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=5.02 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 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.