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

Abstract

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) ±27−32(syst) ± 26 (theor) nb in Pb-p collisions. We measure this process up to about 700 GeV in the γ p 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 γ p 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