Energy dependence of exclusive J/ψ photoproduction off protons in ultra-peripheral p–Pb collisions at √s_NN = 5.02 TeV

The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of J/ψ vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per nucleon pair √s_NN = 5.02 TeV. The e⁺e^- and μ⁺μ^- decay channels are used to measure the cross section as a function of the rapidity of the J/ψ in the range - 2.5 < y < 2.7, corresponding to an energy in the γp centre-of-mass in the interval 40 < W _γp < 550 GeV. The measurements, which are consistent with a power law dependence of the exclusive J/ψ photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements.