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

Abelev, B. and Adam, J. and Adamová, D. and Aggarwal, M. M. and Aglieri, Rinella G. and Agnello, M. and Agostinelli, A. and Agrawal, N. and Ahammed, Z. and Ahmad, N. and Ahmed, I. and Ahn, S. U. and Ahn, S. A. and Aimo, I. and Aiola, S. and Ajaz, M. and Akindinov, A. and Alam, S. N. and Aleksandrov, D. and Alessandro, B. and Alexandre, D. and Alici, A. and Alkin, A. and Alme, J. and Alt, T. and Altinpinar, S. and Altsybeev, I. and Alves, Garcia Prado C. and Andrei, C. and Andronic, A. and Barnaföldi, Gergely Gábor and Bencédi, Gyula and Berényi, Dániel and Boldizsár, László and Dénes, Ervin and Hamar, Gergő and Kiss, Gábor and Varga-Kőfaragó, Mónika and Lévai, Péter and Molnár, Levente and Oláh, László and Pochybova, Sona (2014) Exclusive J/ ψ photoproduction off protons in ultra-peripheral p–Pb collisions at √sNN =5.02 TeV. PHYSICAL REVIEW LETTERS, 113 (23). ISSN 0031-9007

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Abstract

We present the first measurement at the LHC of exclusive J/ ψ photoproduction off protons, in ultra- peripheral proton-lead collisions at √sNN = 5.02 TeV. Events are selected with a dimuon pair pro- duced 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 sectionsσ ( γ + p → J/ ψ + p) are 33.2 ± 2.2 (stat) ± 3.2 (syst) ± 0.7 (theo) nb in p–Pb and 284 ± 36 (stat) +27 −32 (syst) ± 26 (theo) nb in Pb–p collisions. We measure this process up to about 700 GeV in theγp 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 J/ ψ photoproduction cross section in γp energies from about 20 to 700 GeV, or equivalently, from Bjorken-x between ∼ 2 × 10−2 to ∼ 2 × 10−5, thus indicating no significant change in the gluon density behaviour of the proton between HERA and LHC energies.

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