Coherent J/ψ photoproduction in ultra-peripheral PbPb collisions at √(s_NN)=2.76 TeV 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_n0_n, is measured with the CMS experiment in ultra-peripheral PbPb collisions at √(s_NN)= 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 dσ_XnOn^coh)/dy(J/ψ) = 0.36 ± 0.04 (stat)± 0.04 (syst) mb in the rapidity interval 1.8 < absolute value(y) < 2.3. Using a model for the relative rate of coherent photoproduction processes, this X_n0_n measurement gives a total coherent photoproduction cross section of dσ^coh/dy(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. (authors)