Coherent J/ ψ Photoproduction in Ultra-Peripheral PbPb Collisions at √s[subscript NN = 2.76 TeV 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[subscript n]0[subscript n], is measured with the CMS experiment in ultra-peripheral PbPb collisions at √s[subscript NN = 2.76 TeV....
| Summary: | 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[subscript n]0[subscript n], is measured with the CMS experiment in ultra-peripheral PbPb collisions at √s[subscript NN = 2.76 TeV. 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 [subscript Xn0n][superscript 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 n 0 n measurement gives a total coherent photoproduction cross section of dσ[superscript 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. |
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