Photoproduction of J/ψ and dileptons in Pb–Pb collisions with nuclear overlap

Abstract

Photon-photon reactions and the production of J/ψ meson through photonuclear reactions have been extensively studied in ultra-peripheral heavy-ion collisions, in which the impact parameter is larger than twice the nuclear radius. In recent years, coherently photoproduced J/ψ and dilepton production via photon-photon interactions have also been observed in nucleus-nucleus (A–A) collisions with nuclear overlap. The former can help to constrain the nuclear gluon distributions at low Bjorken-x and high energy, while the latter could be used to further map the electromagnetic fields produced in heavy-ion collisions. In addition, these measurements can shed light on the theory behind photon-induced reactions in A–A collisions with nuclear overlap, including possible interactions of the measured probes with the formed and fast expanding quark-gluon plasma.

Since the produced quarkonium is expected to keep the polarization of the incoming photon due to s-channel helicity conservation, the photoproduction origin of the J/ψ yield excess at very low transverse momentum, pT, can be confirmed by the measurement of the J/ψ polarization.

The ALICE detector can perform quarkonium production measurements at both mid (|y|<0.9) and forward (2.5<y<4) rapidities down to pT = 0. In the following, the new ALICE measurements of the J/ψ y-differential cross section and the first polarization results of coherently photoproduced J/ψ via the dimuon decay channel at forward rapidity in Pb–Pb collisions at √sNN = 5.02 TeV are reported. Additionally, the measurement of an excess with respect to expectations from hadronic production in the dielectron yield, at low mass and pT, at midrapidity in Pb–Pb collisions at √sNN = 5.02 TeV, is presented. The results are compared with available theoretical models.