Speaker
Description
Ultrarelativistic heavy-ion collisions generate extremely strong electromagnetic fields that can induce photonuclear reactions. These processes have been extensively studied in ultraperipheral collisions (UPCs), where the impact parameter is larger than twice the nuclear radius. Recently, coherent J/$\psi$ photoproduction has also been observed in nucleus--nucleus (A--A) collisions with nuclear overlap, based on the measurement of an excess of J/$\psi$ production with respect to hadroproduction expectations at very low transverse momentum ($p_{\mathrm{T}}$). Such quarkonium measurements provide valuable insights into the nuclear gluon distribution at low Bjorken-$x$ and high energies. In addition, they offer a unique opportunity to investigate photon-induced reactions in A--A collisions with nuclear overlap, including possible interactions of the produced probes with the formed and rapidly expanding quark--gluon plasma.
To confirm the photoproduction origin of the very low-$p_{\mathrm{T}}$ J/$\psi$ yield excess, polarization measurements represent a golden observable. Due to $s$-channel helicity conservation, the produced quarkonium is expected to retain the polarization of the incoming photon. The ALICE experiment can measure inclusive and exclusive photo produced quarkonium down to zero transverse momentum, both at forward rapidity ($2.5 < y < 4$) and at midrapidity ($|y| < 0.9$).
In this contribution, Run~2 measurements of the rapidity($y$)-differential cross section and polarization of photoproduced J/$\psi$ mesons in peripheral Pb--Pb collisions at the LHC are presented. Both measurements are performed at forward rapidity in the dimuon decay channel. The results are discussed in the context of recent measurements of coherent J/$\psi$ photoproduction as a function of centrality at both midrapidity and forward rapidity. The current status and prospects of these studies in Run~3 are also discussed. Comparisons with available theoretical models are presented.
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