Measurement of an excess in the yield of J/$\psi$ at very low $p_{\mathrm{T}}$ in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV

Abstract

We report on the first measurement of an excess in the yield of J/$\psi$ at very low transverse momentum ($p_{\mathrm{T}}<0.3$ GeV/$c$) in peripheral hadronic Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV, performed by ALICE at the CERN LHC. Remarkably, the measured nuclear modification factor of J/$\psi$ in the rapidity range $2.5<y<4$ reaches about 7 (2) in the $p_{\mathrm{T}}$ range 0-0.3 GeV/$c$ in the 70-90% (50-70%) centrality class. The J/$\psi$ production cross section associated with the observed excess is obtained under the hypothesis that coherent photoproduction of J/$\psi$ is the underlying physics mechanism. If confirmed, the observation of J/$\psi$ coherent photoproduction in Pb-Pb collisions at impact parameters smaller than twice the nuclear radius opens new theoretical and experimental challenges and opportunities. In particular, coherent photoproduction accompanying hadronic collisions may provide insight into the dynamics of photoproduction and nuclear reactions, as well as become a novel probe of the Quark-Gluon Plasma.

18 pages, 3 captioned figures, 1 table, authors from page 13, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/1918

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• Differential studies of inclusive J/$\psi$ and $\psi$(2S) production at forward rapidity in Pb-Pb collisions at $\sqrt{{\text{s}}_{{}_{\mathbf{NN}}}}$ = 2.76 TeV
• Rapidity and transverse-momentum dependence of the inclusive J/$\psi$ nuclear modification factor in p-Pb collisions at $\sqrt{{\text{s}}_{\mathbf{NN}}}=5.02$ TeV
• Inclusive, prompt and non-prompt J/$\psi$ production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV
• Measurement of J/psi production in pp collisions at sqrt(s)=2.76 TeV
• Photoproduction of $J/\psi$ mesons in peripheral and semi-central heavy ion collisions

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