Abstract
The coherent photoproduction of mathrm{J}/psi and {uppsi '} mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy sqrt{s_{mathrm {NN}}}~=~5.02 TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The mathrm{J}/psi is reconstructed using the dilepton (l^{+} l^{-}) and proton–antiproton decay channels, while for the {uppsi '} the dilepton and the l^{+} l^{-} pi ^{+} pi ^{-} decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 mu {mathrm{b}}^{-1}. The results are compared with theoretical models for coherent mathrm{J}/psi and {uppsi '} photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around 6times 10^{-4}, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent mathrm{J}/psi cross section including ALICE measurements at forward rapidity. The ratio of {uppsi '} to mathrm{J}/psi coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.
Highlights
Photonuclear reactions can be studied in ultra-peripheral collisions (UPCs) of heavy ions where the two nuclei pass by with an impact parameter larger than the sum of their radii
We present the first measurement of the coherent J/ψ and ψ photoproduction cross sections
Both the leading twist approximation (LTA) model and the EPS09 curve, corresponding to the EPS09 LO central set, are found to be in a good agreement with the J/ψ and ψ cross sections measured at midrapidity
Summary
Photonuclear reactions can be studied in ultra-peripheral collisions (UPCs) of heavy ions where the two nuclei pass by with an impact parameter larger than the sum of their radii. The extraction of the nuclear suppression factor in UPC measurements is complicated by the fact that the measured vector meson cross section in UPCs is expressed as a sum of two contributions since either of the colliding ions can serve as a photon source. The gluon shadowing factor Rg(x ∼ 10−2) ∼ 0.8 was obtained under assumption that the contribution from high photon-nucleus energies, i.e. low Bjorken x ∼ 10−5, can be neglected in the measured cross sections. UPCs at The J/ψ photoproduction cross section in this measurement is sensitviivoeutsomx i∈dr(a0p.i3d,it1y.4m)×ea1su0r−e3m, aenfatcatto√r 2ssNmNal=ler than 2.76 in the preTeV [24] This data sam√ple is Pb sample at sNN approximately 10 = 2.76 TeV used times larger than Pb– for the ALICE results reported in Refs. Shadowing models are based on a parametrization of previously available data, the leading twist approximation and several variations of the color dipole approach
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