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

TL;DR

This paper presents the first measurements of low-$p_T$ $e^{+}e^{-}$ pair production in non-central Au+Au and U+U collisions at RHIC energies, revealing significant excesses over known hadronic sources and testing theoretical models of in-medium effects and photon-photon interactions.

Contribution

It provides novel experimental data on low-$p_T$ $e^{+}e^{-}$ pairs in heavy-ion collisions and compares these with theoretical models, highlighting discrepancies in $p_T$ distributions.

Findings

Significant excess of $e^{+}e^{-}$ pairs at low $p_T$ in non-central collisions.

Excess peaks at low $p_T$ with a width of 40-60 MeV/$c$.

Photon-photon interaction models describe yields but not $p_T$ distributions.

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 $\rho$ 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.