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

We report first measurements of $e^{+}{e}^{-}$ pair production in the mass region $0.4<M_{ee}<2.6\mathrm{GeV}/c^2$ at low transverse momentum ($p_T<0.15\mathrm{GeV}/c$) in noncentral $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ and $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{NN}}=193\mathrm{GeV}$. 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 ($\sqrt{⟨p_T^2⟩}$) between 40 and $60\mathrm{MeV}/c$. 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 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 $p_T^2$ distributions.

Figure 1

(a) The centrality dependence of $e^{+}{e}^{-}$ invariant mass spectra within the STAR acceptance from $\mathrm{Au}+\mathrm{Au}$ collisions and $\mathrm{U}+\mathrm{U}$ collisions for pair $p_T<0.15\mathrm{GeV}/c$. The vertical bars on data points depict the statistical uncertainties, while the systematic uncertainties are shown as gray boxes. The hadronic cocktail yields from $\mathrm{U}+\mathrm{U}$ collisions are $\sim 5\%–12\%$ higher than those from $\mathrm{Au}+\mathrm{Au}$ collisions in given centrality bins; thus only cocktails for $\mathrm{Au}+\mathrm{Au}$ collisions are shown here as solid lines, with shaded bands representing the systematic uncertainties for clarity. (b) The corresponding ratios of data over cocktail.

Figure 2

The $e^{+}{e}^{-}$ pair $p_T$ distributions within the STAR acceptance for different mass regions in 60%–80% $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$ collisions compared to cocktails. The systematic uncertainties of the data are shown as gray boxes. The gray bands depict the systematic uncertainties of the cocktails.

Figure 3

The low-$p_T$ ($p_T<0.15\mathrm{GeV}/c$) $e^{+}{e}^{-}$ excess mass spectra (data $-$ cocktail) within the STAR acceptance in (a) 60%–80% and (b) 40%–60% for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$ collisions, compared with a broadened $\rho$ model calculation [8]. The contributions of $\rho$ and $\varphi$ from the photonuclear process are shown, as are the contributions of photon-photon process from two models [33, 34]. The model calculations are for $\mathrm{Au}+\mathrm{Au}$ collisions in the corresponding centrality bins. (c) The centrality dependence of integrated excess yields in the mass regions of 0.4–0.76, 0.76–1.2, and $1.2–2.6\mathrm{GeV}/c^2$ in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$ collisions. The centrality dependence of hadronic cocktail yields in the mass region of $0.76–1.2\mathrm{GeV}/c^2$ in both collisions is also shown for comparison. The systematic uncertainties are shown as gray boxes.

Figure 4

The $p_T^2$ distributions of excess yields within the STAR acceptance in the mass regions of (a) 0.4–0.76, (b) 0.76–1.2, and (c) $1.2–2.6\mathrm{GeV}/c^2$ in 60%–80% $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$ collisions. The systematic uncertainties are shown as gray boxes. The solid and dotted lines are exponential fits to the data in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$ collisions, respectively. The dot-dashed and dot-dot-dashed lines represent the $p_T^2$ distributions for the photon-photon process from two models [33, 34] within the STAR acceptance in 60%–80% $\mathrm{Au}+\mathrm{Au}$ collisions. The dashed lines illustrate the corresponding $p_T^2$ distributions for $e^{+}{e}^{-}$ pairs from the model [33] traversing 1 fm in a constant magnetic field of $10^{14}\mathrm{T}$ perpendicular to the beam line. (d) The corresponding $\sqrt{⟨p_T^2⟩}$ of excess yields. The vertical bars on data points are the combined statistical and systematic uncertainties.