Low-$p_T$ $e^{+}e^{-}$ Pair Production in Au +Au Collisions at $\sqrt{s_{NN}}$ =200 GeV and U + U Collisions at $\sqrt{s_{NN}}$ =193 GeV at STAR

Abstract

We report first measurements of $e^{+}e^{-}$ pair production in the mass region 0.4 <$M_{ee}$< 2.6 GeV/c$^2$ at low transverse momentum ($p_T$< 0.15 GeV/$c$) in non-central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV. Significant enhancement factors, expressed as ratios of data over known hadronic contributions, are observed in the 40-80% centrality of these collisions. The excess yields peak distinctly at low-$p_T$ with a width $\sqrt{\langle p^2_T\rangle}$ between 40 to 60 MeV/$c$. The absolute cross section of the excess depends weakly on centrality while those from a theoretical model calculation incorporating an in-medium broadened ρ spectral function and radiation from a Quark Gluon Plasma or hadronic cocktail contributions increase dramatically with increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the $p^{2}_{T}$ distributions.