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Measurement of dielectron production in central Pb-Pb collisions at $\sqrt{{\textit{s}}_{\mathrm{NN}}}$ = 2.76 TeV

The collaboration
No Journal Information, 2018

Abstract
The first measurement of dielectron ($\mathrm{e}^{+}\mathrm{e}^{-}$) production in central (0$-$10$\%$) Pb-Pb collisions at $\mathbf{\sqrt{{\textit{s}}_{\mathrm{NN}}}}$ = 2.76 TeV at the LHC is presented. The dielectron invariant-mass spectrum is compared to the expected contributions from hadron decays in the invariant-mass range $0 < m_{\mathrm{ee}}<3.5\ \mathrm{GeV}/\textit{c}^{2}$. The ratio of data and the cocktail of hadronic contributions without vacuum $\rho^{0}$ is measured in the invariant-mass range $0.15 < m_{\mathrm{ee}}<0.7\ \mathrm{GeV}/\textit{c}^{2}$, where an excess of dielectrons is observed in other experiments, and its value is $1.40 \pm 0.28\ (\mathrm{stat.}) \pm 0.08\ (\mathrm{syst.}) \pm 0.27\ (\mathrm{cocktail})$. The dielectron spectrum measured in the invariant mass range $0 < m_{\mathrm{ee}}<1\ \mathrm{GeV}/\textit{c}^{2}$ is consistent with the predictions from two theoretical model calculations that include thermal dielectron production from both partonic and hadronic phases with in-medium broadened $\rho^{0}$ meson. The fraction of direct virtual photons over inclusive virtual photons is extracted for dielectron pairs with invariant mass $0.1 < m_{\mathrm{ee}}<0.3\ \mathrm{GeV}/\textit{c}^{2}$, and in the transverse-momentum intervals $1<p_{\mathrm{T}, \mathrm{ee}}<2\ \mathrm{GeV}/\textit{c}$ and $2<p_{\mathrm{T}, \mathrm{ee}}<4\ \mathrm{GeV}/\textit{c}$. The measured fraction of virtual direct photons is consistent with the measurement of real direct photons by ALICE and with the expectations from previous dielectron measurements at RHIC within the experimental uncertainties.

• #### Table 1

Figure 10 (upper panel)

10.17182/hepdata.86551.v1/t1

Dielectron invariant-mass spectrum measured in central Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. The statistical and systematic uncertainties of the...

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Figure 10 (lower panel)

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Data-to-cocktail ratio. Statistical uncertainties are represented by vertical bars and systematic uncertainties by boxes.

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Figure 11 (lower panel)

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Dielectron invariant-mass spectrum divided by the hadronic cocktail without the contribution from the vacuum $\rho^{0}$. The statistical and systematic uncertainties...

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Figure 12 (lower panel)

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Data-to-cocktail ratio as a function of the invariant-mass. In the hadronic cocktail, random correlations of dielectrons from charm decays are...

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Figure 13 (left)

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Figure 14

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Direct photon ratio $R_{\gamma^{(*)}}=1+\gamma^{(*)}_{\mathrm{dir}}/\gamma^{(*)}_{\mathrm{decay}}$ measured using virtual photons in the centrality range 0$-$10$\%$. The statistical and systematic uncertainties of the...