Dielectron production in central Pb-Pb collisions at √s_NN = 5.02 TeV

The first measurement of the e⁺e⁻ pair production at midrapidity and low invariant mass in central Pb-Pb collisions at √s_NN = 5.02 TeV at the Large Hadron Collider is presented. The yield of e⁺e⁻ pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass (m_ee) and pair transverse momentum (p_T,ee) ranges m_ee < 3.5 GeV/c² and p_T,ee < 8 GeV/c. For 0.18 < m_ee < 0.5 GeV/c² the ratio of data to the cocktail of hadronic contributions amounts to 1.40 ± 0.11 (stat.) ± 0.23 (syst.) ± 0.16 (cocktail) and 1.42 ± 0.11 (stat.) ± 0.23 (syst.)⁺0.24_−0.29 (cocktail), including or not including medium effects in the estimation of the heavy-flavor background, respectively. It is consistent with predictions from two different models for an additional contribution of thermal e⁺e⁻ pairs from the hadronic and partonic phases. In the intermediate-mass range (1.2 < m_ee < 2.6 GeV/c²), the pair transverse impact parameter of the e⁺e⁻ pairs (DCA_ee, where “DCA” denotes “distance of closest approach”) is used for the first time in Pb-Pb collisions to separate displaced dielectrons from heavy-flavor hadron decays from a possible (thermal) contribution produced at the interaction point. The data are consistent with a suppression of e⁺e⁻ pairs from cc and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at √s_NN = 5.02 TeV is reported via the study of virtual direct photons in the transverse momentum range 1 < p_T < 5 GeV/c. A model including prompt photons, as well as photons from the preequilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties.