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

TL;DR

This paper reports the first observation of an excess in low transverse momentum J/ψ production in peripheral Pb-Pb collisions at the LHC, suggesting possible coherent photoproduction mechanisms in hadronic interactions.

Contribution

It provides the first measurement indicating a significant excess in J/ψ yield at very low pT in peripheral heavy-ion collisions, proposing coherent photoproduction as the underlying cause.

Findings

Nuclear modification factor of J/ψ reaches about 7 in certain centrality classes.

Observation of excess J/ψ yield at very low pT in peripheral collisions.

Implication of coherent photoproduction in hadronic Pb-Pb collisions.

Abstract

We report on the first measurement of an excess in the yield of J/$\psi$ at very low transverse momentum ($p_{\rm T}< 0.3$ GeV/$c$) in peripheral hadronic Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 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 $p_{\rm T}$ 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.