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 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.
Abelev, B., J. Adam, D. Adamova, M. M. Aggarwal, G. Aglieri Rinella, M. Agnello, A. Agostinelli, N. Agrawal, and Daniel Tapia Takaki. "Exclusive J/ψ Photoproduction off Protons in Ultraperipheral P-Pb Collisions at √SNN=5.02 TeV." Exclusive J/ψ Photoproduction off Protons in Ultraperipheral P-Pb Collisions at √sNN = 5.02 TeV 113.23 (2014). http://dx.doi.org/10.1103/PhysRevLett.113.232504
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