Low-\u3cem\u3ep\u3csub\u3eT\u3c/sub\u3e\u3c/em\u3e \u3cem\u3ee\u3c/em\u3e\u3csup\u3e+\u3c/sup\u3e \u3cem\u3ee\u3c/em\u3e\u3csup\u3e-\u3c/sup\u3e Pair Production in Au + Au Collisions at √\u3cem\u3es\u3c/em\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 193 GeV and U + U Collisions at √\u3cem\u3es\u3c/em\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 193 GeV at STAR

Abstract

We report first measurements of e+e− pair production in the mass region 0.4 \u3c Mee \u3c 2.6  GeV/c2 at low transverse momentum (pT \u3c 0.15  GeV/c) in noncentral Au+Au collisions at √sNN = 200  GeV and U+U collisions at √sNN = 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 pT with a width (√⟨p2T⟩) between 40 and 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 an 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 p2T distributions