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Dielectron production in central Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02TeV

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The first measurement of the e$^+$e$^−$ pair production at midrapidity and low invariant mass in central Pb-Pb collisions at $\sqrt{s_{NN}}$=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 (m$_{ee}$) and pair transverse momentum ($p_{T,ee}$) ranges m$_{ee}$ < 3.5 GeV/c$^2$ and p$_{T,ee}$ <8 GeV/c. For 0.18 < m$_{ee}$ < 0.5 GeV/c$^2$ 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.)−0.29+0.24(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/c$^2$), the pair transverse impact parameter of the e$^+$e$^−$ pairs (DCAee, 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 $c\bar{c}$ and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02TeV 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.

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High-Energy Particle Collisions ResearchParticle physics theoretical and experimental studiesEarth Systems and Cosmic Evolution

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