Coherent J/ψ photoproduction in ultra-peripheral PbPb collisions at sqrt SNN = 2.76 TeV with the CMS experiment

Abstract

The cross section for coherent photo production accompanied by at least one neutron on one side of the interaction point and no neutron activity on the other side, , is measured with the CMS experiment in ultra-peripheral PbPb collisions at . The analysis is based on a data sample corresponding to an integrated luminosity of 159, collected during the 2011 PbPb run. The mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is in the rapidity interval . Using a model for the relative rate of coherent photoproduction processes, this measurement gives a total coherent photoproduction cross section of . The data strongly disfavor the impulse approximation model prediction, indicating that nuclear effects are needed to describe coherent photo production in interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing.