Exclusive J/ψ photoproduction off protons in ultra-peripheral p-Pb collisions at √sNN = 5.02 TeV

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5.02 TeV
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We present the first measurement at the LHC of exclusive J/ψ photoproduction off protons, in ultra-peripheral proton-lead collisions at √sNN=5.02 TeV. Events are selected with a dimuon pair produced either in the rapidity interval, in the laboratory frame, 2.5<y<4 (p-Pb) or 3.6<y<2.6 (Pb--p), and no other particles observed in the ALICE acceptance. The measured cross sections σ(γ+pJ/ψ+p) are 33.2 ± 2.2 (stat) ± 3.1 (syst) ± 0.7 (theo) nb in p-Pb and 284 ± 36 (stat) +2732 (syst) ± 26 (theo) nb in Pb-p collisions. We measure this process up to about 700 GeV in the γp centre-of-mass, which is a factor of two larger than the highest energy studied at HERA. The data are consistent with a power law dependence of the J/ψ photoproduction cross section in γp energies from about 20 to 700 GeV, or equivalently, from Bjorken-x between 2×102 to 2×105, thus indicating no significant change in the gluon density behaviour of the proton between HERA and LHC energies.

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