Speaker
Description
Photon-photon reactions and the production of the 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. Such quarkonium measurements can help to constrain the nuclear gluon distributions at low Bjorken-x and high energy, while the continuum dilepton production 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. Furthermore, the produced quarkonium is expected to keep the polarization of the incoming photon due to s-channel helicity conservation. Thus, in order to confirm the photoproduction origin of the very low-pT J/ψ yield excess, polarization measurement is an important observable. 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 this presentation, 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 sqrt(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 sqrt(sNN) = 5.02 TeV is presented. The results are compared with available theoretical models.
