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

We present the first measurement at the LHC of exclusive J/ψ photoproduction off protons, in ultraperipheral 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, 2.5HERA. 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.