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Muon reconstruction efficiency as function of its tranverse impact parameter, $d_0$.

Electron selection efficiency as function of its tranverse momentum, $p_T$.

Muon selection efficiency as function of its tranverse momentum, $p_T$.

The centrality dependence of e+e− invariant mass spectra within the STAR acceptance from Au+Au collisions and U+U collisions for pair pT < 0.15 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 U+U collisions are ∼5%–12% higher than those from Au+Au collisions in given centrality bins; thus only cocktails for Au+Au collisions are shown here as solid lines, with shaded bands representing the systematic uncertainties for clarity.

The centrality dependence of e+e− invariant mass spectra within the STAR acceptance from Au+Au collisions and U+U collisions for pair pT < 0.15 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 U+U collisions are ∼5%–12% higher than those from Au+Au collisions in given centrality bins; thus only cocktails for Au+Au collisions are shown here as solid lines, with shaded bands representing the systematic uncertainties for clarity.

The centrality dependence of e+e− invariant mass spectra within the STAR acceptance from Au+Au collisions and U+U collisions for pair pT < 0.15 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 U+U collisions are ∼5%–12% higher than those from Au+Au collisions in given centrality bins; thus only cocktails for Au+Au collisions are shown here as solid lines, with shaded bands representing the systematic uncertainties for clarity.

The corresponding ratios of data over cocktail for Figure 1.

The corresponding ratios of data over cocktail for Figure 1.

The corresponding ratios of data over cocktail for Figure 1.

The corresponding ratios of data over cocktail for Figure 1.

The corresponding ratios of data over cocktail for Figure 1.

The corresponding ratios of data over cocktail for Figure 1.

The e+e− pair pT distributions within the STAR acceptance for different mass regions in 60%–80% Au+Au and U+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.

The e+e− pair pT distributions within the STAR acceptance for different mass regions in 60%–80% Au+Au and U+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.

The e+e− pair pT distributions within the STAR acceptance for different mass regions in 60%–80% Au+Au and U+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.

The e+e− pair pT distributions within the STAR acceptance for different mass regions in 60%–80% Au+Au and U+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.

The e+e− pair pT distributions within the STAR acceptance for different mass regions in 60%–80% Au+Au and U+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.

The e+e− pair pT distributions within the STAR acceptance for different mass regions in 60%–80% Au+Au and U+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.

The low-pT (pT < 0.15 GeV/c) e+e− excess mass spectra (data − cocktail) within the STAR acceptance in 60%–80% for Au + Au and U + U collisions, compared with a broadened rho meson model calculation [8]. The contributions of rho and phi from the photonuclear process are shown, as are the contributions of photon-photon process from two models [33,34]. The model calculations are for Au + Au collisions in the corresponding centrality bins.

The low-pT (pT < 0.15 GeV/c) e+e− excess mass spectra (data − cocktail) within the STAR acceptance in 60%–80% for Au + Au and U + U collisions, compared with a broadened rho meson model calculation [8]. The contributions of rho and phi from the photonuclear process are shown, as are the contributions of photon-photon process from two models [33,34]. The model calculations are for Au + Au collisions in the corresponding centrality bins.

The low-pT (pT < 0.15 GeV/c) e+e− excess mass spectra (data − cocktail) within the STAR acceptance in 40%–60% for Au + Au and U + U collisions, compared with a broadened rho meson model calculation [8]. The contributions of rho and phi from the photonuclear process are shown, as are the contributions of photon-photon process from two models [33,34]. The model calculations are for Au + Au collisions in the corresponding centrality bins.

The low-pT (pT < 0.15 GeV/c) e+e− excess mass spectra (data − cocktail) within the STAR acceptance in 40%–60% for Au + Au and U + U collisions, compared with a broadened rho meson model calculation [8]. The contributions of rho and phi from the photonuclear process are shown, as are the contributions of photon-photon process from two models [33,34]. The model calculations are for Au + Au collisions in the corresponding centrality bins.

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 GeV=c2 in Au + Au and U + U collisions. The centrality dependence of hadronic cocktail yields in the mass region of 0.76–1.2 GeV/c2 in both collisions is also shown for comparison. The systematic uncertainties are shown as gray boxes.

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 GeV=c2 in Au + Au and U + U collisions. The centrality dependence of hadronic cocktail yields in the mass region of 0.76–1.2 GeV/c2 in both collisions is also shown for comparison. The systematic uncertainties are shown as gray boxes.

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 GeV=c2 in Au + Au and U + U collisions. The centrality dependence of hadronic cocktail yields in the mass region of 0.76–1.2 GeV/c2 in both collisions is also shown for comparison. The systematic uncertainties are shown as gray boxes.

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 GeV=c2 in Au + Au and U + U collisions. The centrality dependence of hadronic cocktail yields in the mass region of 0.76–1.2 GeV/c2 in both collisions is also shown for comparison. The systematic uncertainties are shown as gray boxes.

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 GeV=c2 in Au + Au and U + U collisions. The centrality dependence of hadronic cocktail yields in the mass region of 0.76–1.2 GeV/c2 in both collisions is also shown for comparison. The systematic uncertainties are shown as gray boxes.

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 GeV/c2 in Au + Au and U + U collisions. The centrality dependence of hadronic cocktail yields in the mass region of 0.76–1.2 GeV/c2 in both collisions is also shown for comparison. The systematic uncertainties are shown as gray boxes.

The p2T distributions of excess yields within the STAR acceptance in the mass regions of 0.4–0.76 GeV/c2 in 60%–80% Au + Au and U + U collisions. The systematic uncertainties are shown as gray boxes. The solid and dotted lines are exponential fits to the data in Au + Au and U + U collisions, respectively. The dot-dashed and dot-dot- dashed lines represent the p2T distributions for the photon-photon process from two models [33,34] within the STAR acceptance in 60%–80% Au + Au collisions. The dashed lines illustrate the corresponding p2T distributions for e+e− pairs from the model [33] traversing 1 fm in a constant magnetic field of 10^14 T perpendicular to the beam line.

The p2T distributions of excess yields within the STAR acceptance in the mass regions of 0.4–0.76 GeV/c2 in 60%–80% Au + Au and U + U collisions. The systematic uncertainties are shown as gray boxes. The solid and dotted lines are exponential fits to the data in Au + Au and U + U collisions, respectively. The dot-dashed and dot-dot- dashed lines represent the p2T distributions for the photon-photon process from two models [33,34] within the STAR acceptance in 60%–80% Au + Au collisions. The dashed lines illustrate the corresponding p2T distributions for e+e− pairs from the model [33] traversing 1 fm in a constant magnetic field of 10^14 T perpendicular to the beam line.

The p2T distributions of excess yields within the STAR acceptance in the mass regions of 0.76–1.2 GeV/c2 in 60%–80% Au + Au and U + U collisions. The systematic uncertainties are shown as gray boxes. The solid and dotted lines are exponential fits to the data in Au + Au and U + U collisions, respectively. The dot-dashed and dot-dot- dashed lines represent the p2T distributions for the photon-photon process from two models [33,34] within the STAR acceptance in 60%–80% Au + Au collisions. The dashed lines illustrate the corresponding p2T distributions for e+e− pairs from the model [33] traversing 1 fm in a constant magnetic field of 10^14 T perpendicular to the beam line.

The p2T distributions of excess yields within the STAR acceptance in the mass regions of 0.76–1.2 GeV/c2 in 60%–80% Au + Au and U + U collisions. The systematic uncertainties are shown as gray boxes. The solid and dotted lines are exponential fits to the data in Au + Au and U + U collisions, respectively. The dot-dashed and dot-dot- dashed lines represent the p2T distributions for the photon-photon process from two models [33,34] within the STAR acceptance in 60%–80% Au + Au collisions. The dashed lines illustrate the corresponding p2T distributions for e+e− pairs from the model [33] traversing 1 fm in a constant magnetic field of 10^14 T perpendicular to the beam line.

The p2T distributions of excess yields within the STAR acceptance in the mass regions of 1.2–2.6 GeV/c2 in 60%–80% Au + Au and U + U collisions. The systematic uncertainties are shown as gray boxes. The solid and dotted lines are exponential fits to the data in Au + Au and U + U collisions, respectively. The dot-dashed and dot-dot- dashed lines represent the p2T distributions for the photon-photon process from two models [33,34] within the STAR acceptance in 60%–80% Au + Au collisions. The dashed lines illustrate the corresponding p2T distributions for e+e− pairs from the model [33] traversing 1 fm in a constant magnetic field of 10^14 T perpendicular to the beam line.

The p2T distributions of excess yields within the STAR acceptance in the mass regions of 1.2–2.6 GeV/c2 in 60%–80% Au + Au and U + U collisions. The systematic uncertainties are shown as gray boxes. The solid and dotted lines are exponential fits to the data in Au + Au and U + U collisions, respectively. The dot-dashed and dot-dot- dashed lines represent the p2T distributions for the photon-photon process from two models [33,34] within the STAR acceptance in 60%–80% Au + Au collisions. The dashed lines illustrate the corresponding p2T distributions for e+e− pairs from the model [33] traversing 1 fm in a constant magnetic field of 10^14 T perpendicular to the beam line.

The corresponding sqrt(p2T) of excess yields. The vertical bars on data points are the combined statistical and systematic uncertainties.

The corresponding sqrt(p2T) of excess yields. The vertical bars on data points are the combined statistical and systematic uncertainties.