We present the first measurement at the LHC of exclusive J/ψ photoproduction off protons, in ultraperipheral proton-lead collisions at sNN=5.02TeV. 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+p→J/ψ+p) are 33.2±2.2(stat)±3.2(syst)±0.7(theor)nb in p-Pb and 284±36(stat)-32+27(syst)±26(theor)nb in Pb-p collisions. We measure this process up to about 700 GeV in the γp center 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 scaling variable between ∼2×10-2 and ∼2×10-5, thus indicating no significant change in the gluon density behavior of the proton between HERA and LHC energies.

Original languageEnglish
Article number232504
JournalPhysical Review Letters
Issue number23
Publication statusPublished - 5 Dec 2014


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