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.

Abstract

Measurements of quarkonium production are an important tool to study the properties of the Quark-Gluon Plasma (QGP) formed in relativistic heavy-ion collisions. Quarkonium suppression due to the color-screening effect was proposed as a direct evidence of the QGP formation. However, other effects, such as cold nuclear matter effects and regeneration, add additional complications to the interpretation of the observed suppression. Different quarkonium states with different binding energies are expected to dissociate at different temperatures, and therefore measurement of this “sequential melting” can help constrain the temperature of the medium. In this seminar, I will present and discuss the latest measurements of quarkonium (J/psi and Upsilon) production in p+p, p+Au and Au+Au collisions at √sNN = 200 GeV with the STAR experiment.