# Coherent J/ψ photoproduction in ultra-peripheral PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV with the CMS experiment

## Abstract

The cross section for coherent J/psi 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 $\sqrt{s_{NN}} = 2.76 TeV$. The analysis is based on a data sample corresponding to an integrated luminosity of 159 inverse microbarns, collected during the 2011 PbPb run. The $J/ \psi$ mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is $d \sigma {^{coh}_{X_n 0_n}} / dy(J/ \psi) = 0.36 ± 0.04 (stat) ± 0.04 (syst) mb$ in the rapidity interval $1.8 < |y| < 2.3$. Using a model for the relative rate of coherent photoproduction processes, this X[z,n,z] measurement gives a total coherent photoproduction cross section of $d \sigma^{coh} / dy(J/ \psi) = 1.82 ± 0.22 (stat) ± 0.20 (syst) ± 0.19 (theo) mb$. The data strongly disfavour the impulse approximation model prediction, indicating that nuclear effects are needed to describe coherent $J/ \psi$ photoproduction in $\gamma + Pb$ interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing.

## Abstract

The cross section for coherent J/psi 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 $\sqrt{s_{NN}} = 2.76 TeV$. The analysis is based on a data sample corresponding to an integrated luminosity of 159 inverse microbarns, collected during the 2011 PbPb run. The $J/ \psi$ mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is $d \sigma {^{coh}_{X_n 0_n}} / dy(J/ \psi) = 0.36 ± 0.04 (stat) ± 0.04 (syst) mb$ in the rapidity interval $1.8 < |y| < 2.3$. Using a model for the relative rate of coherent photoproduction processes, this X[z,n,z] measurement gives a total coherent photoproduction cross section of $d \sigma^{coh} / dy(J/ \psi) = 1.82 ± 0.22 (stat) ± 0.20 (syst) ± 0.19 (theo) mb$. The data strongly disfavour the impulse approximation model prediction, indicating that nuclear effects are needed to describe coherent $J/ \psi$ photoproduction in $\gamma + Pb$ interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing.

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