Coherent J/ψ photoproduction in ultra-peripheral PbPb collisions at [formula omitted] with the CMS experiment.

The cross section for coherent J / ψ photoproduction accompanied by at least one neutron on one side of the interaction point and no neutron activity on the other side, X n 0 n , is measured with the CMS experiment in ultra-peripheral PbPb collisions at s N N = 2.76 TeV . The analysis is based on a data sample corresponding to an integrated luminosity of 159 μb − 1 , collected during the 2011 PbPb run. The J / ψ mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is d σ X n 0 n coh / d y ( J / ψ ) = 0.36 ± 0.04 (stat) ± 0.04 (syst)  mb in the rapidity interval 1.8 < | y | < 2.3 . Using a model for the relative rate of coherent photoproduction processes, this X n 0 n measurement gives a total coherent photoproduction cross section of d σ coh / d y ( J / ψ ) = 1.82 ± 0.22 (stat) ± 0.20 (syst) ± 0.19 (theo)  mb . The data strongly disfavor the impulse approximation model prediction, indicating that nuclear effects are needed to describe coherent J / ψ photoproduction in γ + Pb interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing. [ABSTRACT FROM AUTHOR]