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 n 0 n , is measured with the CMS experiment in ultra-peripheral PbPb collisions at √ s NN = 2.76 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 159 µb −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 coh X n 0 n /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 coh /dy(J/ψ) = 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/ψ photoproduction in γ + Pb interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing. IntroductionPhoton-induced reactions are dominant in Ultra-Peripheral Collisions (UPC) of heavy ions, which involve electromagnetic interactions at large impact parameters of the colliding nuclei. Because of the extremely high photon flux in ultra-peripheral heavy-ion collisions which is proportional to Z 2 , where Z is the charge of the nucleus, photon-nucleus collisions at the LHC are abundant [1][2][3]. Furthermore, in UPCs the LHC can reach unprecedented photon-lead and photon-proton center-of-mass energies.Vector meson photoproduction in UPCs has received recent interest [3]. Exclusive J/ψ photoproduction off protons is defined by the reaction γ + p → J/ψ + p, with the characteristic features that, apart from the vector meson in the final state, no other particles are produced and the vector meson has a mean transverse momentum significantly lower than in inclusive reactions. Another characteristic feature is that in exclusive photoproduction the quantum numbers of the final state can be studied unambiguously. The γ + p → J/ψ + p production process has been studied by H1 and ZEUS collaborations at the electron-proton collider HERA [4][5][6], by the CDF collaboration in proton-antiproton collisions at the Tevatron [7], and by the ALICE and LHCb collaborations at the LHC, in proton-lead [8] and proton-proton collisions [9], respectively. Since the cross section of photoproduced vector mesons such as J/ψ, ψ(2S), and Υ(nS), in leading order perturbative QCD, is proportional to the gluon density squared in the target [10,11], the study of such diffractive processes in high-energy collisions is expected to provide insights into the role played by gluons in hadronic matter. As an example, a J/ψ produced at rapidity y is sensitive to the gluon distribution at x = (M J/ψ / √ s)e ±y at hard scales Q 2 ∼ M 2 J/ψ /4, where M J/ψ is the J/ψ mass, The CMS detectorThe...

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The nuclear modification factors of and mesons are measured in collisions at a centre-of-mass energy per nucleon pair of . The analysis is based on and data samples collected by CMS at the LHC in 2015, corresponding to integrated luminosities of 464 and 28, respectively. The measurements are performed in the dimuon rapidity range of as a function of centrality, rapidity, and transverse momentum ( ) from in the most forward region and up to 50. Both prompt and nonprompt (coming from b hadron decays) mesons are observed to be increasingly suppressed with centrality, with a magnitude similar to the one observed at for the two meson components. No dependence on rapidity is observed for either prompt or nonprompt mesons. An indication of a lower prompt meson suppression at is seen with respect to that observed at intermediate . The prompt meson yield is found to be more suppressed than that of the prompt mesons in the entire range.

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