Measurement of dielectron production in central Pb-Pb collisions at √sNN=2.76 TeV

Abstract

The first measurement of dielectron (e+e−) production in central (0–10%) Pb–Pb collisions at √sNN =2.76 TeV at the LHC is presented. The dielectron invariant-mass spectrum is compared to the expectedcontributions from hadron decays in the invariant-mass range 0 < mee < 3.5 GeV/c2. The ratio of dataand the cocktail of hadronic contributions without vacuum ρ0 is measured in the invariant-mass range0.15 < mee < 0.7 GeV/c2, where an excess of dielectrons is observed in other experiments, and its value is1.40 ± 0.28 (stat.) ± 0.08 (syst.) ± 0.27 (cocktail). The dielectron spectrum measured in the invariant massrange 0 < mee < 1 GeV/c2 is consistent with the predictions from two theoretical model calculations thatinclude thermal dielectron production from both partonic and hadronic phases with in-medium broadened ρ0meson. The fraction of direct virtual photons over inclusive virtual photons is extracted for dielectron pairs withinvariant mass 0.1 < mee < 0.3 GeV/c2 and in the transverse-momentum intervals 1 < pT,ee < 2 GeV/c and2 < pT,ee < 4 GeV/c. The measured fraction of virtual direct photons is consistent with the measurement of realdirect photons by ALICE and with the expectations from previous dielectron measurements at RHIC within theexperimental uncertainties.