Coherent J/ψ photoproduction in ultra-peripheral PbPb collisions at sNN=2.76TeV 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=2.76TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 159μb⁻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_Xn0n ^coh/dy(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_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.