Title: Measurements of photon interactions in hadronic heavy-ion collisions at STAR

Speaker: Shuai Yang (BNL)

Abstract

Photon-photon and photonuclear interactions can be induced by the strong electromagnetic fields arising from relativistic heavy ions. These two types of interactions are conventionally studied in ultra-peripheral collisions (UPC). The ALICE collaboration has observed a significant excess of $J/\psi$ yields at low transverse momenta ($p_T$) in peripheral Pb+Pb collisions, which can be qualitatively explained by coherent photonuclear production mechanism. Such an explanation implies that photon-photon interactions would be also measurable and contribute to the $l^+l^-$ pair production in hadronic heavy-ion collisions. Since the nuclei break up in peripheral heavy-ion collisions unlike in the UPCs, it is non-trivial to incorporate the coherence condition for the aforementioned photon interactions in such collisions. Measurements of $J/\psi$ and $e^+e^-$ pair productions at very low $p_T$ for different collision systems and energies, discussed in this talk, are thus important to verify and further understand photon interactions and their possible impacts on emerging phenomena in hadronic heavy-ion collisions.