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Coherent J/psi and psi ' photoproduction at midrapidity in ultra-peripheral Pb-Pb collisions at root s(NN)=5.02 TeV

The coherent photoproduction of J/ψ and ψ′ mesons was measured in ultra-peripheral Pb-Pb collisions at a center-of-mass energy √sNN = 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 J/ψ is reconstructed using the dilepton (l+l−) and proton-antiproton decay channels, while for the ψ′, the dilepton and the l+l−π+π− decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 μb^{; ; ; ; ; -1}; ; ; ; ; . The results are compared with theoretical models for coherent J/ψ and ψ′ photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.65 at a Bjorken-xx of around 6×10^{; ; ; ; ; -4}; ; ; ; ; , such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent J/ψ cross section including ALICE measurements at forward rapidity. The ratio of ψ′ to J/ψ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.

J/psi(3100): photoproduction ; psi(3685): photoproduction ; heavy ion: scattering ; gluon: shadowing ; coherence ; dilepton ; rapidity ; ALICE ; rapidity dependence ; parametrization ; charmonium ; experimental results ; 5020 GeV-cms/nucleon ; psi(3836): photo ; production ; suppression ; anti-p p ; 5020 GeV-cms/nucleon

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