  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 dsigma[coh,X[n]0[n]] / dy(J/psi) =
0.36 +/- 0.04 (stat) +/- 0.04 (syst) mb in the rapidity interval 1.8 < abs(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 dsigma[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.
