions are the same, so the measured
cross sections can be unambiguously compared with models predicting the
neutron emission probability. ALICE can measure beam-rapidity neutrons at both
sides of the nominal interaction point. The sides are called A and C, with the
latter one hosting the ALICE muon spectrometer [14]. ALICE has previously
reported [10] the cross section for the coherent production of $\rho^{0}$
vector mesons in Pb–Pb UPC for events with no beam-rapidity neutrons (0n0n,
where the first 0n refers to the A-side and the second to the C-side), with
one or more neutrons in one side only (0nXn+Xn0n), or in both sides (XnXn).
The comparison of the corresponding cross section fractions with calculations
of the emission of beam-rapidity neutrons based on the STARlight [15, 16] and
n$\mathbf{{}_{O}^{O}}$n [17] models suggests that the method works, but it is
important to test it further, for example with the large data sets expected
from the LHC Run 3 and 4 [18].

In this letter, the first measurement of the coherent photoproduction of
$\rho^{0}$ vector mesons in Xe–Xe UPCs at $\sqrt{s_{\mathrm{\rm NN}}}=5.44$
TeV is presented. The cross section of this process is measured at midrapidity
through the decay channel $\rho^{0}\to\pi^{+}\pi^{-}$. The ratio of the
continuum to resonant contributions for the production of pion pairs is also
measured. In addition, the fraction of events in the 0n0n, 0nXn+Xn0n, and XnXn
classes are reported. Finally, using data from HERA and from Pb–Pb UPC
collisions measured by ALICE, the $A$ dependence of the cross section is
studied at a centre-of-mass energy per nucleon of the $\gamma A$ system of
$W_{\gamma A,n}=65$ GeV.

##  2 Experimental set-up

During a 6-hour pilot run in October 2017, the LHC collided xenon nuclei for
the first time. These collisions took place at $\sqrt{s_{\mathrm{\rm
NN}}}=5.44$ TeV. A complete description of the ALICE detector and its
performance can be found in Ref. [14, 19]; here, just a brief description of
the systems involved in the measurement is given.

The decay products of the $\rho^{0}$ vector meson are measured in the central-
barrel region of ALICE with the ITS and TPC detectors. The ALICE Inner
Tracking System (ITS) [20] is made of six layers of silicon sensors. Each
layer has a cylindrical geometry concentric around the beam line. Three
different technologies are used: pixel, drift and strip sensors. Each
technology is used in two consecutive layers. All six layers are used for
tracking in this analysis. The Time-Projection Chamber (TPC) [21] surrounds
the ITS. It is a large cylindrical gas detector with a central membrane at
high voltage and two readout planes, composed of multiwire proportional
chambers, at the end caps. It is the main tracking detector and it also offers
particle identification through the measurement of ionisation energy loss. The
TPC and ITS cover a pseudorapidity interval $|\eta|<0.9$ and the full azimuth;
they are situated inside a large solenoid magnet which in Xe–Xe collisions
provided a B = 0.2 T field.

The neutrons at beam rapidity are measured by two neutron zero-degree
calorimeters, ZNA and ZNC, located at $\pm 112.5$ m from the nominal
interaction point along the beam line and covering the pseudorapidity range
$|\eta|>8.8$ [19]. The energy resolution for single neutrons is around 20%
which allows measuring of events with either zero or a few neutrons at beam
rapidities.

The trigger used the Silicon Pixel Detector (SPD), the Time-of-Flight detector
(TOF) and the V0 detectors. The SPD forms the two innermost layers of the ITS,
covering pseudorapidity ranges $|\eta|<2$ and $|\eta|<1.4$, respectively. The
SPD has about $10^{7}$ pixels which are read out by 400 (800) chips in the
inner (outer) layer. Each of the readout chips fires a trigger if at least one
of its pixels has a signal. TOF surrounds the TPC and matches its
pseudorapidity coverage. The TOF is a large cylindrical barrel of multigap
resistive plate chambers with some $1.5\times 10^{5}$ readout channels
arranged in 1608 pads that are capable of triggering [22]. The V0 [23] is a
set of two arrays made of 32 scintillator cells each. The arrays cover the
pseudorapidity ranges $-3.7<\eta<-1.7$ and $2.8<\eta<5.1$, respectively. The
time resolution of the V0 is better than 500 ps and provides a trigger if it
registers a signal. The trigger requires at least two hits in the inner and in
the outer layer of the SPD, at least two pads fired in TOF and no signal in
the V0.

The determination of the luminosity is based on reference trigger counts in
the V0 detector, while the reference trigger cross section is derived from
estimates based on a Glauber model [24]. For xenon the following values are
used: $A=129$, the radius of the nuclear-charge distribution $r=(5.36\pm 0.1)$
fm, a skin depth of $(0.59\pm 0.07)$ fm, and a deformation parameter
$\beta^{2}=0.18\pm 0.02$ [25]. The integrated luminosity used in this analysis
is $(279.5\pm 29.9)$ mb-1, where the quoted uncertainty is systematic and is
described later.

##  3 Analysis procedure

Figure 1:  (Colour online) Uncorrected invariant mass (left) and transverse
momentum (right) distribution of selected candidates. Also shown are track
pairs that have the same electric charge and fulfil all other requirements.
The STARlight templates for coherent and incoherent production are normalised
to the corresponding luminosity of data.

###  3.1 Event selection

Events are selected for the analysis if ($i$) the trigger described above is
active, ($ii$) there are no signals in the V0 detectors as determined by an
offline selection, and ($iii$) they have exactly two tracks.

Offline, a more refined algorithm to quantify the V0 timing signal is used,
consisting of a larger time window. For this reason, this analysis requires
the V0 offline reconstruction for selecting events.

The tracks are required to have contributions from both the ITS and the TPC.
Both layers of the SPD have to have a signal associated both to the track and
to a SPD trigger signal, the tracks should also have at least 50 (out of 159)
space points reconstructed by the TPC. Both tracks are required to be fully
within the acceptance of the detector ($|\eta_{\rm trk}|<0.8$). They have to
originate from a primary vertex whose coordinate along the beam line fulfils
$|z_{\rm trk}|<10$ cm, and their associated electric charge should be of
opposite sign. Particle identification of a track is determined by the number
of standard deviations ($n_{\sigma}$) by which the energy loss measurement
deviates from the pion hypothesis. The quadratic sum of $n_{\sigma_{1,2}}$ for
the $\pi^{+}$ and $\pi^{-}$ candidates has to be less than five squared
($n^{2}_{\sigma_{1}}+n^{2}_{\sigma_{2}}<5^{2}$). Finally, the transverse
momentum of the pion pair has to be less than 0.15 GeV/$c$. After the
aforementioned selections, 1827 events remain in the data sample.

Figure 1 shows the distributions of the invariant mass of the pion pairs as
well as their transverse momentum. In addition, the figure also shows the
corresponding distribution of an alternate data sample obtained by applying
all the criteria described above except that the electric charge associated to
both tracks has to be of the same sign. The mass distribution shows a clear
signal of a $\rho^{0}$ vector meson over a very small background represented
by the same-sign distribution. The transverse momentum distribution shows a
pronounced peak at values of few tens of MeV/$c$ as expected by coherent
production accompanied by a tail towards larger momenta produced by incoherent
production and the small remaining hadronic background.

The presence of one or more neutrons at beam rapidity is determined by using
the timing capabilities of the ZNA and ZNC which allow for the selection of
events with a signal within $\pm 2$ ns from the time expected for neutrons
produced in the interaction.

###  3.2 Signal extraction

A Monte Carlo (MC) sample of pion pairs from continuum and $\rho^{0}$ resonant
production, generated with the STARlight program [16], is used to extract
mass-dependent efficiency correction factors to account for the acceptance and
the efficiency of the detector and the selection criteria. All events in this
sample are passed by a detailed simulation of the ALICE apparatus and
subjected to the same analysis procedure as in data. The correction factor at
each mass of the pion pair is used to correct the mass distribution shown in
Fig. 1. The correction factor increases from about 0.025 at 550 MeV/$c^{2}$ to
0.055 at 1.1 GeV/$c^{2}$. The number of $\rho^{0}$ vector mesons is extracted
from the corrected invariant mass distribution normalised by the luminosity of
the sample and after applying other corrections, described below, to take into
account pile-up and the contribution from incoherent events.

The corrected mass distribution is fitted to a model describing the resonant
and continuum production of pion pairs according to the Söding prescription
[26] and a term $M$ that takes into account the contribution of the
$\gamma\gamma\to\mu^{+}\mu^{-}$ process:

$\frac{{\rm d}\sigma}{{\rm d}m\,{\rm d}y}=|A\times BW_{\rho}+B|^{2}+M.$ (1)

Here $A$ is the normalisation factor of the $\rho^{0}$ Breit–Wigner
($BW_{\rho}$) function, and $B$ is the non-resonant amplitude. The
relativistic Breit–Wigner function of the $\rho^{0}$ vector meson is

$BW_{\rho}=\frac{\sqrt{m\times
m_{\rho^{0}}\times\Gamma(m)}}{m^{2}-m^{2}_{\rho^{0}}+im_{\rho^{0}}\times\Gamma(m)},$
(2)

where $m_{\rho^{0}}$ is the pole mass of the $\rho^{0}$ vector meson. The
mass-dependent width $\Gamma(m)$ is given by

$\Gamma(m)=\Gamma(m_{\rho^{0}})\times\frac{m_{\rho^{0}}}{m}\times\left(\frac{m^{2}-4m^{2}_{\pi}}{m^{2}_{\rho^{0}}-m^{2}_{\pi}}\right)^{3/2},$
(3)

with $\Gamma(m_{\rho^{0}})$ the width of the $\rho^{0}$ vector meson and
$m_{\pi}$ the mass of the pion [27]. The shape of
$\gamma\gamma\to\mu^{+}\mu^{-}$ process $M$ is taken from STARlight and passed
through the same selection procedure as data. The fitted parameters are $A$,
$B$ and $M$, the mass and width of the $\rho^{0}$ were fixed to the PDG values
[28].

Figure 2:  (Colour online) Invariant mass distribution of pion pairs with the
different components of the fit represented by lines. The number of $\rho^{0}$
candidates is integrated over the resonant Breit–Wigner part (green, full
line), the interference term between $A$ and $B$ of Eq. (1) is shown by the
dash-dotted blue line and the muon template $M$ (red, dashed line) is taken
from STARlight. See text for more details.

An example fit of data with this model is shown in Fig. 2, where a clear
resonance structure is seen in data and decomposed by the fit into a small
background contribution, the contribution of the continuum production of pion
pairs, the interference term, and the $\rho^{0}$ signal.

###  3.3 Systematic uncertainties

The contributions to the systematic uncertainty are listed in Table 1 and
discussed one by one in the following paragraphs. The total uncertainty is
obtained as the quadratic sum of the various contributions.

Table 1: Summary of the systematic uncertainties for the measured cross section. See text for details. Source | Uncertainty
---|---
Variations to the fit procedure |  $\pm 2.5$%
Ross–Stodolsky fit model | $+3.5\%$
Acceptance and efficiency |  $\pm 0.5$%
Track selection |  $\pm 3.0$%
Track ITS–TPC matching |  $\pm 4.0$%
SPD trigger-to-track matching |  $\pm 2.0$%
TOF trigger efficiencies |  $\pm 2.8$%
Vertex selection |  $\pm 1.5$%
Incoherent contribution |  $\pm 2.0$%
Pile-up |  $\pm 1.0$%
Muon background ($\gamma\gamma\to\mu^{+}\mu^{-}$) |  ${}^{+({\textrm{0}.5})}_{-({\textrm{0}.2})}$%
Electromagnetic dissociation |  $\pm 0.2$%
Luminosity |  $\pm 10.7$%
Total |  ${}^{+({\textrm{1}3.3})}_{-({\textrm{1}2.8})}$ %

The signal extraction procedure is performed many times by varying the lower
and upper limit of the fit range as well as the bin width. The variations are
within 0.55 to 0.65 GeV$/c^{2}$, 0.9 to 1.4 GeV$/c^{2}$, and 10 to 50
MeV$/c^{2}$, respectively. The average value of the pole mass for the
$\rho^{0}$ vector meson as well as its width are found to agree with the value
reported by the Particle Data Group [28]. As the precision of our data sample
is limited, the values for the pole mass and width of the $\rho^{0}$ vector
meson are then fixed to the values measured in the $\rho^{0}$ photoproduction
process [28] and the signal extraction procedure is repeated. The standard
deviation of the distribution of extracted number of $\rho^{0}$ vector mesons
from all the different fits is considered as a systematic uncertainty, while
the mean is taken as the signal. The mean of all statistical uncertainties is
taken as the statistical uncertainty. Fits are performed using a log-
likelihood as well as a $\chi^{2}$ approach; both producing the same results.
The systematic uncertainty of the cross section from those variations amounts
to 2.5%.

The Ross–Stodolsky prescription [29] is used as an alternative model to fit
the resonance and continuum contribution, which results in a yield
systematically higher by 3.5%. Pure MC studies, where signal is generated with
a Söding function and fitted with a Ross–Stodolsky model, and vice versa, show
a similar behaviour. As the underlying distribution is not known, this
difference is taken as a systematic uncertainty.

Two MC samples are used to account for the acceptance and efficiency
correction. One simulates only the Breit–Wigner distribution for a pure
$\rho^{0}$ signal and the other includes the effect of the pion-pair
continuum. The full variation of 0.5% on the cross section obtained by using
these two samples is considered a systematic uncertainty.

All the analysis steps are repeated by varying the tracking selection criteria
within reasonable values. In particular, a test is performed where events with
tracks in some parts of the detector, known to have reduced performance, are
rejected. The full variation of the results amounts to 3% and it is taken as a
systematic uncertainty. Likewise for the uncertainty when matching track
segments in the ITS to their counterparts in the TPC. This uncertainty amounts
to 4%.

The matching of the SPD signal in the tracks to SPD trigger signal is studied
by comparing its effect in data with that in MC. A discrepancy of 2.0% is
found and assigned as a systematic uncertainty. The uncertainty on the trigger
efficiency of TOF is obtained by comparing the acceptance-times-efficiency
correction obtained from MC under different assumptions and assigning the full
2.8% difference.

There is a small discrepancy between data and the MC description of the
coordinate of the interaction vertex along the beam line for collisions
happening $\pm 10$ cm and more beyond the nominal interaction point. The full
difference in the cross section when retaining (or not) events beyond $\pm 10$
cm is found to be $\pm 1.5$% and assigned as a systematic uncertainty.

The contribution from incoherent production of $\rho^{0}$ vector mesons for
the region $p_{\rm T}<0.15$ GeV/$c$ is determined by fitting the corresponding
template from STARlight to the transverse momentum distribution in a range
from 0.15 to 1.0 GeV/$c$ and extrapolating to the region covered by the
measurement. The fit is repeated many times varying the fit ranges, within the
stated interval, and the bin widths. The mean of the results from these fits
amounts to 10.2% and it is subtracted from the cross section. The standard
deviation of all fits is taken as a systematic uncertainty ($\pm 2.0$%). It is
worth noting that the contribution from incoherent production varies across
different classes. As the data sample of events with neutrons at beam
rapidities is small, the incoherent contribution is estimated for the full
0nXn+Xn0n+XnXn sample. The incoherent background amounts to $(6.1\pm
3.0{\rm(syst.)})$% for the 0n0n and $(35.8\pm 4.3{\rm(syst.)})$% for the
0nXn+Xn0n+XnXn event classes, respectively. This is taken into account when
computing the fractions of the cross section in each class reported below.

The V0 veto could be invalidated if this detector shows a signal which
originates from a separate interaction, an effect called pile-up.
Electromagnetic $e^{+}+e^{-}$ pair production is the main source of these
signals. The probability for pile-up is obtained from an unbiased sample
triggered by the timing of expected bunch crossings at the interaction point
surrounded by the ALICE detector. This probability is used, assuming a Poisson
process, to correct for the events lost due to pile-up. The correction factor
is $0.89\pm 0.01$; the uncertainty from this procedure is taken as systematic
uncertainty ($\pm 1.0$%).

The statistical uncertainty of the $\gamma\gamma\to{\rm e}^{+}{\rm e}^{-}$
cross section in our previous measurement [30] is around 10% and within this
precision it agrees with the prediction from STARlight. Changing the
normalisation of the $\gamma\gamma\to\mu^{+}\mu^{-}$ template in the fit by
$\pm 10$%, produces a $-0.2$% and $+0.5$% systematic uncertainty on the
extracted $\rho^{0}$ cross section.

Electromagnetic dissociation producing the beam-rapidity neutrons is
accompanied on occasion by other charged particles [31]. These charged
particles, if they hit the V0, may cause the event to be lost. The probability
for this to happen is estimated to be $(1.7\pm 0.2)$% using the unbiased
sample just mentioned. The statistical precision of this procedure is taken as
systematic uncertainty ($\pm 0.2$%).

The uncertainty on the luminosity is determined by computing the prediction of
the Glauber model varying each parameter within their reported uncertainty.
This is the dominant source of uncertainty in the measurement and amounts to
10.7%. It is worth noting that a variation in the normalisation of the $M$
term in Eq. (1) within the $\pm 10$% uncertainty mentioned before produces a
change in the cross section which is negligible with respect to the other
effects already mentioned.

The extraction of fractions of the cross section in the 0n0n, 0nXn+Xn0n, and
XnXn classes is also affected by pile-up in the ZNA and ZNC and by the
efficiency of these calorimeters to detected neutrons. The pile-up
probabilities are $(0.47\pm 0.02)$% and $(0.44\pm 0.02)$%, while the
efficiencies are $0.91\pm 0.01$ and $0.92\pm 0.02$ for the ZNA and ZNC,
respectively. The uncertainties in these numbers, along with the uncertainty
on the subtraction of the incoherent contribution, are taken into account to
obtain the uncertainty on the fractions quoted below.

##  4 Cross section results

The cross section for the coherent photoproduction of $\rho^{0}$ vector mesons
in ultra-peripheral Xe–Xe collisions at $\sqrt{s_{\mathrm{\rm NN}}}=5.44$ TeV
measured at midrapidity is

$\frac{{\rm d}\sigma}{{\rm d}y}=131.5\pm 5.6\ {\rm(stat.)}^{+17.5}_{-16.9}\
{\rm(syst.)}\ {\rm mb.}$ (4)

Figure 3:  (Colour online) Cross section for the coherent photoproduction of
$\rho^{0}$ vector mesons in Xe–Xe UPC. The lines show the predictions of the
different models described in the text.

Figure 3 shows the measured cross section and compares it with the prediction
of the following models. STARlight [16], which is based on ($i$) a
phenomenological description of existing data on exclusive production of
$\rho^{0}$ vector mesons off protons, ($ii$) the optical theorem, and ($iii$)
a Glauber-like eikonal formalism which neglects the elastic part of the
elementary $\rho^{0}$–nucleon cross section. The prediction by Guzey, Kryshen
and Zhalov (GKZ) [32] relies on a modified vector dominance model, where
hadronic fluctuations of the photon are taken into account according to the
Gribov–Glauber model of nuclear shadowing; the band shows the variation on the
predictions when varying the parameters of the model. The model by Gonçalves
et al. (GMMNS) [33] uses the colour dipole approach with amplitudes obtained
from the IIM model [34] coupled to a Glauber prescription to go from the
nucleon to the nuclear case. The two lines shown for GMMNS bracket the changes
in their predictions when varying different ingredients of their model [33].
Finally, the model from Cepila et al. (CCKT) [35, 36] also uses the colour
dipole approach, but in this case the structure of the nucleon in the
transverse plane is described by so-called hot spots, regions of high gluonic
density, whose number increases with increasing energy [35]; nuclear effects
are implemented along the ideas of the Glauber model proposed in Ref. [37]. At
midrapidity, all models are relatively close to one another and overestimate
the data. The lower band of GMMNS as well as the STARlight and CCKT
predictions are slightly more than one standard deviation above the data. Only
the upper band of GMMNS is disfavoured by more than three standard deviations.

The ratio of non-resonant to resonant pion production, see Eq. (1), is
measured to be $|B/A|=0.58\pm 0.04\ ({\rm stat.)}\pm 0.03\ ({\rm syst.)}$
$({\rm GeV}/c^{2})^{-\frac{1}{2}}$. The main uncertainty comes from the
correction for acceptance and efficiency, closely followed by variations from
the signal extraction procedure. This value is consistent with those obtained
in Pb–Pb UPC at $\sqrt{s_{\mathrm{\rm NN}}}=2.76$ TeV [9] and
$\sqrt{s_{\mathrm{\rm NN}}}=5.02$ TeV [10], namely $|B/A|=0.50\pm 0.04\ ({\rm
stat.)}\pm^{0.10}_{0.04}\ ({\rm syst.)}$ $({\rm GeV}/c^{2})^{-\frac{1}{2}}$
and $|B/A|=0.57\pm 0.01\ ({\rm stat.)}\pm 0.02\ ({\rm syst.)}$ $({\rm
GeV}/c^{2})^{-\frac{1}{2}}$, respectively. The corresponding ratio in coherent
Au–Au UPC measured by STAR at $\sqrt{s_{\mathrm{\rm NN}}}=200$ GeV is $0.79\pm
0.01\ ({\rm stat.)}\pm 0.08\ ({\rm syst.)}$ $({\rm GeV}/c^{2})^{-\frac{1}{2}}$
[8]. The CMS Collaboration measured $0.50\pm 0.06\ ({\rm stat.)}$ $({\rm
GeV}/c^{2})^{-\frac{1}{2}}$ in p–Pb UPC at $\sqrt{s_{\mathrm{\rm NN}}}=5.02$
TeV [38] for $|t|<0.5$ GeV2. The ZEUS Collaboration, using a sample of
positron–proton collisions at a centre-of-mass energy of 300 GeV, reports
$0.67\pm 0.02\ ({\rm stat.)}\pm 0.04\ ({\rm syst.)}$ $({\rm
GeV}/c^{2})^{-\frac{1}{2}}$ for their full analysed sample, and $\approx 0.8$
$({\rm GeV}/c^{2})^{-\frac{1}{2}}$ for $t$ values similar to those of coherent
$\rho^{0}$ production in Pb–Pb UPC [39].

The fraction of the cross section in each one of the classes defined by the
presence or absence of beam-rapidity neutrons is shown in Table 2, where the
measurement is also compared with the prediction from the
n$\mathbf{{}_{O}^{O}}$n MC [17]. This program generates neutrons emitted due
to the electromagnetic dissociation (EMD) of two interacting nuclei. It is
based on photon fluxes computed in the semi-classical approximation, and on
all existing data on EMD complemented by phenomenological extrapolations where
data is not available. It can easily be interfaced to theoretical predictions
of coherent vector meson production. The agreement of the model with data, as
well as the satisfactory description of the corresponding cross sections
observed in Pb–Pb UPC at $\sqrt{s_{\mathrm{\rm NN}}}=5.02$ TeV [10], suggests
that the emission of neutrons at beam-rapidity is well understood for the
coherent photoproduction of $\rho^{0}$ vector mesons off nuclei with such
different atomic mass number as Pb and Xe.

Table 2: Fraction of the cross section in each one of the classes defined by the presence or absence of beam-rapidity neutrons compared with the predictions from the n$\mathbf{{}_{O}^{O}}$n model [17]. The first uncertainty is statistical, the second comes from the variations in the ZNA and ZNC pile-up factors and efficiencies, while the third comes from the variation in the number of events which is dominated by the subtraction of incoherent contribution. See text for details.  Class | Measured fraction |  n$\mathbf{{}_{O}^{O}}$n prediction
---|---|---
0n0n | ($90.46\pm 0.70\pm 0.17\mp 0.68$)% | 92.4%
0nXn+Xn0n | ($8.48\pm 0.66\mp 0.13\pm 0.64$)% | 6.9%
XnXn | ($1.07\pm 0.25\mp 0.04\pm 0.07$)% | 0.7%
Figure 4:  $A$ dependence of the $\gamma A$ cross section for the coherent
production of a $\rho^{0}$ meson and the corresponding power low fit shown as
a band. The general expectations for three extreme cases are represented by
the dashed, dotted-dashed, and dotted lines, respectively. The red band
corresponds to the GKZ predictions when varying the parameters of the model. A
power-law fit to the CCKT model is shown by the blue band. See text for
details.

The measurements of the UPC cross section for coherent production of
$\rho^{0}$ vector mesons at midrapdity for Pb–Pb [10] and for Xe–Xe have been
converted into a $\gamma A$ measurement by dividing the cross sections by two
times the corresponding photon fluxes of 58.6 (Xe) and 128.1 (Pb). These
numbers are obtained following the prescription detailed in Ref. [12]. A flux
uncertainty of 2% is considered, which is uncorrelated between both nuclei
(because it mainly originates in the knowledge of the nuclear geometries). The
uncertainties coming from the Ross–Stodolsky fit model and from the ITS-TPC
matching are correlated between the Xe and Pb results. The midrapidity
photon–nucleus centre-of-mass energy per nucleon is given by $W_{\gamma
A,n}^{2}=m\sqrt{s_{\mathrm{\rm NN}}}$ (with $m$ the mass of the vector meson),
so it is slightly different in both systems (62 GeV in Pb–Pb and 65 GeV in
Xe–Xe); as the $\gamma$–Pb cross section is expected to change around 1%
between these two values, well within the experimental uncertainties, both
measurements are taken as having $W_{\gamma A,n}=65$ GeV.

The dependence of these cross sections on $A$ is fitted by a power-law model,
$\sigma_{\gamma A}(A)=\sigma_{0}A^{\alpha}$, using also the cross section
measured by H1 at this energy [40]: $(11.8\pm 0.9(\rm{syst.}))$ $\mu$b. The
value reported by H1 is consistent with the corresponding cross section found
by the ZEUS [39] and CMS [38] collaborations. The fit is shown in Fig. 4. It
has a $\chi^{2}=1.48$ (for one degree of freedom). The parameters from the fit
when using only uncorrelated uncertainties are $\sigma_{0}=0.0117\pm 0.0009$
mb and $\alpha=0.963\pm 0.019$. The correlation between them is ${-0.78}$.
Varying the flux by $\pm 2$% produces a change in the exponent $\alpha$ of
$0.005$. Varying the cross sections by the correlated uncertainties from the
fit model and the ITS–TPC matching does not modify the $\sigma_{0}$ parameter
and causes a change in the exponent $\alpha$ of $\pm 0.007$ and $+0.006$,
respectively.

The fit is compared with three generic expectations having different
dependence on $A$ resulting on slopes $\alpha$ of 4/3, 1, and 2/3 for full
coherence disregarding any other dynamical effect, for a total incoherent
behaviour, and for the black-disc limit, respectively. The slope found in data
is significantly different from 4/3 signalling important shadowing effects.
The closeness of data to a slope of 1 does not imply incoherent behaviour; it
is just a coincidence produced by the large shadowing suppression. The black-
disc limit seems to be quite distant at this energy of $W_{\gamma A}=65$ GeV.

Fitting to the same functional form the predictions of the Gribov–Glauber
approach (GKZ [41, 32]) and of the colour dipole model with subnucleon degrees
of freedom (CCKT [36, 42]) yields slopes of $0.985\pm 0.007$ and $0.984\pm
0.003$, respectively, where in both cases the parameter $\sigma_{0}$ has been
fixed to the corresponding prediction for the $\gamma$p cross section. Both
slopes are in good agreement with that found in data. This was to be expected
given that both approaches give a reasonable description of the different
available data.

##  5 Summary

The cross section for the coherent photoproduction of $\rho^{0}$ vector mesons
in Xe–Xe UPC at $\sqrt{s_{\mathrm{\rm NN}}}=5.44$ TeV has been measured and
compared with existing models of this process. The theoretical predictions
slightly overestimate the measurement. The ratio of the continuum-to-resonant
contributions for the production of pion pairs is also measured and found to
agree with previous measurements in Pb–Pb UPC. The fraction of events
accompanied by electromagnetic dissociation of either one or both colliding
nuclei is reported and compared with the predictions of the
n$\mathbf{{}_{O}^{O}}$n model. The fair agreement between data and predictions
suggest that this process is well understood within the current experimental
uncertainties and can be used as a tool to disentangle the different $\gamma
A$ contributions to the UPC cross sections.

The dependence on $A$ of the cross section for the coherent $\rho^{0}$
photoproduction at a centre-of-mass energy per nucleon of the $\gamma A$
system of 65 GeV is found to be consistent with a power-law behaviour with a
slope of $0.96\pm 0.02$. This exponent is substantially smaller than what is
expected from a purely coherent process, taking into account the geometry, but
disregarding any dynamic effect. A fair description of Pb–Pb and Xe–Xe data is
obtained in models based on hadronic degrees of freedom in the Gribov–Glauber
approach (GKZ) as well as in partonic-level models (CCKT). In this context,
the $A$ dependence of the cross section is a strong indicator that QCD effects
are important and relatively well modelled.

## Acknowledgements

The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences, Austrian Science Fund (FWF): [M 2467-N36] and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC) , Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation, Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; Czech Science Foundation; The Danish Council for Independent Research | Natural Sciences, the VILLUM FONDEN and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l’Energie Atomique (CEA) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany; General Secretariat for Research and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy Government of India (DAE), Department of Science and Technology, Government of India (DST), University Grants Commission, Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Science, Indonesia; Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology , Nagasaki Institute of Applied Science (IIST), Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Science and Higher Education, National Science Centre and WUT ID-UB, Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics and Ministry of Research and Innovation and Institute of Atomic Physics, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation, National Research Centre Kurchatov Institute, Russian Science Foundation and Russian Foundation for Basic Research, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSDTA) and Office of the Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America.

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##  Appendix A The ALICE Collaboration

S. Acharya142, D. Adamová97, A. Adler75, J. Adolfsson82, G. Aglieri Rinella35,
M. Agnello31, N. Agrawal55, Z. Ahammed142, S. Ahmad16, S.U. Ahn77, Z. Akbar52,
A. Akindinov94, M. Al-Turany109, D.S.D. Albuquerque124, D. Aleksandrov90, B.
Alessandro60, H.M. Alfanda7, R. Alfaro Molina72, B. Ali16, Y. Ali14, A.
Alici26, N. Alizadehvandchali127, A. Alkin35, J. Alme21, T. Alt69, L.
Altenkamper21, I. Altsybeev115, M.N. Anaam7, C. Andrei49, D. Andreou92, A.
Andronic145, V. Anguelov106, T. Antičić110, F. Antinori58, P. Antonioli55, C.
Anuj16, N. Apadula81, L. Aphecetche117, H. Appelshäuser69, S. Arcelli26, R.
Arnaldi60, M. Arratia81, I.C. Arsene20, M. Arslandok147,106, A. Augustinus35,
R. Averbeck109, S. Aziz79, M.D. Azmi16, A. Badalà57, Y.W. Baek42, X. Bai109,
R. Bailhache69, R. Bala103, A. Balbino31, A. Baldisseri139, M. Ball44, D.
Banerjee4, R. Barbera27, L. Barioglio25, M. Barlou86, G.G. Barnaföldi146, L.S.
Barnby96, V. Barret136, C. Bartels129, K. Barth35, E. Bartsch69, F.
Baruffaldi28, N. Bastid136, S. Basu82,144, G. Batigne117, B. Batyunya76, D.
Bauri50, J.L. Bazo Alba114, I.G. Bearden91, C. Beattie147, I. Belikov138,
A.D.C. Bell Hechavarria145, F. Bellini35, R. Bellwied127, S. Belokurova115, V.
Belyaev95, G. Bencedi70,146, S. Beole25, A. Bercuci49, Y. Berdnikov100, A.
Berdnikova106, D. Berenyi146, L. Bergmann106, M.G. Besoiu68, L. Betev35, P.P.
Bhaduri142, A. Bhasin103, I.R. Bhat103, M.A. Bhat4, B. Bhattacharjee43, P.
Bhattacharya23, A. Bianchi25, L. Bianchi25, N. Bianchi53, J. Bielčík38, J.
Bielčíková97, A. Bilandzic107, G. Biro146, S. Biswas4, J.T. Blair121, D.
Blau90, M.B. Blidaru109, C. Blume69, G. Boca29, F. Bock98, A. Bogdanov95, S.
Boi23, J. Bok62, L. Boldizsár146, A. Bolozdynya95, M. Bombara39, P.M. Bond35,
G. Bonomi141, H. Borel139, A. Borissov83,95, H. Bossi147, E. Botta25, L.
Bratrud69, P. Braun-Munzinger109, M. Bregant123, M. Broz38, G.E. Bruno108,34,
M.D. Buckland129, D. Budnikov111, H. Buesching69, S. Bufalino31, O. Bugnon117,
P. Buhler116, P. Buncic35, Z. Buthelezi73,133, J.B. Butt14, S.A. Bysiak120, D.
Caffarri92, A. Caliva109, E. Calvo Villar114, J.M.M. Camacho122, R.S.
Camacho46, P. Camerini24, F.D.M. Canedo123, A.A. Capon116, F. Carnesecchi26,
R. Caron139, J. Castillo Castellanos139, E.A.R. Casula23, F. Catalano31, C.
Ceballos Sanchez76, P. Chakraborty50, S. Chandra142, W. Chang7, S.
Chapeland35, M. Chartier129, S. Chattopadhyay142, S. Chattopadhyay112, A.
Chauvin23, T.G. Chavez46, C. Cheshkov137, B. Cheynis137, V. Chibante
Barroso35, D.D. Chinellato124, S. Cho62, P. Chochula35, P. Christakoglou92,
C.H. Christensen91, P. Christiansen82, T. Chujo135, C. Cicalo56, L.
Cifarelli26, F. Cindolo55, M.R. Ciupek109, G. ClaiII,55, J. Cleymans126, F.
Colamaria54, J.S. Colburn113, D. Colella54,146, A. Collu81, M. Colocci35,26,
M. ConcasIII,60, G. Conesa Balbastre80, Z. Conesa del Valle79, G. Contin24,
J.G. Contreras38, T.M. Cormier98, P. Cortese32, M.R. Cosentino125, F. Costa35,
S. Costanza29, P. Crochet136, E. Cuautle70, P. Cui7, L. Cunqueiro98, A.
Dainese58, F.P.A. Damas117,139, M.C. Danisch106, A. Danu68, I. Das112, P.
Das88, P. Das4, S. Das4, S. Dash50, S. De88, A. De Caro30, G. de Cataldo54, L.
De Cilladi25, J. de Cuveland40, A. De Falco23, D. De Gruttola30, N. De
Marco60, C. De Martin24, S. De Pasquale30, S. Deb51, H.F. Degenhardt123, K.R.
Deja143, L. Dello Stritto30, S. Delsanto25, W. Deng7, P. Dhankher19, D. Di
Bari34, A. Di Mauro35, R.A. Diaz8, T. Dietel126, Y. Ding7, R. Divià35, D.U.
Dixit19, Ø. Djuvsland21, U. Dmitrieva64, J. Do62, A. Dobrin68, B. Dönigus69,
O. Dordic20, A.K. Dubey142, A. Dubla109,92, S. Dudi102, M. Dukhishyam88, P.
Dupieux136, T.M. Eder145, R.J. Ehlers98, V.N. Eikeland21, D. Elia54, B.
Erazmus117, F. Ercolessi26, F. Erhardt101, A. Erokhin115, M.R. Ersdal21, B.
Espagnon79, G. Eulisse35, D. Evans113, S. Evdokimov93, L. Fabbietti107, M.
Faggin28, J. Faivre80, F. Fan7, A. Fantoni53, M. Fasel98, P. Fecchio31, A.
Feliciello60, G. Feofilov115, A. Fernández Téllez46, A. Ferrero139, A.
Ferretti25, A. Festanti35, V.J.G. Feuillard106, J. Figiel120, S. Filchagin111,
D. Finogeev64, F.M. Fionda21, G. Fiorenza54, F. Flor127, A.N. Flores121, S.
Foertsch73, P. Foka109, S. Fokin90, E. Fragiacomo61, U. Fuchs35, N.
Funicello30, C. Furget80, A. Furs64, M. Fusco Girard30, J.J. Gaardhøje91, M.
Gagliardi25, A.M. Gago114, A. Gal138, C.D. Galvan122, P. Ganoti86, C.
Garabatos109, J.R.A. Garcia46, E. Garcia-Solis10, K. Garg117, C. Gargiulo35,
A. Garibli89, K. Garner145, P. Gasik107, E.F. Gauger121, M.B. Gay Ducati71, M.
Germain117, J. Ghosh112, P. Ghosh142, S.K. Ghosh4, M. Giacalone26, P.
Gianotti53, P. Giubellino109,60, P. Giubilato28, A.M.C. Glaenzer139, P.
Glässel106, V. Gonzalez144, L.H. González-Trueba72, S. Gorbunov40, L.
Görlich120, S. Gotovac36, V. Grabski72, L.K. Graczykowski143, K.L. Graham113,
L. Greiner81, A. Grelli63, C. Grigoras35, V. Grigoriev95, A. GrigoryanI,1, S.
Grigoryan76,1, O.S. Groettvik21, F. Grosa60, J.F. Grosse-Oetringhaus35, R.
Grosso109, R. Guernane80, M. Guilbaud117, M. Guittiere117, K. Gulbrandsen91,
T. Gunji134, A. Gupta103, R. Gupta103, I.B. Guzman46, R. Haake147, M.K.
Habib109, C. Hadjidakis79, H. Hamagaki84, G. Hamar146, M. Hamid7, R.
Hannigan121, M.R. Haque143,88, A. Harlenderova109, J.W. Harris147, A.
Harton10, J.A. Hasenbichler35, H. Hassan98, D. Hatzifotiadou55, P. Hauer44,
L.B. Havener147, S. Hayashi134, S.T. Heckel107, E. Hellbär69, H. Helstrup37,
T. Herman38, E.G. Hernandez46, G. Herrera Corral9, F. Herrmann145, K.F.
Hetland37, H. Hillemanns35, C. Hills129, B. Hippolyte138, B. Hohlweger107, J.
Honermann145, G.H. Hong148, D. Horak38, S. Hornung109, R. Hosokawa15, P.
Hristov35, C. Huang79, C. Hughes132, P. Huhn69, T.J. Humanic99, H. Hushnud112,
L.A. Husova145, N. Hussain43, D. Hutter40, J.P. Iddon35,129, R. Ilkaev111, H.
Ilyas14, M. Inaba135, G.M. Innocenti35, M. Ippolitov90, A. Isakov38,97, M.S.
Islam112, M. Ivanov109, V. Ivanov100, V. Izucheev93, B. Jacak81, N.
Jacazio35,55, P.M. Jacobs81, S. Jadlovska119, J. Jadlovsky119, S. Jaelani63,
C. Jahnke123, M.J. Jakubowska143, M.A. Janik143, T. Janson75, M. Jercic101, O.
Jevons113, M. Jin127, F. Jonas98,145, P.G. Jones113, J. Jung69, M. Jung69, A.
Junique35, A. Jusko113, P. Kalinak65, A. Kalweit35, V. Kaplin95, S. Kar7, A.
Karasu Uysal78, D. Karatovic101, O. Karavichev64, T. Karavicheva64, P.
Karczmarczyk143, E. Karpechev64, A. Kazantsev90, U. Kebschull75, R. Keidel48,
M. Keil35, B. Ketzer44, Z. Khabanova92, A.M. Khan7, S. Khan16, A.
Khanzadeev100, Y. Kharlov93, A. Khatun16, A. Khuntia120, B. Kileng37, B.
Kim62, D. Kim148, D.J. Kim128, E.J. Kim74, H. Kim17, J. Kim148, J.S. Kim42, J.
Kim106, J. Kim148, J. Kim74, M. Kim106, S. Kim18, T. Kim148, S. Kirsch69, I.
Kisel40, S. Kiselev94, A. Kisiel143, J.L. Klay6, J. Klein35,60, S. Klein81, C.
Klein-Bösing145, M. Kleiner69, T. Klemenz107, A. Kluge35, A.G. Knospe127, C.
Kobdaj118, M.K. Köhler106, T. Kollegger109, A. Kondratyev76, N. Kondratyeva95,
E. Kondratyuk93, J. Konig69, S.A. Konigstorfer107, P.J. Konopka2,35, G.
Kornakov143, S.D. Koryciak2, L. Koska119, O. Kovalenko87, V. Kovalenko115, M.
Kowalski120, I. Králik65, A. Kravčáková39, L. Kreis109, M. Krivda113,65, F.
Krizek97, K. Krizkova Gajdosova38, M. Kroesen106, M. Krüger69, E. Kryshen100,
M. Krzewicki40, V. Kučera35, C. Kuhn138, P.G. Kuijer92, T. Kumaoka135, L.
Kumar102, S. Kundu88, P. Kurashvili87, A. Kurepin64, A.B. Kurepin64, A.
Kuryakin111, S. Kushpil97, J. Kvapil113, M.J. Kweon62, J.Y. Kwon62, Y.
Kwon148, S.L. La Pointe40, P. La Rocca27, Y.S. Lai81, A. Lakrathok118, M.
Lamanna35, R. Langoy131, K. Lapidus35, P. Larionov53, E. Laudi35, L.
Lautner35, R. Lavicka38, T. Lazareva115, R. Lea24, J. Lee135, J. Lehrbach40,
R.C. Lemmon96, I. León Monzón122, E.D. Lesser19, M. Lettrich35, P. Lévai146,
X. Li11, X.L. Li7, J. Lien131, R. Lietava113, B. Lim17, S.H. Lim17, V.
Lindenstruth40, A. Lindner49, C. Lippmann109, A. Liu19, J. Liu129, I.M.
Lofnes21, V. Loginov95, C. Loizides98, P. Loncar36, J.A. Lopez106, X.
Lopez136, E. López Torres8, J.R. Luhder145, M. Lunardon28, G. Luparello61,
Y.G. Ma41, A. Maevskaya64, M. Mager35, S.M. Mahmood20, T. Mahmoud44, A.
Maire138, R.D. MajkaI,147, M. Malaev100, Q.W. Malik20, L. MalininaIV,76, D.
Mal’Kevich94, N. Mallick51, P. Malzacher109, G. Mandaglio33,57, V. Manko90, F.
Manso136, V. Manzari54, Y. Mao7, J. Mareš67, G.V. Margagliotti24, A.
Margotti55, A. Marín109, C. Markert121, M. Marquard69, N.A. Martin106, P.
Martinengo35, J.L. Martinez127, M.I. Martínez46, G. Martínez García117, S.
Masciocchi109, M. Masera25, A. Masoni56, L. Massacrier79, A.
Mastroserio140,54, A.M. Mathis107, O. Matonoha82, P.F.T. Matuoka123, A.
Matyja120, C. Mayer120, A.L. Mazuecos35, F. Mazzaschi25, M. Mazzilli35,54,
M.A. Mazzoni59, A.F. Mechler69, F. Meddi22, Y. Melikyan64, A. Menchaca-
Rocha72, C. Mengke28,7, E. Meninno116,30, A.S. Menon127, M. Meres13, S.
Mhlanga126, Y. Miake135, L. Micheletti25, L.C. Migliorin137, D.L. Mihaylov107,
K. Mikhaylov76,94, A.N. Mishra146,70, D. Miśkowiec109, A. Modak4, N.
Mohammadi35, A.P. Mohanty63, B. Mohanty88, M. Mohisin Khan16, Z. Moravcova91,
C. Mordasini107, D.A. Moreira De Godoy145, L.A.P. Moreno46, I. Morozov64, A.
Morsch35, T. Mrnjavac35, V. Muccifora53, E. Mudnic36, D. Mühlheim145, S.
Muhuri142, J.D. Mulligan81, A. Mulliri23, M.G. Munhoz123, R.H. Munzer69, H.
Murakami134, S. Murray126, L. Musa35, J. Musinsky65, C.J. Myers127, J.W.
Myrcha143, B. Naik50, R. Nair87, B.K. Nandi50, R. Nania55, E. Nappi54, M.U.
Naru14, A.F. Nassirpour82, C. Nattrass132, S. Nazarenko111, A. Neagu20, L.
Nellen70, S.V. Nesbo37, G. Neskovic40, D. Nesterov115, B.S. Nielsen91, S.
Nikolaev90, S. Nikulin90, V. Nikulin100, F. Noferini55, S. Noh12, P.
Nomokonov76, J. Norman129, N. Novitzky135, P. Nowakowski143, A. Nyanin90, J.
Nystrand21, M. Ogino84, A. Ohlson82, J. Oleniacz143, A.C. Oliveira Da
Silva132, M.H. Oliver147, A. Onnerstad128, C. Oppedisano60, A. Ortiz
Velasquez70, T. Osako47, A. Oskarsson82, J. Otwinowski120, K. Oyama84, Y.
Pachmayer106, S. Padhan50, D. Pagano141, G. Paić70, A. Palasciano54, J.
Pan144, S. Panebianco139, P. Pareek142, J. Park62, J.E. Parkkila128, S.
Parmar102, S.P. Pathak127, B. Paul23, J. Pazzini141, H. Pei7, T. Peitzmann63,
X. Peng7, L.G. Pereira71, H. Pereira Da Costa139, D. Peresunko90, G.M. Perez8,
S. Perrin139, Y. Pestov5, V. Petráček38, M. Petrovici49, R.P. Pezzi71, S.
Piano61, M. Pikna13, P. Pillot117, O. Pinazza55,35, L. Pinsky127, C. Pinto27,
S. Pisano53, M. Płoskoń81, M. Planinic101, F. Pliquett69, M.G. Poghosyan98, B.
Polichtchouk93, N. Poljak101, A. Pop49, S. Porteboeuf-Houssais136, J.
Porter81, V. Pozdniakov76, S.K. Prasad4, R. Preghenella55, F. Prino60, C.A.
Pruneau144, I. Pshenichnov64, M. Puccio35, S. Qiu92, L. Quaglia25, R.E.
Quishpe127, S. Ragoni113, A. Rakotozafindrabe139, L. Ramello32, F. Rami138,
S.A.R. Ramirez46, A.G.T. Ramos34, R. Raniwala104, S. Raniwala104, S.S.
Räsänen45, R. Rath51, I. Ravasenga92, K.F. Read98,132, A.R. Redelbach40, K.
RedlichV,87, A. Rehman21, P. Reichelt69, F. Reidt35, R. Renfordt69, Z.
Rescakova39, K. Reygers106, A. Riabov100, V. Riabov100, T. Richert82,91, M.
Richter20, P. Riedler35, W. Riegler35, F. Riggi27, C. Ristea68, S.P. Rode51,
M. Rodríguez Cahuantzi46, K. Røed20, R. Rogalev93, E. Rogochaya76, T.S.
Rogoschinski69, D. Rohr35, D. Röhrich21, P.F. Rojas46, P.S. Rokita143, F.
Ronchetti53, A. Rosano33,57, E.D. Rosas70, A. Rossi58, A. Rotondi29, A. Roy51,
P. Roy112, N. Rubini26, O.V. Rueda82, R. Rui24, B. Rumyantsev76, A.
Rustamov89, E. Ryabinkin90, Y. Ryabov100, A. Rybicki120, H. Rytkonen128, W.
Rzesa143, O.A.M. Saarimaki45, R. Sadek117, S. Sadovsky93, J. Saetre21, K.
Šafařík38, S.K. Saha142, S. Saha88, B. Sahoo50, P. Sahoo50, R. Sahoo51, S.
Sahoo66, D. Sahu51, P.K. Sahu66, J. Saini142, S. Sakai135, S. Sambyal103, V.
SamsonovI,100,95, D. Sarkar144, N. Sarkar142, P. Sarma43, V.M. Sarti107,
M.H.P. Sas147,63, J. Schambach98,121, H.S. Scheid69, C. Schiaua49, R.
Schicker106, A. Schmah106, C. Schmidt109, H.R. Schmidt105, M.O. Schmidt106, M.
Schmidt105, N.V. Schmidt98,69, A.R. Schmier132, R. Schotter138, J.
Schukraft35, Y. Schutz138, K. Schwarz109, K. Schweda109, G. Scioli26, E.
Scomparin60, J.E. Seger15, Y. Sekiguchi134, D. Sekihata134, I.
Selyuzhenkov109,95, S. Senyukov138, J.J. Seo62, D. Serebryakov64, L.
Šerkšnytė107, A. Sevcenco68, A. Shabanov64, A. Shabetai117, R. Shahoyan35, W.
Shaikh112, A. Shangaraev93, A. Sharma102, H. Sharma120, M. Sharma103, N.
Sharma102, S. Sharma103, O. Sheibani127, A.I. Sheikh142, K. Shigaki47, M.
Shimomura85, S. Shirinkin94, Q. Shou41, Y. Sibiriak90, S. Siddhanta56, T.
Siemiarczuk87, T.F.D. Silva123, D. Silvermyr82, G. Simatovic92, G.
Simonetti35, B. Singh107, R. Singh88, R. Singh103, R. Singh51, V.K. Singh142,
V. Singhal142, T. Sinha112, B. Sitar13, M. Sitta32, T.B. Skaali20, G.
Skorodumovs106, M. Slupecki45, N. Smirnov147, R.J.M. Snellings63, C.
Soncco114, J. Song127, A. Songmoolnak118, F. Soramel28, S. Sorensen132, I.
Sputowska120, J. Stachel106, I. Stan68, P.J. Steffanic132, S.F.
Stiefelmaier106, D. Stocco117, M.M. Storetvedt37, C.P. Stylianidis92, A.A.P.
Suaide123, T. Sugitate47, C. Suire79, M. Suljic35, R. Sultanov94, M.
Šumbera97, V. Sumberia103, S. Sumowidagdo52, S. Swain66, A. Szabo13, I.
Szarka13, U. Tabassam14, S.F. Taghavi107, G. Taillepied136, J. Takahashi124,
G.J. Tambave21, S. Tang136,7, Z. Tang130, M. Tarhini117, M.G. Tarzila49, A.
Tauro35, G. Tejeda Muñoz46, A. Telesca35, L. Terlizzi25, C. Terrevoli127, G.
Tersimonov3, S. Thakur142, D. Thomas121, R. Tieulent137, A. Tikhonov64, A.R.
Timmins127, M. Tkacik119, A. Toia69, N. Topilskaya64, M. Toppi53, F. Torales-
Acosta19, S.R. Torres38, A. Trifiró33,57, S. Tripathy70, T. Tripathy50, S.
Trogolo28, G. Trombetta34, L. Tropp39, V. Trubnikov3, W.H. Trzaska128, T.P.
Trzcinski143, B.A. Trzeciak38, A. Tumkin111, R. Turrisi58, T.S. Tveter20, K.
Ullaland21, E.N. Umaka127, A. Uras137, M. Urioni141, G.L. Usai23, M. Vala39,
N. Valle29, S. Vallero60, N. van der Kolk63, L.V.R. van Doremalen63, M. van
Leeuwen92, P. Vande Vyvre35, D. Varga146, Z. Varga146, M. Varga-Kofarago146,
A. Vargas46, M. Vasileiou86, A. Vasiliev90, O. Vázquez Doce107, V.
Vechernin115, E. Vercellin25, S. Vergara Limón46, L. Vermunt63, R. Vértesi146,
M. Verweij63, L. Vickovic36, Z. Vilakazi133, O. Villalobos Baillie113, G.
Vino54, A. Vinogradov90, T. Virgili30, V. Vislavicius91, A. Vodopyanov76, B.
Volkel35, M.A. Völkl105, K. Voloshin94, S.A. Voloshin144, G. Volpe34, B. von
Haller35, I. Vorobyev107, D. Voscek119, J. Vrláková39, B. Wagner21, M.
Weber116, A. Wegrzynek35, S.C. Wenzel35, J.P. Wessels145, J. Wiechula69, J.
Wikne20, G. Wilk87, J. Wilkinson109, G.A. Willems145, E. Willsher113, B.
Windelband106, M. Winn139, W.E. Witt132, J.R. Wright121, Y. Wu130, R. Xu7, S.
Yalcin78, Y. Yamaguchi47, K. Yamakawa47, S. Yang21, S. Yano47,139, Z. Yin7, H.
Yokoyama63, I.-K. Yoo17, J.H. Yoon62, S. Yuan21, A. Yuncu106, V. Yurchenko3,
V. Zaccolo24, A. Zaman14, C. Zampolli35, H.J.C. Zanoli63, N. Zardoshti35, A.
Zarochentsev115, P. Závada67, N. Zaviyalov111, H. Zbroszczyk143, M. Zhalov100,
S. Zhang41, X. Zhang7, Y. Zhang130, V. Zherebchevskii115, Y. Zhi11, D. Zhou7,
Y. Zhou91, J. Zhu7,109, Y. Zhu7, A. Zichichi26, G. Zinovjev3, N. Zurlo141

Affiliation Notes

I Deceased

II Also at: Italian National Agency for New Technologies, Energy and
Sustainable Economic Development (ENEA), Bologna, Italy

III Also at: Dipartimento DET del Politecnico di Torino, Turin, Italy

IV Also at: M.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute
of Nuclear, Physics, Moscow, Russia

V Also at: Institute of Theoretical Physics, University of Wroclaw, Poland

Collaboration Institutes

1 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute)
Foundation, Yerevan, Armenia

2 AGH University of Science and Technology, Cracow, Poland

3 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences
of Ukraine, Kiev, Ukraine

4 Bose Institute, Department of Physics and Centre for Astroparticle Physics
and Space Science (CAPSS), Kolkata, India

5 Budker Institute for Nuclear Physics, Novosibirsk, Russia

6 California Polytechnic State University, San Luis Obispo, California, United
States

7 Central China Normal University, Wuhan, China

8 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana,
Cuba

9 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and
Mérida, Mexico

10 Chicago State University, Chicago, Illinois, United States

11 China Institute of Atomic Energy, Beijing, China

12 Chungbuk National University, Cheongju, Republic of Korea

13 Comenius University Bratislava, Faculty of Mathematics, Physics and
Informatics, Bratislava, Slovakia

14 COMSATS University Islamabad, Islamabad, Pakistan

15 Creighton University, Omaha, Nebraska, United States

16 Department of Physics, Aligarh Muslim University, Aligarh, India

17 Department of Physics, Pusan National University, Pusan, Republic of Korea

18 Department of Physics, Sejong University, Seoul, Republic of Korea

19 Department of Physics, University of California, Berkeley, California,
United States

20 Department of Physics, University of Oslo, Oslo, Norway

21 Department of Physics and Technology, University of Bergen, Bergen, Norway

22 Dipartimento di Fisica dell’Università ’La Sapienza’ and Sezione INFN,
Rome, Italy

23 Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy

24 Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy

25 Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy

26 Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Bologna, Italy

27 Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Catania, Italy

28 Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN,
Padova, Italy

29 Dipartimento di Fisica e Nucleare e Teorica, Università di Pavia and
Sezione INFN, Pavia, Italy

30 Dipartimento di Fisica ‘E.R. Caianiello’ dell’Università and Gruppo
Collegato INFN, Salerno, Italy

31 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy

32 Dipartimento di Scienze e Innovazione Tecnologica dell’Università del
Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy

33 Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy

34 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari,
Italy

35 European Organization for Nuclear Research (CERN), Geneva, Switzerland

36 Faculty of Electrical Engineering, Mechanical Engineering and Naval
Architecture, University of Split, Split, Croatia

37 Faculty of Engineering and Science, Western Norway University of Applied
Sciences, Bergen, Norway

38 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical
University in Prague, Prague, Czech Republic

39 Faculty of Science, P.J. Šafárik University, Košice, Slovakia

40 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-
Universität Frankfurt, Frankfurt, Germany

41 Fudan University, Shanghai, China

42 Gangneung-Wonju National University, Gangneung, Republic of Korea

43 Gauhati University, Department of Physics, Guwahati, India

44 Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-
Wilhelms-Universität Bonn, Bonn, Germany

45 Helsinki Institute of Physics (HIP), Helsinki, Finland

46 High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico

47 Hiroshima University, Hiroshima, Japan

48 Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation
(ZTT), Worms, Germany

49 Horia Hulubei National Institute of Physics and Nuclear Engineering,
Bucharest, Romania

50 Indian Institute of Technology Bombay (IIT), Mumbai, India

51 Indian Institute of Technology Indore, Indore, India

52 Indonesian Institute of Sciences, Jakarta, Indonesia

53 INFN, Laboratori Nazionali di Frascati, Frascati, Italy

54 INFN, Sezione di Bari, Bari, Italy

55 INFN, Sezione di Bologna, Bologna, Italy

56 INFN, Sezione di Cagliari, Cagliari, Italy

57 INFN, Sezione di Catania, Catania, Italy

58 INFN, Sezione di Padova, Padova, Italy

59 INFN, Sezione di Roma, Rome, Italy

60 INFN, Sezione di Torino, Turin, Italy

61 INFN, Sezione di Trieste, Trieste, Italy

62 Inha University, Incheon, Republic of Korea

63 Institute for Gravitational and Subatomic Physics (GRASP), Utrecht
University/Nikhef, Utrecht, Netherlands

64 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia

65 Institute of Experimental Physics, Slovak Academy of Sciences, Košice,
Slovakia

66 Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India

67 Institute of Physics of the Czech Academy of Sciences, Prague, Czech
Republic

68 Institute of Space Science (ISS), Bucharest, Romania

69 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt,
Frankfurt, Germany

70 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México,
Mexico City, Mexico

71 Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS),
Porto Alegre, Brazil

72 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City,
Mexico

73 iThemba LABS, National Research Foundation, Somerset West, South Africa

74 Jeonbuk National University, Jeonju, Republic of Korea

75 Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik,
Fachbereich Informatik und Mathematik, Frankfurt, Germany

76 Joint Institute for Nuclear Research (JINR), Dubna, Russia

77 Korea Institute of Science and Technology Information, Daejeon, Republic of
Korea

78 KTO Karatay University, Konya, Turkey

79 Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France

80 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-
Alpes, CNRS-IN2P3, Grenoble, France

81 Lawrence Berkeley National Laboratory, Berkeley, California, United States

82 Lund University Department of Physics, Division of Particle Physics, Lund,
Sweden

83 Moscow Institute for Physics and Technology, Moscow, Russia

84 Nagasaki Institute of Applied Science, Nagasaki, Japan

85 Nara Women’s University (NWU), Nara, Japan

86 National and Kapodistrian University of Athens, School of Science,
Department of Physics , Athens, Greece

87 National Centre for Nuclear Research, Warsaw, Poland

88 National Institute of Science Education and Research, Homi Bhabha National
Institute, Jatni, India

89 National Nuclear Research Center, Baku, Azerbaijan

90 National Research Centre Kurchatov Institute, Moscow, Russia

91 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

92 Nikhef, National institute for subatomic physics, Amsterdam, Netherlands

93 NRC Kurchatov Institute IHEP, Protvino, Russia

94 NRC «Kurchatov»Institute - ITEP, Moscow, Russia

95 NRNU Moscow Engineering Physics Institute, Moscow, Russia

96 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom

97 Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy,
Czech Republic

98 Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States

99 Ohio State University, Columbus, Ohio, United States

100 Petersburg Nuclear Physics Institute, Gatchina, Russia

101 Physics department, Faculty of science, University of Zagreb, Zagreb,
Croatia

102 Physics Department, Panjab University, Chandigarh, India

103 Physics Department, University of Jammu, Jammu, India

104 Physics Department, University of Rajasthan, Jaipur, India

105 Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen,
Germany

106 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg,
Heidelberg, Germany

107 Physik Department, Technische Universität München, Munich, Germany

108 Politecnico di Bari and Sezione INFN, Bari, Italy

109 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum
für Schwerionenforschung GmbH, Darmstadt, Germany

110 Rudjer Bošković Institute, Zagreb, Croatia

111 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia

112 Saha Institute of Nuclear Physics, Homi Bhabha National Institute,
Kolkata, India

113 School of Physics and Astronomy, University of Birmingham, Birmingham,
United Kingdom

114 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica
del Perú, Lima, Peru

115 St. Petersburg State University, St. Petersburg, Russia

116 Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria

117 SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France

118 Suranaree University of Technology, Nakhon Ratchasima, Thailand

119 Technical University of Košice, Košice, Slovakia

120 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of
Sciences, Cracow, Poland

121 The University of Texas at Austin, Austin, Texas, United States

122 Universidad Autónoma de Sinaloa, Culiacán, Mexico

123 Universidade de São Paulo (USP), São Paulo, Brazil

124 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil

125 Universidade Federal do ABC, Santo Andre, Brazil

126 University of Cape Town, Cape Town, South Africa

127 University of Houston, Houston, Texas, United States

128 University of Jyväskylä, Jyväskylä, Finland

129 University of Liverpool, Liverpool, United Kingdom

130 University of Science and Technology of China, Hefei, China

131 University of South-Eastern Norway, Tonsberg, Norway

132 University of Tennessee, Knoxville, Tennessee, United States

133 University of the Witwatersrand, Johannesburg, South Africa

134 University of Tokyo, Tokyo, Japan

135 University of Tsukuba, Tsukuba, Japan

136 Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France

137 Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon
, Lyon, France

138 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France,
Strasbourg, France

139 Université Paris-Saclay Centre d’Etudes de Saclay (CEA), IRFU, Départment
de Physique Nucléaire (DPhN), Saclay, France

140 Università degli Studi di Foggia, Foggia, Italy

141 Università di Brescia and Sezione INFN, Brescia, Italy

142 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata,
India

143 Warsaw University of Technology, Warsaw, Poland

144 Wayne State University, Detroit, Michigan, United States

145 Westfälische Wilhelms-Universität Münster, Institut für Kernphysik,
Münster, Germany

146 Wigner Research Centre for Physics, Budapest, Hungary

147 Yale University, New Haven, Connecticut, United States

148 Yonsei University, Seoul, Republic of Korea