Photoproduction of the coherent $J/\psi$ accompanied by the forward neutron emission in ultra-peripheral PbPb collisions at 2.76~TeV

The cross section of coherent $J/\psi$ photoproduction accompanied by forward neutrons is measured in ultra-peripheral PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV with the CMS experiment at the LHC. $J/\psi$ mesons are reconstructed by their decay into muon pairs while the neutrons are measured by Zero Degree Calorimeters. The analysis is based on a data sample collected during the 2011 PbPb run, corresponding to an integrated luminosity of 159 $\mu$b$^{-1}$. The cross section for coherent $J/\psi$ photoproduction in the rapidity region 1.8~$<|y|<$~2.3, in association with at least one neutron, is measured to be $\frac{d\sigma^{coh}_{X_{n}0_{n}}}{dy}($J/\psi$) = $ 0.37 $\pm$ 0.04 (stat) $\pm$ 0.04 (syst)~mb. This result is found to be in agreement with models that include nuclear gluon shadowing.