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

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.

Figures

## Figure 1

 . (A) Distributions of the difference between the measured d$\textit{E}$/d$\textit{x}$ in the TPC and the expected average value for electrons (in units of $\sigma_{\mathrm{TPC}}$) after the selection of the electron candidates in the TOF ($|n\sigma_{\mathrm{TOF}}(\mathrm{e})|< 3$) and in the ITS ($-5< n\sigma_{\mathrm{ITS}}(\mathrm{e})< 1$). (B) Electron purity as a function of the track momentum ($p$) estimated from the multiple skewed Gaussian fit.

## Figure 2

 . Invariant-mass distribution of correlated electron-hadron and hadron-hadron pairs, where hadrons are wrongly identified as electrons, in comparison with the invariant-mass spectrum of all pairs (dielectron signal pairs and `contaminated' pairs) obtained from MC simulations. In the bottom panel, the ratio between the invariant-mass spectrum of electron-hadron and hadron-hadron pairs to that of all pairs is shown. Boxes represent the systematic uncertainties on the hadron contamination.

## Figure 3

 Average fraction of shared clusters in the ITS for secondary tracks as a function of the radial distance ($R$) between the true production vertex and the primary vertex obtained from MC simulations.

## Figure 4

 . (A) $\psi_{\mathrm{V}}$ distribution for correlated $\mathrm{e}^{+}\mathrm{e}^{-}$ pairs originating from different dielectron sources. (B) Correlation between $\psi_{\mathrm{V}}$ and $m_{\mathrm{ee}}$ for electron-positron pairs produced by photon conversion in the detector material. Their tracks have passed the single-track conversion rejection. The regions used to select the conversion pair candidates are indicated by the red boxes.

## Figure 5

 . (A) Rejection efficiency of conversion electrons as a function of $p_{\mathrm{T}, \mathrm{e}}$ using single-track rejection, pair rejection and using both methods. (B) Fraction of signal tracks rejected by the conversion rejection methods as a function of $p_{\mathrm{T}, \mathrm{e}}$.

## Figure 6

 . (A) Dielectron raw yield, in the low-mass region, in comparison with the residual contribution of conversion electrons, estimated using MC simulations. (B) Relative contribution to the dielectron spectrum of conversion electrons surviving the photon conversion rejection. Only results for the negative magnetic field polarity are shown, with similar values obtained for the positive magnetic field configuration.

## Figure 7

 . (A) Invariant-mass distributions of unlike-sign (black), like-sign (red) and signal (blue) dielectron pairs. (B) Acceptance correction factor ($R$-factor) for both magnetic field orientations. Only statistical uncertainties are shown.

## Figure 8

 Signal-to-background ratio of the dielectron measurement as a function of the invariant mass for the negative magnetic field polarity. Similar values are obtained for the positive magnetic field configuration.

## Figure 9

 . Reconstruction efficiency of dielectrons as a function of the invariant mass for different $p_{\mathrm{T}, \mathrm{ee}}$ intervals. The statistical uncertainties are represented by the vertical bars. Only results for the negative magnetic field polarity are shown, with similar values obtained for the positive magnetic field configuration.

## Figure 10

 . Dielectron invariant-mass spectrum measured in central (0$-$10$\%$) Pb--Pb collisions at $\mathbf{\sqrt{{\textit{s}}_{\mathrm{NN}}}}$ = 2.76 TeV in comparison with the hadronic cocktail. The statistical and systematic uncertainties of the data are represented by vertical bars and boxes. The arrow for one mass range represents the upper limit at $90\%$ C.L. The blue band represents the systematic uncertainties on the hadronic cocktail.

## Figure 11

 . Dielectron invariant-mass spectrum for $m_{\mathrm{ee}}< 1\ \mathrm{GeV/\textit{c}^{2}}$ in comparison with the predictions from R. Rapp and PHSD theoretical models. The statistical and systematic uncertainties of data are represented by vertical bars and boxes. The systematic uncertainties on the theory curves are due to the energy loss correction that is applied based on a cocktail simulation (see text for details).

## Figure 12

 . Dielectron invariant-mass spectrum compared to the two different versions of the hadronic cocktail. In one version, shown with a dash-dotted magenta line, the charm contribution is obtained from PYTHIA via $\textit{N}_{\mathrm{coll}}$-scaling, while in the other version, shown with a dotted red line, a random correlation of dielectrons from charm decays is assumed. The statistical and systematic uncertainties of data are represented by vertical bars and boxes.

## Figure 13

 . Fit function (Eq.~\ref{eq:ThreeComponentFunc}), its individual components and dielectron invariant-mass spectra measured in central collisions for the transverse-momentum intervals $1< p_{\mathrm{T}, \mathrm{ee}}< 2\ \mathrm{GeV}/\textit{c}$ (A) and $2< p_{\mathrm{T}, \mathrm{ee}}< 4\ \mathrm{GeV}/\textit{c}$ (B). The hadronic cocktail is $f_{\mathrm{LF}} (m_{\mathrm{ee}}) + f_{\mathrm{HF}} (m_{\mathrm{ee}})$.

## Figure 14

 . Comparison between the direct photon ratio $R_{\gamma^{(*)}}=1+\gamma^{(*)}_{\mathrm{dir}}/\gamma^{(*)}_{\mathrm{decay}}$ measured using virtual photons in the centrality range 0$-$10$\%$ to that obtained using real photons in the centrality range 0$-$20$\%$ and several theoretical model calculations (see text for references). The real photon measurement is taken from . The statistical and systematic uncertainties of the data are represented by vertical bars and boxes.