PHYSICAL REVIEW C, vol.112, no.5, 2025 (SCI-Expanded, Scopus)
The first measurement of the e(+) e(-) pair production at midrapidity and low invariant mass in central Pb-Pb collisions at root sNN = 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/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.)(+0.24) (-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 < mee < 2.6 GeV/c(2)), 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 root sNN = 5.02 TeV is reported via the study of virtual direct photons in the transverse momentum range 1 < pT < 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.