Exclusive J/$ψ$ photoproduction off protons in ultra-peripheral p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV

Paper

Abstract

We present the first measurement at the LHC of exclusive J/

photoproduction off protons, in ultra-peripheral proton-lead collisions at

TeV. Events are selected with a dimuon pair produced either in the rapidity interval, in the laboratory frame,

(p-Pb) or

(Pb--p), and no other particles observed in the ALICE acceptance. The measured cross sections

are 33.2

2.2 (stat)

3.1 (syst)

0.7 (theo) nb in p-Pb and 284

36 (stat)

(syst)

26 (theo) nb in Pb-p collisions. We measure this process up to about 700 GeV in the

centre-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

photoproduction cross section in

energies from about 20 to 700 GeV, or equivalently, from Bjorken-

between

, thus indicating no significant change in the gluon density behaviour of the proton between HERA and LHC energies.