# Measurement of an excess in the yield of J/$\psi$ at very low-$p_{\rm T}$ in Pb--Pb collisions with the ALICE detector

Abstract : We report on the measurement of J/$\psi$ production at very low transverse momentum ($p_{\rm T} <$ 300 MeV/$c$) in Pb--Pb collisions performed with the ALICE detector at the LHC. We find an excess in the yield of J/$\psi$ with respect to expectations from hadronic production. Coherent photo-production of J/$\psi$ is proposed as a plausible origin of this excess. We show the nuclear modification factor of very low-$p_{\rm T}$ J/$\psi$ as a function of centrality. Then we measure the J/$\psi$ coherent photoproduction cross section in peripheral events assuming that it is the mechanism at the origin of the measured excess. It's worth noting that the observation of J/$\psi$ coherent photoproduction in Pb--Pb collisions at impact parameters smaller than twice the nuclear radius has never been observed so far and would open new theoretical challenges.
Document type :
Conference papers
Domain :

http://hal.in2p3.fr/in2p3-01223191
Contributor : Sabine Starita <>
Submitted on : Monday, November 2, 2015 - 11:14:23 AM
Last modification on : Tuesday, June 30, 2020 - 10:00:54 AM

### Identifiers

• HAL Id : in2p3-01223191, version 1
• ARXIV : 1510.08315

### Citation

L. Massacrier. Measurement of an excess in the yield of J/$\psi$ at very low-$p_{\rm T}$ in Pb--Pb collisions with the ALICE detector. EDS Blois Conference, Jun 2015, Borgo, France. ⟨in2p3-01223191⟩

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