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 pffiffiffiffiffiffiffi ultraperipheral proton-lead collisions at sNN 1⁄4 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 σðγ þ p → J=ψ þ pÞ are 33.2 ` 2.2ðstatÞ ` 3.2ðsystÞ ` 0.7ðtheorÞ nb in p-Pb and 284 ` 36ðstatÞþ27ðsystÞ ` −32 26ðtheorÞ 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.