Small $x$ gluon shadowing from LHC data on coherent $J/\psi$ photoproduction

Abstract

The cross section for coherent $J/\psi$ photoproduction in Pb-Pb collisions is the sum of two contributions, one from low-, the other from high-energy photon-nucleus interactions. A novel method to disentangle both contributions allowing one to extract the coherent photo-nuclear cross section for coherent $J/\psi$ production, ${\sigma }_{\gamma \mathrm{Pb}}$, is presented. The utility of the method is demonstrated using measurements from peripheral and ultra-peripheral Pb-Pb collisions at the LHC. Applying the proposed method to the available data it is possible to obtain ${\sigma }_{\gamma \mathrm{Pb}}$ up to a photon-lead centre-of-mass energy of 470 GeV, which corresponds to $x$ of 4.4x10${}^{-5}$. To illustrate a possible use of the extracted photo-nuclear cross sections, the corresponding nuclear suppression factors are computed and they are compared to predictions of gluon shadowing calculated in the leading-twist approximation approach.

Extended version including more details. Results unchanged. Accepted for publication by PRC

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• Coherent J/psi photoproduction in ultra-peripheral PbPb collisions at sqrt(s[NN]) = 2.76 TeV with the CMS experiment
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• Heavy Mesons Photoproduction in Peripheral AA Collisions
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