Dielectron Production in Central Pb-Pb Collisions at √sNN = 5.02 TeV

The first measurement of the 𝑒+⁢𝑒− pair production at midrapidity and low invariant mass in central Pb-Pb collisions at √𝑠𝑁⁢𝑁=5.02TeV 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 (𝑚𝑒⁢𝑒) and pair transverse momentum (𝑝T,𝑒⁢𝑒) ranges 𝑚𝑒⁢𝑒<3.5GeV/𝑐2 and 𝑝T,𝑒⁢𝑒<8GeV/𝑐. For 0.18<𝑚𝑒⁢𝑒<0.5GeV/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<𝑚𝑒⁢𝑒<2.6GeV/𝑐2), the pair transverse impact parameter of the 𝑒+⁢𝑒− pairs (DCA𝑒⁢𝑒, 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 𝑐⁢ ‾ 𝑐 and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at √𝑠𝑁⁢𝑁=5.02TeV is reported via the study of virtual direct photons in the transverse momentum range 1<𝑝T<5GeV/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.