Speaker
Description
In high-energy heavy-ion collisions, dileptons can be produced via photon-photon and photonuclear processes, induced by the strong electromagnetic field accompanying highly charged relativistic heavy ions. These two processes were conventionally and extensively studied in ultra-peripheral collisions (UPCs). Recently, a significant excess of $J/\psi$ yields at low transverse momenta ($p_T$) was observed by the ALICE and STAR collaborations in peripheral heavy-ion collisions, qualitatively explained by coherent photonuclear production mechanisms. If indeed coherent photonuclear production was the underlying mechanism for the observed $J/\psi$ excess, the energetic and high-density photon flux would be there and coherent photon-photon production would also contribute to the $e^{+}e^{-}$ pair production. Measurements of $e^+e^-$ pair production at low $p_T$ for different collision systems and energies thus are important to verify and further constrain the existence of strong electromagnetic field and its possible impact on emerging phenomena in hadronic heavy-ion collisions.
In this talk, we will present centrality and invariant mass dependences of inclusive $e^{+}e^{-}$ pair production at $p_T<$ 0.15 GeV/$c$ in Au$+$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U$+$U collisions at $\sqrt{s_{NN}}$= 193 GeV. The observed excess of $e^{+}e^{-}$ yields with respect to the known hadronic sources will be presented as a function of centrality and $p_T^2$. Physics implications will be discussed together with model comparisons.