Charmonium and e+e- pair photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at √sNN = 2.76 Tev

Abstract

The ALICE Collaboration at the LHC has mea[1]sured the J/ψ and ψ[1] photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at √sNN = 2.76 TeV. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be mini[1]mal. The analysis is based on an event sample correspond[1]ing to an integrated luminosity of about 23 µb−1. The cross section for coherent and incoherent J/ψ production in the rapidity interval −0.9 <y< 0.9, are dσcoh J/ψ /dy = 2.38+0.34 −0.24(sta + sys) mb and dσinc J/ψ /dy = 0.98+0.19 −0.17(sta + sys) mb, respectively. The results are compared to theoret[1]ical models for J/ψ production and the coherent cross sec[1]tion is found to be in good agreement with those models in[1]corporating moderate nuclear gluon shadowing at Bjorken[1]x around 10−3, such as EPS09 parametrization. In addition the cross section for the process γγ → e+e− has been mea[1]sured and found to be in agreement with models implement[1]ing QED at leading order.