Photoproduction of J/$\psi$ and dileptons in Pb-Pb collisions with nuclear overlap

TL;DR

This paper reports new measurements of photoproduced J/ψ mesons and dileptons in Pb-Pb collisions with nuclear overlap, providing insights into photon-induced reactions, nuclear gluon distributions, and quark-gluon plasma interactions.

Contribution

It presents the first polarization results of coherently photoproduced J/ψ and detailed cross section measurements at both mid and forward rapidities in heavy-ion collisions with nuclear overlap.

Findings

Observation of J/ψ polarization consistent with photon polarization

Measurement of J/ψ and dilepton yields exceeding hadronic expectations

Comparison with theoretical models to understand photon-induced processes

Abstract

Photon-photon reactions and the production of J/$\psi$ 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/$\psi$ 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/$\psi$ yield excess at very low transverse momentum, $p_{\rm T}$, can be confirmed by the measurement of the J/$\psi$ polarization. The ALICE detector can perform quarkonium production measurements at both mid ($|y|<0.9$) and forward ($2.5<y<4$) rapidities down to $p_{\rm T} = 0$. In the following, the new ALICE measurements of the J/$\psi$ $y$-differential cross section and the first polarization results of coherently photoproduced J/$\psi$ via the dimuon decay channel at forward rapidity in Pb-Pb collisions at $\sqrt{s_{NN}}=$ 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 $p_{\rm T}$, at midrapidity in Pb-Pb collisions at $\sqrt{s_{NN}}=$ 5.02~TeV, is presented. The results are compared with available theoretical models.