Coherent J/ψ photoproduction in peripheral heavy-ion collisions

O Bugnon

doi:10.1209/0295-5075/129/42001

Abstract

In 2015, the ALICE Collaboration reported the first measurement of an excess in the yield of at very low transverse momentum () in the forward rapidity region (2.5 < y < 4) in peripheral lead-lead (Pb-Pb) collisions at at the CERN LHC. The coherent photoproduction was proposed as the potential underlying physics mechanism. This is known to be the main production mechanism for low- production in ultra-peripheral collisions, which are dominated by electromagnetic interactions. However, the observation of a large effect in more central collisions that are dominated by the hadronic interactions was quite surprising. This article represents a proceeding contribution on the preliminary results from Pb-Pb collisions at shown in a poster during the National Congress of the French Physics Society in July 2019.

Highlights

In 2015, the ALICE Collaboration reported the first measurement of an excess in the yield of J/ψ at very low transverse momentum in the forward rapidity region (2.5 < y < 4) in peripheral lead-lead (Pb-Pb) collisions at sNN = 2.76 TeV at the CERN Large Hadron Collider (LHC)
In presence of a Quark-Gluon Plasma (QGP), a color-charge screening mechanism is expected to prevent the formation of the charmonium states
Effects related to the initial state of the partons in the nuclei but unrelated to the QGP formation, the cold nuclear matter effects, significantly affect the charmonium production in heavy-ion collisions and must be taken into account when studying the medium properties [4,5]

Summary

Introduction

The corresponding photonuclear cross section should be proportional to the square of the nuclear gluon distribution function (nP√DF) at a given value of Bjorkenx with x(±y) = (mJ/ψ/ sNN) exp(±y), mJ/ψ being the J/ψ mass and y the rapidity. The electromagnetic continuum process γγ → μ+μ− contributes to low-pt dimuons as the main background source in UPC which is subtracted using the fit to the dimuon invariant mass distribution.

Results

Conclusion