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The cross section for coherent J/psi photoproduction accompanied by at leastone neutron on one side of the interaction point and no neutron activity on theother side, X[n]0[n], is measured with the CMS experiment in ultra-peripheralPbPb collisions at sqrt(s[NN]) = 2.76 TeV. The analysis is based on a datasample corresponding to an integrated luminosity of 159 inverse microbarns,collected during the 2011 PbPb run. The J/psi mesons are reconstructed in thedimuon decay channel, while neutrons are detected using zero degreecalorimeters. 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 sectionof dsigma[coh] / dy(J/psi) = 1.82 +/- 0.22 (stat) +/- 0.20 (syst) +/- 0.19(theo) mb. The data strongly disfavour the impulse approximation modelprediction, indicating that nuclear effects are needed to describe coherentJ/psi photoproduction in gamma + Pb interactions. The data are found to beconsistent with the leading twist approximation, which includes nuclear gluonshadowing.

Coherent J/psi photoproduction in ultra-peripheral PbPb collisions at sqrt(s[NN]) = 2.76 TeV with the CMS experiment