ALICE investigates photon-induced processes in Pb–Pb collisions with nuclear overlap

About

Photon-photon and photonuclear reactions are induced by the strong electromagnetic field generated by ultra-relativistic heavy-ion collisions. These processes have been extensively stud- ied in ultra-peripheral collisions, in which the impact parameter is larger than twice the nuclear radius. In recent years, both the photoproduction of the J/$\psi$ vector meson and the production of dileptons via photon-photon interactions have been observed in A-A collisions with nuclear overlap. Photoproduced quarkonia can probe the nuclear gluon distributions at low Bjorken-$x$, while the continuum dilepton production could be used to further map the electromagnetic fields produced in heavy-ion collisions. These measurements are complementary to constrain the theory behind photon induced reactions in A-A collisions with nuclear overlap and the measured probes can possibly interact with the formed and fast-expanding quark-gluon plasma medium. In this seminar, measurements of coherent J/$\psi$ photoproduction cross sections in Pb-Pb collisions in the 40-90% centrality range, measured by ALICE at midrapidity in the dielectron channel will be presented, as well as final results on coherent J/$\psi$ photoproduction cross-sections at forward rapidity in the dimuon decay channel in the 30-90% centrality range. Finally, the measurement of an excess with respect to hadronic production, in the midrapidity dielectron yield at low mass and $p_T$, in the centrality interval 50-90% will be presented. Results will be compared with available models.

ALICE Collaboration, arXiv:2204.10684

ALICE Collaboration, arXiv:2204.11732

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