Coherent J/y photoproduction in ultra-peripheral Pb-Pb collisions at √sNN= 2.76 TeV

The ALICE collaboration has made the first measurement at the LHC of J/ψ photoproduction in ultra-peripheral Pb-Pb-collisions at √ s NN = 2.76 TeV.The J/ψ is identified via its dimuon decay in the forward rapidity region with the muon spectrometer for events where the hadronic activity is required to be minimal.The analysis is based on an event sample corresponding to an integrated luminosity of about 55 µb -1 .The cross section for coherent J/ψ production in the rapidity interval -3.6 < y < -2.6 is measured to be dσ coh J/ψ /dy = 1.00 ± 0.18(stat) +0.24 -0.26 (syst) mb.The result is compared to theoretical models for coherent J/ψ production and found to be in good agreement with those models which include nuclear gluon shadowing.* See Appendix A for the list of collaboration members J/ψ photoproduction in ultra-peripheral Pb-Pb collisions ALICE Collaboration Two-photon and photonuclear interactions at unprecedentedly high energies can be studied in ultraperipheral heavy-ion collisions (UPC) at the LHC.In such collisions the nuclei are separated by impact parameters larger than the sum of their radii and therefore hadronic interactions are strongly suppressed.The cross sections for photon induced reactions remain large because the strong electromagnetic field of the nucleus enhances the intensity of the virtual photon flux, which grows as Z 2 , where Z is the charge of the nucleus.The virtuality of the photons is restricted by the nuclear form factor to be of the order 1/R ≈ 30 MeV/c (R is the radius of the nucleus).The physics of ultra-peripheral collisions is reviewed in [1,2].Exclusive photoproduction of vector mesons, where a vector meson but no other particles are produced in the event, is of particular interest.Exclusive production of J/ψ in photon-proton interactions, γ + p → J/ψ + p, has been successfully modelled in perturbative QCD in terms of the exchange of two gluons with no net-colour transfer [3].Experimental data on this process from HERA have been used to constrain the proton gluon-distribution at low Bjorken-x [4].Exclusive vector meson production in heavy-ion interactions is expected to probe the nuclear gluon-distribution [5], for which there is considerable uncertainty in the low-x region [6].A J/ψ produced at rapidity y is sensitive to the gluon distribution at x = (M J/ψ / √ s NN ) exp(±y) at hard scales Q 2 ≈ M 2 J/ψ /4 [7].The two-fold ambiguity in x is due to the fact that either nucleus can serve as photon emitter or photon target.At the forward rapidities studied here (-3.6 < y < -2.6), the relevant values of x are ≈ 10 -2 and ≈ 10 -5 , respectively.Exclusive ρ 0 [8] and J/ψ [9] production have been studied in Au-Au collisions at RHIC.The ρ 0 is too light to provide a hard scale, and the J/ψ analysis suffered from very low statistics, so no conclusions concerning nuclear shadowing were made from these studies.Exclusive J/ψ production has also been studied by the CDF collaboration in proton-antiproton collisions at the Tevatron [10].The availability of such measurements has led to an increase in interest in ultra-peripheral collisions, stimulating several new model calculations.In this Letter, the first LHC results on exclusive photoproduction of J/ψ vector mesons are presented.J/ψ mesons produced in Pb-Pb collisions at √ s NN = 2.76 TeV have been measured at forward rapidities through their dimuon decay.Exclusive photoproduction can be either coherent, where the photon couples coherently to all nucleons, or incoherent, where the photon couples to a single nucleon.Coherent production is characterized by low vector meson transverse momentum ( p T ≃ 60 MeV/c) and the target nucleus normally does not break up.Incoherent production, corresponding to quasi-elastic scattering off a single nucleon, is characterized by a somewhat higher transverse momentum ( p T ≃ 500 MeV/c) and the target nucleus normally breaks up, but except for single nucleons or nuclear fragments in the very forward region no other particles are produced.This analysis is focussed on coherently produced J/ψ mesons.The experimental definition of coherent production, which must take into consideration also the finite detector resolution, is here p T < 0.3 GeV/c.The measured cross section is compared to model predictions [5,11,12,13,14].The ALICE detector consists of a central barrel placed inside a large solenoid magnet (B = 0.5 T), covering the pseudorapidity region |η| < 0.9 [15], and a muon spectrometer covering the range -4.0< η <-2.5.The spectrometer consists of a ten interaction length (λ I ) thick absorber filtering the muons, in front of five tracking stations containing two planes of cathode pad multi-wire proportional chambers (MWPC) each, with the third station placed inside a dipole magnet with a Bdl = 3 Tm integrated field.The forward muon spectrometer includes a triggering system, used to select muon candidates with a transverse momentum larger than a given programmable threshold.It has four planes of resistive plate chambers (RPC) downstream of a 1.2 m thick iron wall (7.2 λ I ), which absorbs secondary punch- through hadrons from the front absorber and low momentum muons from π and K weak decays.This analysis uses the VZERO counters for triggering and event selection.These consist of two arrays of 32 scintillator tiles each, covering the range 2.8< η <5.1 (VZERO-A, on the opposite side of the muon arm) and -3.7<η <-1.7 (VZERO-C) and positioned at z = 329 cm and z = -87 cm from the interaction

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