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| Format: | Preprint |
| Published: |
2024
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2406.12630 |
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Table of Contents:
- 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, $p_{\rm T}$, 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 $p_{\rm T} = 0$. 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 $\sqrt{s_{NN}}=$ 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.