First measurement of coherent ρ^{0} photoproduction in ultra-peripheral Xe-Xe collisions at \mathbf{\sqrt{s_{\rm {\scriptscriptstyle \mathbf{NN}}}} = 5.44} TeV
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First measurement of coherent ρ photoproduction in ultra-peripheralXe–Xe collisions at √ s NN = .
44 TeV
ALICE Collaboration * Abstract
The first measurement of the coherent photoproduction of ρ vector mesons in ultra-peripheral Xe–Xe collisions at √ s NN = .
44 TeV is presented. This result, together with previous γ p and γ –Pbmeasurements, describes the atomic number ( A ) dependence of this process, which is particularlysensitive 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 Xe + Xe → ρ + Xe + Xe process, measured at midrapiditythrough the decay channel ρ → π + π − , is found to be d σ / d y = . ± . ( stat . ) + . − . ( syst . ) mb.The ratio of the continuum to resonant contributions for the production of pion pairs is also mea-sured. In addition, the fraction of events accompanied by electromagnetic dissociation of either oneor both colliding nuclei is reported. The dependence on A of cross section for the coherent ρ pho-toproduction at a centre-of-mass energy per nucleon of the γ A system of W γ A , n =
65 GeV is found tobe consistent with a power-law behaviour σ ( γ A → ρ A ) ∝ A α with a slope α = . ± . ( syst . ) .This slope signals important shadowing effects, but it is still far from the behaviour expected in theblack-disc limit. * See Appendix A for the list of collaboration members oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaboration The Large Hadron Collider (LHC) is a source of photon-induced processes. The electromagnetic fieldsof the relativistic particles are strongly contracted allowing for their interpretation as a flux of quasi-realphotons, which interact with the particles travelling in the opposite direction. When the impact parameterof the collision is larger than the sum of the radii of the incoming particles, purely strong interactionsare suppressed due to the short range of this force and photon-induced processes dominate. These areultra-peripheral collisions (UPCs) [1–3].Among all the possible processes in UPC, the coherent production of vector mesons stands out due to thelarge associated cross sections and the cleanliness of its experimental signature: the quasi-real photoninteracts 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, h p T i , is relatedto the transverse size of the nucleus R A as h p T i ∼ ℏ / R A [1], yielding h p T i ∼
37 (30) MeV/ c for a Xe(Pb) nucleus. A related process is the incoherent production where the photon interacts with a nucleonin the nucleus, which implies a larger average transverse momentum of the produced vector meson. Inaddition, secondary electromagnetic interactions of the colliding nuclei may excite one or both of themand upon de-excitation produce neutrons at beam rapidities [4]. This effect depends on the square of theelectric 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 ρ vector me-son, which offers the opportunity to study the approach to the black-disc limit of QCD with a semi-hardscale [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 Collaborationat centre-of-mass energies per nucleon pair ( √ s 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 5.02TeV [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 andmore precise data motivated an improvement of the different theoretical approaches, which in generalare now closer to data [10]. The situation, although better than a few years ago, still calls for moredata to improve our understanding of this process. Furthermore, the coherent production of a ρ vectormeson off a nucleus allows for the study of shadowing, the experimental fact that the nuclear structurefunctions are suppressed compared to the superposition of those of their constituent nucleons [11]. Thisphenomenon is expected to depend on the atomic number A of the nucleus so measurements for differentvalues of A offer another tool to test our understanding of shadowing at high energies and semi-hardscales.In collisions of two heavy ions with atomic number A , either nucleus can be a source of photons, whichresults in two contributions to the cross section. At midrapidity both contributions are the same, butat 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 theenergy dependence of the process, allowing for the study of the energy dependence of the underlyingQCD dynamics. Two techniques have been put forward to this end [12, 13]. The first one make use ofUPCs and peripheral collisions. The one described in Ref. [13] proposes to classify the measured eventsdepending on the presence of the beam-rapidity neutrons mentioned earlier, and use these different crosssections to disentangle the low and high energy contributions. Measurements at midrapidity are idealto test this proposal because both contributions are the same, so the measured cross sections can beunambiguously compared with models predicting the neutron emission probability. ALICE can measurebeam-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 ρ vector mesons in Pb–Pb UPC for events with nobeam-rapidity neutrons (0n0n, where the first 0n refers to the A-side and the second to the C-side), with2oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaborationone or more neutrons in one side only (0nXn+Xn0n), or in both sides (XnXn). The comparison of thecorresponding cross section fractions with calculations of the emission of beam-rapidity neutrons basedon the STARlight [15, 16] and n OO n [17] models suggests that the method works, but it is important totest 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 ρ vector mesons in Xe–Xe UPCsat √ s NN = .
44 TeV is presented. The cross section of this process is measured at midrapidity throughthe decay channel ρ → π + π − . The ratio of the continuum to resonant contributions for the productionof pion pairs is also measured. In addition, the fraction of events in the 0n0n, 0nXn+Xn0n, and XnXnclasses are reported. Finally, using data from HERA and from Pb–Pb UPC collisions measured byALICE, the A dependence of the cross section is studied at a centre-of-mass energy per nucleon of the γ A system of W γ A , n =
65 GeV.
During a 6-hour pilot run in October 2017, the LHC collided xenon nuclei for the first time. These colli-sions took place at √ s NN = .
44 TeV. A complete description of the ALICE detector and its performancecan be found in Ref. [14, 19]; here, just a brief description of the systems involved in the measurementis given.The decay products of the ρ vector meson are measured in the central-barrel region of ALICE withthe ITS and TPC detectors. The ALICE Inner Tracking System (ITS) [20] is made of six layers ofsilicon sensors. Each layer has a cylindrical geometry concentric around the beam line. Three differenttechnologies 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] surroundsthe ITS. It is a large cylindrical gas detector with a central membrane at high voltage and two readoutplanes, composed of multiwire proportional chambers, at the end caps. It is the main tracking detectorand it also offers particle identification through the measurement of ionisation energy loss. The TPCand ITS cover a pseudorapidity interval | η | < . ± . | η | > . | η | < | η | < .
4, respectively. The SPD has about 10 pixels which are read out by 400 (800) chips in theinner (outer) layer. Each of the readout chips fires a trigger if at least one of its pixels has a signal. TOFsurrounds the TPC and matches its pseudorapidity coverage. The TOF is a large cylindrical barrel ofmultigap resistive plate chambers with some 1 . × readout channels arranged in 1608 pads that arecapable of triggering [22]. The V0 [23] is a set of two arrays made of 32 scintillator cells each. Thearrays cover the pseudorapidity ranges − . < η < − . . < η < .
1, respectively. The time res-olution of the V0 is better than 500 ps and provides a trigger if it registers a signal. The trigger requiresat least two hits in the inner and in the outer layer of the SPD, at least two pads fired in TOF and nosignal in the V0.The determination of the luminosity is based on reference trigger counts in the V0 detector, while thereference trigger cross section is derived from estimates based on a Glauber model [24]. For xenon thefollowing values are used: A = r = ( . ± . ) fm,a skin depth of ( . ± . ) fm, and a deformation parameter β = . ± .
02 [25]. The integrated3oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaboration c ) (GeV/ ππ ( m c C oun t s pe r M e V / = 5.44 TeV NN s Xe UPC − ALICE Xe
Opposite-sign pairsSame-sign pairs c < 0.15 GeV/ T p | < 0.8 y | c ) (GeV/ ππ ( T p c C oun t s pe r M e V / = 5.44 TeV NN s Xe UPC − ALICE Xe
Opposite-sign pairsSame-sign pairsSTARlight coherentSTARlight incoherent c < 1.5 GeV/ m y | Figure 1: (Colour online) Uncorrected invariant mass (left) and transverse momentum (right) distribution of se-lected 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 luminosityof data. luminosity used in this analysis is ( . ± . ) mb − , where the quoted uncertainty is systematic andis described later. Events are selected for the analysis if ( i ) the trigger described above is active, ( ii ) there are no signals inthe 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 timewindow. 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 SPDhave to have a signal associated both to the track and to a SPD trigger signal, the tracks should alsohave at least 50 (out of 159) space points reconstructed by the TPC. Both tracks are required to be fullywithin the acceptance of the detector ( | η trk | < . | z trk | <
10 cm, and their associated electric charge should be ofopposite sign. Particle identification of a track is determined by the number of standard deviations ( n σ )by which the energy loss measurement deviates from the pion hypothesis. The quadratic sum of n σ , for the π + and π − candidates has to be less than five squared ( n σ + n σ < ). Finally, the transversemomentum of the pion pair has to be less than 0.15 GeV/ c . After the aforementioned selections, 1827events remain in the data sample.Figure 1 shows the distributions of the invariant mass of the pion pairs as well as their transverse mo-mentum. In addition, the figure also shows the corresponding distribution of an alternate data sampleobtained by applying all the criteria described above except that the electric charge associated to bothtracks has to be of the same sign. The mass distribution shows a clear signal of a ρ vector meson over avery small background represented by the same-sign distribution. The transverse momentum distributionshows a pronounced peak at values of few tens of MeV/ c as expected by coherent production accom-panied by a tail towards larger momenta produced by incoherent production and the small remaininghadronic background.The presence of one or more neutrons at beam rapidity is determined by using the timing capabilitiesof the ZNA and ZNC which allow for the selection of events with a signal within ± ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaborationexpected for neutrons produced in the interaction. A Monte Carlo (MC) sample of pion pairs from continuum and ρ resonant production, generated withthe STARlight program [16], is used to extract mass-dependent efficiency correction factors to accountfor the acceptance and the efficiency of the detector and the selection criteria. All events in this sample arepassed by a detailed simulation of the ALICE apparatus and subjected to the same analysis procedure asin data. The correction factor at each mass of the pion pair is used to correct the mass distribution shownin Fig. 1. The correction factor increases from about 0.025 at 550 MeV/ c to 0.055 at 1.1 GeV/ c . Thenumber of ρ vector mesons is extracted from the corrected invariant mass distribution normalised bythe luminosity of the sample and after applying other corrections, described below, to take into accountpile-up and the contribution from incoherent events.The corrected mass distribution is fitted to a model describing the resonant and continuum production ofpion pairs according to the Söding prescription [26] and a term M that takes into account the contributionof the γγ → µ + µ − process: d σ d m d y = | A × BW ρ + B | + M . (1)Here A is the normalisation factor of the ρ Breit–Wigner ( BW ρ ) function, and B is the non-resonantamplitude. The relativistic Breit–Wigner function of the ρ vector meson is BW ρ = q m × m ρ × Γ ( m ) m − m ρ + im ρ × Γ ( m ) , (2)where m ρ is the pole mass of the ρ vector meson. The mass-dependent width Γ ( m ) is given by Γ ( m ) = Γ ( m ρ ) × m ρ m × m − m π m ρ − m π ! / , (3)with Γ ( m ρ ) the width of the ρ vector meson and m π the mass of the pion [27]. The shape of γγ → µ + µ − process M is taken from STARlight and passed through the same selection procedure as data. The fittedparameters are A , B and M , the mass and width of the ρ were fixed to the PDG values [28].An example fit of data with this model is shown in Fig. 2, where a clear resonance structure is seen indata and decomposed by the fit into a small background contribution, the contribution of the continuumproduction of pion pairs, the interference term, and the ρ signal. The contributions to the systematic uncertainty are listed in Table 1 and discussed one by one in thefollowing paragraphs. The total uncertainty is obtained as the quadratic sum of the various contributions.The signal extraction procedure is performed many times by varying the lower and upper limit of the fitrange as well as the bin width. The variations are within 0.55 to 0.65 GeV / c , 0.9 to 1.4 GeV / c , and10 to 50 MeV / c , respectively. The average value of the pole mass for the ρ vector meson as well as itswidth are found to agree with the value reported by the Particle Data Group [28]. As the precision of ourdata sample is limited, the values for the pole mass and width of the ρ vector meson are then fixed to thevalues measured in the ρ photoproduction process [28] and the signal extraction procedure is repeated.The standard deviation of the distribution of extracted number of ρ vector mesons from all the differentfits is considered as a systematic uncertainty, while the mean is taken as the signal. The mean of allstatistical uncertainties is taken as the statistical uncertainty. Fits are performed using a log-likelihood as5oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaboration c ) (GeV/ ππ ( m )) c ) ( b / ( G e V / y d m / ( d σ d c < 0.15 GeV/ T p | < 0.8 y | ALICE dataFull fit Wigner − Breit ρ interference ππ− ρ Muon template = 5.44 TeV NN s Xe UPC − ALICE Xe
Figure 2: (Colour online) Invariant mass distribution of pion pairs with the different components of the fit repre-sented by lines. The number of ρ 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. Table 1:
Summary of the systematic uncertainties for the measured cross section. See text for details.
Source UncertaintyVariations to the fit procedure ± . + . ± . ± . ± . ± . ± . ± . ± . ± . γγ → µ + µ − ) +( . ) − ( . ) %Electromagnetic dissociation ± . ± . +( . ) − ( . ) %well as a χ approach; both producing the same results. The systematic uncertainty of the cross sectionfrom those variations amounts to 2.5%.The Ross–Stodolsky prescription [29] is used as an alternative model to fit the resonance and continuumcontribution, which results in a yield systematically higher by 3.5%. Pure MC studies, where signalis generated with a Söding function and fitted with a Ross–Stodolsky model, and vice versa, show a6oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaborationsimilar behaviour. As the underlying distribution is not known, this difference is taken as a systematicuncertainty.Two MC samples are used to account for the acceptance and efficiency correction. One simulates onlythe Breit–Wigner distribution for a pure ρ signal and the other includes the effect of the pion-paircontinuum. The full variation of 0.5% on the cross section obtained by using these two samples isconsidered 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 havereduced performance, are rejected. The full variation of the results amounts to 3% and it is taken as asystematic uncertainty. Likewise for the uncertainty when matching track segments in the ITS to theircounterparts 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 effectin data with that in MC. A discrepancy of 2.0% is found and assigned as a systematic uncertainty. Theuncertainty on the trigger efficiency of TOF is obtained by comparing the acceptance-times-efficiencycorrection 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 interactionvertex along the beam line for collisions happening ±
10 cm and more beyond the nominal interactionpoint. The full difference in the cross section when retaining (or not) events beyond ±
10 cm is found tobe ± .
5% and assigned as a systematic uncertainty.The contribution from incoherent production of ρ vector mesons for the region p T < .
15 GeV/ c is de-termined by fitting the corresponding template from STARlight to the transverse momentum distributionin a range from 0.15 to 1.0 GeV/ c and extrapolating to the region covered by the measurement. The fitis repeated many times varying the fit ranges, within the stated interval, and the bin widths. The meanof the results from these fits amounts to 10.2% and it is subtracted from the cross section. The standarddeviation of all fits is taken as a systematic uncertainty ( ± . ( . ± . ( syst . )) % for the 0n0n and ( . ± . ( syst . )) % for the0nXn+Xn0n+XnXn event classes, respectively. This is taken into account when computing the fractionsof 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 in-teraction, an effect called pile-up. Electromagnetic e + + e − pair production is the main source of thesesignals. The probability for pile-up is obtained from an unbiased sample triggered by the timing of ex-pected bunch crossings at the interaction point surrounded by the ALICE detector. This probability isused, assuming a Poisson process, to correct for the events lost due to pile-up. The correction factor is0 . ± .
01; the uncertainty from this procedure is taken as systematic uncertainty ( ± . γγ → e + e − cross section in our previous measurement [30] is around10% and within this precision it agrees with the prediction from STARlight. Changing the normalisationof the γγ → µ + µ − template in the fit by ± − .
2% and + .
5% systematic uncertaintyon the extracted ρ cross section.Electromagnetic dissociation producing the beam-rapidity neutrons is accompanied on occasion by othercharged particles [31]. These charged particles, if they hit the V0, may cause the event to be lost. Theprobability for this to happen is estimated to be ( . ± . ) % using the unbiased sample just mentioned.The statistical precision of this procedure is taken as systematic uncertainty ( ± . ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaboration − − − − y ) ( m b ) ρ X e + X e + → ( X e + X e y / d σ d ALICE dataCCKT
GMMNSGKZSTARlight = 5.44 TeV NN s Xe UPC − ALICE Xe
Figure 3: (Colour online) Cross section for the coherent photoproduction of ρ vector mesons in Xe–Xe UPC.The lines show the predictions of the different models described in the text. varying each parameter within their reported uncertainty. This is the dominant source of uncertainty inthe 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 ±
10% uncertainty mentioned before produces a change in the cross sectionwhich 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 alsoaffected by pile-up in the ZNA and ZNC and by the efficiency of these calorimeters to detected neutrons.The pile-up probabilities are ( . ± . ) % and ( . ± . ) %, while the efficiencies are 0 . ± . . ± .
02 for the ZNA and ZNC, respectively. The uncertainties in these numbers, along withthe uncertainty on the subtraction of the incoherent contribution, are taken into account to obtain theuncertainty on the fractions quoted below.
The cross section for the coherent photoproduction of ρ vector mesons in ultra-peripheral Xe–Xe colli-sions at √ s NN = .
44 TeV measured at midrapidity isd σ d y = . ± . ( stat . ) + . − . ( syst . ) mb . (4)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 exclusiveproduction of ρ vector mesons off protons, ( ii ) the optical theorem, and ( iii ) a Glauber-like eikonalformalism which neglects the elastic part of the elementary ρ –nucleon cross section. The prediction byGuzey, Kryshen and Zhalov (GKZ) [32] relies on a modified vector dominance model, where hadronicfluctuations of the photon are taken into account according to the Gribov–Glauber model of nuclearshadowing; the band shows the variation on the predictions when varying the parameters of the model.8oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaboration 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 OO n model [17]. The first uncertainty is statistical, thesecond comes from the variations in the ZNA and ZNC pile-up factors and efficiencies, while the third comes fromthe variation in the number of events which is dominated by the subtraction of incoherent contribution. See textfor details. Class Measured fraction n OO n prediction0n0n (90 . ± . ± . ∓ . . ± . ∓ . ± . . ± . ∓ . ± . | B / A | = . ± . ( stat . ) ± . ( syst . ) ( GeV / c ) − . The main uncertainty comes from the correction for acceptanceand efficiency, closely followed by variations from the signal extraction procedure. This value is con-sistent with those obtained in Pb–Pb UPC at √ s NN = .
76 TeV [9] and √ s NN = .
02 TeV [10], namely | B / A | = . ± . ( stat . ) ± . . ( syst . ) ( GeV / c ) − and | B / A | = . ± . ( stat . ) ± . ( syst . )( GeV / c ) − , respectively. The corresponding ratio in coherent Au–Au UPC measured by STAR at √ s NN =
200 GeV is 0 . ± . ( stat . ) ± . ( syst . ) ( GeV / c ) − [8]. The CMS Collaboration mea-sured 0 . ± . ( stat . ) ( GeV / c ) − in p–Pb UPC at √ s NN = .
02 TeV [38] for | t | < . . TheZEUS Collaboration, using a sample of positron–proton collisions at a centre-of-mass energy of 300GeV, reports 0 . ± . ( stat . ) ± . ( syst . ) ( GeV / c ) − for their full analysed sample, and ≈ . ( GeV / c ) − for t values similar to those of coherent ρ 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 fromthe n OO 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 approxima-tion, and on all existing data on EMD complemented by phenomenological extrapolations where data isnot 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 crosssections observed in Pb–Pb UPC at √ s NN = .
02 TeV [10], suggests that the emission of neutrons atbeam-rapidity is well understood for the coherent photoproduction of ρ vector mesons off nuclei withsuch different atomic mass number as Pb and Xe.The measurements of the UPC cross section for coherent production of ρ vector mesons at midrapdityfor Pb–Pb [10] and for Xe–Xe have been converted into a γ A measurement by dividing the cross sectionsby two times the corresponding photon fluxes of 58.6 (Xe) and 128.1 (Pb). These numbers are obtainedfollowing the prescription detailed in Ref. [12]. A flux uncertainty of 2% is considered, which is uncorre-lated between both nuclei (because it mainly originates in the knowledge of the nuclear geometries). The9oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaboration A = G e V ) ( m b ) A , n γ ( W A γ σ Xe −γ ALICE Pb −γ ALICE p γ H1 α A σ fit: CCKTGKZ A σ coherent: A σ incoherent: A σ black disk: Figure 4: A dependence of the γ A cross section for the coherent production of a ρ meson and the correspondingpower 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 theparameters of the model. A power-law fit to the CCKT model is shown by the blue band. See text for details. uncertainties coming from the Ross–Stodolsky fit model and from the ITS-TPC matching are correlatedbetween the Xe and Pb results. The midrapidity photon–nucleus centre-of-mass energy per nucleon isgiven by W γ A , n = m √ s 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 γ –Pb cross section is expected to change around 1%between these two values, well within the experimental uncertainties, both measurements are taken ashaving W γ A , n =
65 GeV.The dependence of these cross sections on A is fitted by a power-law model, σ γ A ( A ) = σ A α , using alsothe cross section measured by H1 at this energy [40]: ( . ± . ( syst . )) µ b. The value reported by H1is 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 χ = .
48 (for one degree of freedom). The parameters from thefit when using only uncorrelated uncertainties are σ = . ± . α = . ± . − .
78. Varying the flux by ±
2% produces a change in the exponent α of 0 . σ parameter and causes a change in the exponent α of ± .
007 and + . A resulting on slopes α of 4/3, 1, and 2/3 for full coherence disregarding any other dynamical effect, for a total incoherentbehaviour, and for the black-disc limit, respectively. The slope found in data is significantly differentfrom 4/3 signalling important shadowing effects. The closeness of data to a slope of 1 does not implyincoherent 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 γ A =
65 GeV.Fitting to the same functional form the predictions of the Gribov–Glauber approach (GKZ [32, 41])and of the colour dipole model with subnucleon degrees of freedom (CCKT [36, 42]) yields slopes of10oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaboration0 . ± .
007 and 0 . ± . σ has been fixed tothe corresponding prediction for the γ p cross section. Both slopes are in good agreement with that foundin data. This was to be expected given that both approaches give a reasonable description of the differentavailable data. The cross section for the coherent photoproduction of ρ vector mesons in Xe–Xe UPC at √ s NN = . n OO n model. The fair agreement between dataand predictions suggest that this process is well understood within the current experimental uncertaintiesand can be used as a tool to disentangle the different γ A contributions to the UPC cross sections.The dependence on A of the cross section for the coherent ρ photoproduction at a centre-of-mass energyper nucleon of the γ A system of 65 GeV is found to be consistent with a power-law behaviour with aslope of 0 . ± .
02. This exponent is substantially smaller than what is expected from a purely coherentprocess, 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–Glauberapproach (GKZ) as well as in partonic-level models (CCKT). In this context, the A dependence of thecross 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 con-tributions to the construction of the experiment and the CERN accelerator teams for the outstandingperformance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources andsupport 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 andrunning 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 fürForschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technologies,National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tec-nológico (CNPq), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estadode São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry ofEducation of China (MOEC) , Ministry of Science & Technology of China (MSTC) and National NaturalScience Foundation of China (NSFC), China; Ministry of Science and Education and Croatian ScienceFoundation, Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaen-ergía, Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; CzechScience Foundation; The Danish Council for Independent Research | Natural Sciences, the VILLUMFONDEN and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics(HIP), Finland; Commissariat à l’Energie Atomique (CEA) and Institut National de Physique Nucléaireet de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France;Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionen-forschung GmbH, Germany; General Secretariat for Research and Technology, Ministry of Education,Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; De-partment of Atomic Energy Government of India (DAE), Department of Science and Technology, Gov-11oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaborationernment of India (DST), University Grants Commission, Government of India (UGC) and Council ofScientific and Industrial Research (CSIR), India; Indonesian Institute of Science, Indonesia; IstitutoNazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology , NagasakiInstitute of Applied Science (IIST), Japanese Ministry of Education, Culture, Sports, Science and Tech-nology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; ConsejoNacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Cien-cia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA),Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The ResearchCouncil of Norway, Norway; Commission on Science and Technology for Sustainable Development inthe South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Scienceand Higher Education, National Science Centre and WUT ID-UB, Poland; Korea Institute of Scienceand 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 andInnovation and Institute of Atomic Physics, Romania; Joint Institute for Nuclear Research (JINR), Min-istry 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 Educa-tion, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation ofSouth Africa, South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Founda-tion (KAW), Sweden; European Organization for Nuclear Research, Switzerland; Suranaree Universityof Technology (SUT), National Science and Technology Development Agency (NSDTA) and Office ofthe Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic EnergyAgency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and TechnologyFacilities Council (STFC), United Kingdom; National Science Foundation of the United States of Amer-ica (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United Statesof America. References [1] A. J. Baltz, “The Physics of Ultraperipheral Collisions at the LHC”,
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S. Acharya , D. Adamová , A. Adler , J. Adolfsson , G. Aglieri Rinella , M. Agnello ,N. Agrawal , Z. Ahammed , S. Ahmad , S.U. Ahn , Z. Akbar , A. Akindinov ,M. Al-Turany , D.S.D. Albuquerque , D. Aleksandrov , B. Alessandro , H.M. Alfanda ,R. Alfaro Molina , B. Ali , Y. Ali , A. Alici , N. Alizadehvandchali , A. Alkin , J. Alme ,T. Alt , L. Altenkamper , I. Altsybeev , M.N. Anaam , C. Andrei , D. Andreou , A. Andronic ,V. Anguelov , T. Antiˇci´c , F. Antinori , P. Antonioli , C. Anuj , N. Apadula , L. Aphecetche ,H. Appelshäuser , S. Arcelli , R. Arnaldi , M. Arratia , I.C. Arsene , M. Arslandok , ,A. Augustinus , R. Averbeck , S. Aziz , M.D. Azmi , A. Badalà , Y.W. Baek , X. Bai ,R. Bailhache , R. Bala , A. Balbino , A. Baldisseri , M. Ball , D. Banerjee , R. Barbera ,L. Barioglio , M. Barlou , G.G. Barnaföldi , L.S. Barnby , V. Barret , C. Bartels , K. Barth ,E. Bartsch , F. Baruffaldi , N. Bastid , S. Basu , , G. Batigne , B. Batyunya , D. Bauri ,J.L. Bazo Alba , I.G. Bearden , C. Beattie , I. Belikov , A.D.C. Bell Hechavarria , F. Bellini ,R. Bellwied , S. Belokurova , V. Belyaev , G. Bencedi , , S. Beole , A. Bercuci ,Y. Berdnikov , A. Berdnikova , D. Berenyi , L. Bergmann , M.G. Besoiu , L. Betev ,P.P. Bhaduri , A. Bhasin , I.R. Bhat , M.A. Bhat , B. Bhattacharjee , P. Bhattacharya ,A. Bianchi , L. Bianchi , N. Bianchi , J. Bielˇcík , J. Bielˇcíková , A. Bilandzic , G. Biro ,S. Biswas , J.T. Blair , D. Blau , M.B. Blidaru , C. Blume , G. Boca , F. Bock ,A. Bogdanov , S. Boi , J. Bok , L. Boldizsár , A. Bolozdynya , M. Bombara , P.M. Bond ,G. Bonomi , H. Borel , A. Borissov , , H. Bossi , E. Botta , L. Bratrud ,P. Braun-Munzinger , M. Bregant , M. Broz , G.E. Bruno , , M.D. Buckland ,D. Budnikov , H. Buesching , S. Bufalino , O. Bugnon , P. Buhler , P. Buncic ,Z. Buthelezi , , J.B. Butt , S.A. Bysiak , D. Caffarri , A. Caliva , E. Calvo Villar ,J.M.M. Camacho , R.S. Camacho , P. Camerini , F.D.M. Canedo , A.A. Capon ,F. Carnesecchi , R. Caron , J. Castillo Castellanos , E.A.R. Casula , F. Catalano , C. CeballosSanchez , P. Chakraborty , S. Chandra , W. Chang , S. Chapeland , M. Chartier ,S. Chattopadhyay , S. Chattopadhyay , A. Chauvin , T.G. Chavez , C. Cheshkov ,B. Cheynis , V. Chibante Barroso , D.D. Chinellato , S. Cho , P. Chochula , P. Christakoglou ,C.H. Christensen , P. Christiansen , T. Chujo , C. Cicalo , L. Cifarelli , F. Cindolo ,M.R. Ciupek , G. Clai II , , J. Cleymans , F. Colamaria , J.S. Colburn , D. Colella , ,A. Collu , M. Colocci , , M. Concas III , , G. Conesa Balbastre , Z. Conesa del Valle , G. Contin ,J.G. Contreras , T.M. Cormier , P. Cortese , M.R. Cosentino , F. Costa , S. Costanza ,P. Crochet , E. Cuautle , P. Cui , L. Cunqueiro , A. Dainese , F.P.A. Damas , ,M.C. Danisch , A. Danu , I. Das , P. Das , P. Das , S. Das , S. Dash , S. De , A. De Caro ,G. de Cataldo , L. De Cilladi , J. de Cuveland , A. De Falco , D. De Gruttola , N. De Marco ,C. De Martin , S. De Pasquale , S. Deb , H.F. Degenhardt , K.R. Deja , L. Dello Stritto ,S. Delsanto , W. Deng , P. Dhankher , D. Di Bari , A. Di Mauro , R.A. Diaz , T. Dietel ,Y. Ding , R. Divià , D.U. Dixit , Ø. Djuvsland , U. Dmitrieva , J. Do , A. Dobrin , B. Dönigus ,O. Dordic , A.K. Dubey , A. Dubla , , S. Dudi , M. Dukhishyam , P. Dupieux ,T.M. Eder , R.J. Ehlers , V.N. Eikeland , D. Elia , B. Erazmus , F. Ercolessi , F. Erhardt ,A. Erokhin , M.R. Ersdal , B. Espagnon , G. Eulisse , D. Evans , S. Evdokimov ,L. Fabbietti , M. Faggin , J. Faivre , F. Fan , A. Fantoni , M. Fasel , P. Fecchio , A. Feliciello ,G. Feofilov , A. Fernández Téllez , A. Ferrero , A. Ferretti , A. Festanti , V.J.G. Feuillard ,J. Figiel , S. Filchagin , D. Finogeev , F.M. Fionda , G. Fiorenza , F. Flor , A.N. Flores ,S. Foertsch , P. Foka , S. Fokin , E. Fragiacomo , U. Fuchs , N. Funicello , C. Furget ,A. Furs , M. Fusco Girard , J.J. Gaardhøje , M. Gagliardi , A.M. Gago , A. Gal ,C.D. Galvan , P. Ganoti , C. Garabatos , J.R.A. Garcia , E. Garcia-Solis , K. Garg ,C. Gargiulo , A. Garibli , K. Garner , P. Gasik , E.F. Gauger , M.B. Gay Ducati ,M. Germain , J. Ghosh , P. Ghosh , S.K. Ghosh , M. Giacalone , P. Gianotti ,16oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE CollaborationP. Giubellino , , P. Giubilato , A.M.C. Glaenzer , P. Glässel , V. Gonzalez ,L.H. González-Trueba , S. Gorbunov , L. Görlich , S. Gotovac , V. Grabski ,L.K. Graczykowski , K.L. Graham , L. Greiner , A. Grelli , C. Grigoras , V. Grigoriev ,A. Grigoryan I , , S. Grigoryan , , O.S. Groettvik , F. Grosa , J.F. Grosse-Oetringhaus ,R. Grosso , R. Guernane , M. Guilbaud , M. Guittiere , K. Gulbrandsen , T. Gunji ,A. Gupta , R. Gupta , I.B. Guzman , R. Haake , M.K. Habib , C. Hadjidakis ,H. Hamagaki , G. Hamar , M. Hamid , R. Hannigan , M.R. Haque , , A. Harlenderova ,J.W. Harris , A. Harton , J.A. Hasenbichler , H. Hassan , D. Hatzifotiadou , P. Hauer ,L.B. Havener , S. Hayashi , S.T. Heckel , E. Hellbär , H. Helstrup , T. Herman ,E.G. Hernandez , G. Herrera Corral , F. Herrmann , K.F. Hetland , H. Hillemanns , C. Hills ,B. Hippolyte , B. Hohlweger , J. Honermann , G.H. Hong , D. Horak , S. Hornung ,R. Hosokawa , P. Hristov , C. Huang , C. Hughes , P. Huhn , T.J. Humanic , H. Hushnud ,L.A. Husova , N. Hussain , D. Hutter , J.P. Iddon , , R. Ilkaev , H. Ilyas , M. Inaba ,G.M. Innocenti , M. Ippolitov , A. Isakov , , M.S. Islam , M. Ivanov , V. Ivanov ,V. Izucheev , B. Jacak , N. Jacazio , , P.M. Jacobs , S. Jadlovska , J. Jadlovsky , S. Jaelani ,C. Jahnke , M.J. Jakubowska , M.A. Janik , T. Janson , M. Jercic , O. Jevons , M. Jin ,F. Jonas , , P.G. Jones , J. Jung , M. Jung , A. Junique , A. Jusko , P. Kalinak , A. Kalweit ,V. Kaplin , S. Kar , A. Karasu Uysal , D. Karatovic , O. Karavichev , T. Karavicheva ,P. Karczmarczyk , E. Karpechev , A. Kazantsev , U. Kebschull , R. Keidel , M. Keil ,B. Ketzer , Z. Khabanova , A.M. Khan , S. Khan , A. Khanzadeev , Y. Kharlov , A. Khatun ,A. Khuntia , B. Kileng , B. Kim , D. Kim , D.J. Kim , E.J. Kim , H. Kim , J. Kim ,J.S. Kim , J. Kim , J. Kim , J. Kim , M. Kim , S. Kim , T. Kim , S. Kirsch , I. Kisel ,S. Kiselev , A. Kisiel , J.L. Klay , J. Klein , , S. Klein , C. Klein-Bösing , M. Kleiner ,T. Klemenz , A. Kluge , A.G. Knospe , C. Kobdaj , M.K. Köhler , T. Kollegger ,A. Kondratyev , N. Kondratyeva , E. Kondratyuk , J. Konig , S.A. Konigstorfer ,P.J. Konopka , , G. Kornakov , S.D. Koryciak , L. Koska , O. Kovalenko , V. Kovalenko ,M. Kowalski , I. Králik , A. Kravˇcáková , L. Kreis , M. Krivda , , F. Krizek ,K. Krizkova Gajdosova , M. Kroesen , M. Krüger , E. Kryshen , M. Krzewicki , V. Kuˇcera ,C. Kuhn , P.G. Kuijer , T. Kumaoka , L. Kumar , S. Kundu , P. Kurashvili , A. Kurepin ,A.B. Kurepin , A. Kuryakin , S. Kushpil , J. Kvapil , M.J. Kweon , J.Y. Kwon , Y. Kwon ,S.L. La Pointe , P. La Rocca , Y.S. Lai , A. Lakrathok , M. Lamanna , R. Langoy ,K. Lapidus , P. Larionov , E. Laudi , L. Lautner , R. Lavicka , T. Lazareva , R. Lea , J. Lee ,J. Lehrbach , R.C. Lemmon , I. León Monzón , E.D. Lesser , M. Lettrich , P. Lévai , X. Li ,X.L. Li , J. Lien , R. Lietava , B. Lim , S.H. Lim , V. Lindenstruth , A. Lindner ,C. Lippmann , A. Liu , J. Liu , I.M. Lofnes , V. Loginov , C. Loizides , P. Loncar ,J.A. Lopez , X. Lopez , E. López Torres , J.R. Luhder , M. Lunardon , G. Luparello ,Y.G. Ma , A. Maevskaya , M. Mager , S.M. Mahmood , T. Mahmoud , A. Maire ,R.D. Majka I , , M. Malaev , Q.W. Malik , L. Malinina IV , , D. Mal’Kevich , N. Mallick ,P. Malzacher , G. Mandaglio , , V. Manko , F. Manso , V. Manzari , Y. Mao , J. Mareš ,G.V. Margagliotti , A. Margotti , A. Marín , C. Markert , M. Marquard , N.A. Martin ,P. Martinengo , J.L. Martinez , M.I. Martínez , G. Martínez García , S. Masciocchi ,M. Masera , A. Masoni , L. Massacrier , A. Mastroserio , , A.M. Mathis , O. Matonoha ,P.F.T. Matuoka , A. Matyja , C. Mayer , A.L. Mazuecos , F. Mazzaschi , M. Mazzilli , ,M.A. Mazzoni , A.F. Mechler , F. Meddi , Y. Melikyan , A. Menchaca-Rocha , C. Mengke , ,E. Meninno , , A.S. Menon , M. Meres , S. Mhlanga , Y. Miake , L. Micheletti ,L.C. Migliorin , D.L. Mihaylov , K. Mikhaylov , , A.N. Mishra , , D. Mi´skowiec ,A. Modak , N. Mohammadi , A.P. Mohanty , B. Mohanty , M. Mohisin Khan , Z. Moravcova ,C. Mordasini , D.A. Moreira De Godoy , L.A.P. Moreno , I. Morozov , A. Morsch ,T. Mrnjavac , V. Muccifora , E. Mudnic , D. Mühlheim , S. Muhuri , J.D. Mulligan ,A. Mulliri , M.G. Munhoz , R.H. Munzer , H. Murakami , S. Murray , L. Musa ,17oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE CollaborationJ. Musinsky , C.J. Myers , J.W. Myrcha , B. Naik , R. Nair , B.K. Nandi , R. Nania ,E. Nappi , M.U. Naru , A.F. Nassirpour , C. Nattrass , S. Nazarenko , A. Neagu , L. Nellen ,S.V. Nesbo , G. Neskovic , D. Nesterov , B.S. Nielsen , S. Nikolaev , S. Nikulin , V. Nikulin ,F. Noferini , S. Noh , P. Nomokonov , J. Norman , N. Novitzky , P. Nowakowski ,A. Nyanin , J. Nystrand , M. Ogino , A. Ohlson , J. Oleniacz , A.C. Oliveira Da Silva ,M.H. Oliver , A. Onnerstad , C. Oppedisano , A. Ortiz Velasquez , T. Osako , A. Oskarsson ,J. Otwinowski , K. Oyama , Y. Pachmayer , S. Padhan , D. Pagano , G. Pai´c ,A. Palasciano , J. Pan , S. Panebianco , P. Pareek , J. Park , J.E. Parkkila , S. Parmar ,S.P. Pathak , B. Paul , J. Pazzini , H. Pei , T. Peitzmann , X. Peng , L.G. Pereira , H. Pereira DaCosta , D. Peresunko , G.M. Perez , S. Perrin , Y. Pestov , V. Petráˇcek , M. Petrovici ,R.P. Pezzi , S. Piano , M. Pikna , P. Pillot , O. Pinazza , , L. Pinsky , C. Pinto , S. Pisano ,M. Płosko´n , M. Planinic , F. Pliquett , M.G. Poghosyan , B. Polichtchouk , N. Poljak ,A. Pop , S. Porteboeuf-Houssais , J. Porter , V. Pozdniakov , S.K. Prasad , R. Preghenella ,F. Prino , C.A. Pruneau , I. Pshenichnov , M. Puccio , S. Qiu , L. Quaglia , R.E. Quishpe ,S. Ragoni , A. Rakotozafindrabe , L. Ramello , F. Rami , S.A.R. Ramirez , A.G.T. Ramos ,R. Raniwala , S. Raniwala , S.S. Räsänen , R. Rath , I. Ravasenga , K.F. Read , ,A.R. Redelbach , K. Redlich V , , A. Rehman , P. Reichelt , F. Reidt , R. Renfordt ,Z. Rescakova , K. Reygers , A. Riabov , V. Riabov , T. Richert , , M. Richter , P. Riedler ,W. Riegler , F. Riggi , C. Ristea , S.P. Rode , M. Rodríguez Cahuantzi , K. Røed , R. Rogalev ,E. Rogochaya , T.S. Rogoschinski , D. Rohr , D. Röhrich , P.F. Rojas , P.S. Rokita ,F. Ronchetti , A. Rosano , , E.D. Rosas , A. Rossi , A. Rotondi , A. Roy , P. Roy ,N. Rubini , O.V. Rueda , R. Rui , B. Rumyantsev , A. Rustamov , E. Ryabinkin , Y. Ryabov ,A. Rybicki , H. Rytkonen , W. Rzesa , O.A.M. Saarimaki , R. Sadek , S. Sadovsky ,J. Saetre , K. Šafaˇrík , S.K. Saha , S. Saha , B. Sahoo , P. Sahoo , R. Sahoo , S. Sahoo ,D. Sahu , P.K. Sahu , J. Saini , S. Sakai , S. Sambyal , V. Samsonov I , , , D. Sarkar ,N. Sarkar , P. Sarma , V.M. Sarti , M.H.P. Sas , , J. Schambach , , H.S. Scheid ,C. Schiaua , R. Schicker , A. Schmah , C. Schmidt , H.R. Schmidt , M.O. Schmidt ,M. Schmidt , N.V. Schmidt , , A.R. Schmier , R. Schotter , J. Schukraft , Y. Schutz ,K. Schwarz , K. Schweda , G. Scioli , E. Scomparin , J.E. Seger , Y. Sekiguchi ,D. Sekihata , I. Selyuzhenkov , , S. Senyukov , J.J. Seo , D. Serebryakov , L. Šerkšnyt˙e ,A. Sevcenco , A. Shabanov , A. Shabetai , R. Shahoyan , W. Shaikh , A. Shangaraev ,A. Sharma , H. Sharma , M. Sharma , N. Sharma , S. Sharma , O. Sheibani ,A.I. Sheikh , K. Shigaki , M. Shimomura , S. Shirinkin , Q. Shou , Y. Sibiriak , S. Siddhanta ,T. Siemiarczuk , T.F.D. Silva , D. Silvermyr , G. Simatovic , G. Simonetti , B. Singh ,R. Singh , R. Singh , R. Singh , V.K. Singh , V. Singhal , T. Sinha , B. Sitar , M. Sitta ,T.B. Skaali , G. Skorodumovs , M. Slupecki , N. Smirnov , R.J.M. Snellings , C. Soncco ,J. Song , A. Songmoolnak , F. Soramel , S. Sorensen , I. Sputowska , J. Stachel , I. Stan ,P.J. Steffanic , S.F. Stiefelmaier , D. Stocco , M.M. Storetvedt , C.P. Stylianidis ,A.A.P. Suaide , T. Sugitate , C. Suire , M. Suljic , R. Sultanov , M. Šumbera , V. Sumberia ,S. Sumowidagdo , S. Swain , A. Szabo , I. Szarka , U. Tabassam , S.F. Taghavi ,G. Taillepied , J. Takahashi , G.J. Tambave , S. Tang , , Z. Tang , M. Tarhini ,M.G. Tarzila , A. Tauro , G. Tejeda Muñoz , A. Telesca , L. Terlizzi , C. Terrevoli ,G. Tersimonov , S. Thakur , D. Thomas , R. Tieulent , A. Tikhonov , A.R. Timmins ,M. Tkacik , A. Toia , N. Topilskaya , M. Toppi , F. Torales-Acosta , S.R. Torres ,A. Trifiró , , S. Tripathy , T. Tripathy , S. Trogolo , G. Trombetta , L. Tropp , V. Trubnikov ,W.H. Trzaska , T.P. Trzcinski , B.A. Trzeciak , A. Tumkin , R. Turrisi , T.S. Tveter ,K. Ullaland , E.N. Umaka , A. Uras , M. Urioni , G.L. Usai , M. Vala , N. Valle ,S. Vallero , N. van der Kolk , L.V.R. van Doremalen , M. van Leeuwen , P. Vande Vyvre ,D. Varga , Z. Varga , M. Varga-Kofarago , A. Vargas , M. Vasileiou , A. Vasiliev , O. VázquezDoce , V. Vechernin , E. Vercellin , S. Vergara Limón , L. Vermunt , R. Vértesi ,18oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE CollaborationM. Verweij , L. Vickovic , Z. Vilakazi , O. Villalobos Baillie , G. Vino , A. Vinogradov ,T. Virgili , V. Vislavicius , A. Vodopyanov , B. Volkel , M.A. Völkl , K. Voloshin ,S.A. Voloshin , G. Volpe , B. von Haller , I. Vorobyev , D. Voscek , J. Vrláková ,B. Wagner , M. Weber , A. Wegrzynek , S.C. Wenzel , J.P. Wessels , J. Wiechula , J. Wikne ,G. Wilk , J. Wilkinson , G.A. Willems , E. Willsher , B. Windelband , M. Winn ,W.E. Witt , J.R. Wright , Y. Wu , R. Xu , S. Yalcin , Y. Yamaguchi , K. Yamakawa ,S. Yang , S. Yano , , Z. Yin , H. Yokoyama , I.-K. Yoo , J.H. Yoon , S. Yuan , A. Yuncu ,V. Yurchenko , V. Zaccolo , A. Zaman , C. Zampolli , H.J.C. Zanoli , N. Zardoshti ,A. Zarochentsev , P. Závada , N. Zaviyalov , H. Zbroszczyk , M. Zhalov , S. Zhang ,X. Zhang , Y. Zhang , V. Zherebchevskii , Y. Zhi , D. Zhou , Y. Zhou , J. Zhu , , Y. Zhu ,A. Zichichi , G. Zinovjev , N. Zurlo Affiliation Notes I Deceased II Also at: Italian National Agency for New Technologies, Energy and Sustainable EconomicDevelopment (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 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan,Armenia AGH University of Science and Technology, Cracow, Poland Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev,Ukraine Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science(CAPSS), Kolkata, India Budker Institute for Nuclear Physics, Novosibirsk, Russia California Polytechnic State University, San Luis Obispo, California, United States Central China Normal University, Wuhan, China Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico Chicago State University, Chicago, Illinois, United States China Institute of Atomic Energy, Beijing, China Chungbuk National University, Cheongju, Republic of Korea Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava,Slovakia COMSATS University Islamabad, Islamabad, Pakistan Creighton University, Omaha, Nebraska, United States Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics, Pusan National University, Pusan, Republic of Korea Department of Physics, Sejong University, Seoul, Republic of Korea Department of Physics, University of California, Berkeley, California, United States Department of Physics, University of Oslo, Oslo, Norway19oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE Collaboration Department of Physics and Technology, University of Bergen, Bergen, Norway Dipartimento di Fisica dell’Università ’La Sapienza’ and Sezione INFN, Rome, Italy Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Padova, Italy Dipartimento di Fisica e Nucleare e Teorica, Università di Pavia and Sezione INFN, Pavia, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell’Università and Gruppo Collegato INFN, Salerno, Italy Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy Dipartimento di Scienze e Innovazione Tecnologica dell’Università del Piemonte Orientale and INFNSezione di Torino, Alessandria, Italy Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy European Organization for Nuclear Research (CERN), Geneva, Switzerland Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University ofSplit, Split, Croatia Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen,Norway Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague,Prague, Czech Republic Faculty of Science, P.J. Šafárik University, Košice, Slovakia Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt,Germany Fudan University, Shanghai, China Gangneung-Wonju National University, Gangneung, Republic of Korea Gauhati University, Department of Physics, Guwahati, India Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn,Bonn, Germany Helsinki Institute of Physics (HIP), Helsinki, Finland High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico Hiroshima University, Hiroshima, Japan Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation (ZTT), Worms,Germany Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Indore, Indore, India Indonesian Institute of Sciences, Jakarta, Indonesia INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Sezione di Bari, Bari, Italy INFN, Sezione di Bologna, Bologna, Italy INFN, Sezione di Cagliari, Cagliari, Italy INFN, Sezione di Catania, Catania, Italy INFN, Sezione di Padova, Padova, Italy INFN, Sezione di Roma, Rome, Italy INFN, Sezione di Torino, Turin, Italy INFN, Sezione di Trieste, Trieste, Italy Inha University, Incheon, Republic of Korea Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht,20oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE CollaborationNetherlands Institute for Nuclear Research, Academy of Sciences, Moscow, Russia Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Institute of Space Science (ISS), Bucharest, Romania Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico iThemba LABS, National Research Foundation, Somerset West, South Africa Jeonbuk National University, Jeonju, Republic of Korea Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik undMathematik, Frankfurt, Germany Joint Institute for Nuclear Research (JINR), Dubna, Russia Korea Institute of Science and Technology Information, Daejeon, Republic of Korea KTO Karatay University, Konya, Turkey Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3,Grenoble, France Lawrence Berkeley National Laboratory, Berkeley, California, United States Lund University Department of Physics, Division of Particle Physics, Lund, Sweden Moscow Institute for Physics and Technology, Moscow, Russia Nagasaki Institute of Applied Science, Nagasaki, Japan Nara Women’s University (NWU), Nara, Japan National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens,Greece National Centre for Nuclear Research, Warsaw, Poland National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India National Nuclear Research Center, Baku, Azerbaijan National Research Centre Kurchatov Institute, Moscow, Russia Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Nikhef, National institute for subatomic physics, Amsterdam, Netherlands NRC Kurchatov Institute IHEP, Protvino, Russia NRC «Kurchatov»Institute - ITEP, Moscow, Russia NRNU Moscow Engineering Physics Institute, Moscow, Russia Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Institute of the Czech Academy of Sciences, ˇRež u Prahy, Czech Republic Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States Ohio State University, Columbus, Ohio, United States
Petersburg Nuclear Physics Institute, Gatchina, Russia
Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia
Physics Department, Panjab University, Chandigarh, India
Physics Department, University of Jammu, Jammu, India
Physics Department, University of Rajasthan, Jaipur, India
Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany
Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
Physik Department, Technische Universität München, Munich, Germany
Politecnico di Bari and Sezione INFN, Bari, Italy
Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für21oherent ρ photoproduction in ultra-peripheral Xe—Xe collisions ALICE CollaborationSchwerionenforschung GmbH, Darmstadt, Germany Rudjer Boškovi´c Institute, Zagreb, Croatia
Russian Federal Nuclear Center (VNIIEF), Sarov, Russia
Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India
School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom
Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru
St. Petersburg State University, St. Petersburg, Russia
Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria
SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France
Suranaree University of Technology, Nakhon Ratchasima, Thailand
Technical University of Košice, Košice, Slovakia
The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow,Poland
The University of Texas at Austin, Austin, Texas, United States
Universidad Autónoma de Sinaloa, Culiacán, Mexico
Universidade de São Paulo (USP), São Paulo, Brazil
Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil
Universidade Federal do ABC, Santo Andre, Brazil
University of Cape Town, Cape Town, South Africa
University of Houston, Houston, Texas, United States
University of Jyväskylä, Jyväskylä, Finland
University of Liverpool, Liverpool, United Kingdom
University of Science and Technology of China, Hefei, China
University of South-Eastern Norway, Tonsberg, Norway
University of Tennessee, Knoxville, Tennessee, United States
University of the Witwatersrand, Johannesburg, South Africa
University of Tokyo, Tokyo, Japan
University of Tsukuba, Tsukuba, Japan
Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France
Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon , Lyon, France
Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France
Université Paris-Saclay Centre d’Etudes de Saclay (CEA), IRFU, Départment de PhysiqueNucléaire (DPhN), Saclay, France
Università degli Studi di Foggia, Foggia, Italy
Università di Brescia and Sezione INFN, Brescia, Italy
Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India
Warsaw University of Technology, Warsaw, Poland
Wayne State University, Detroit, Michigan, United States
Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany
Wigner Research Centre for Physics, Budapest, Hungary
Yale University, New Haven, Connecticut, United States148