Dielectron production in central Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02TeV
The first measurement of the π + β’π β pair production at midrapidity and low invariant mass in central Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV at the Large Hadron Collider is presented. The yield of π + β’π β pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass (ππβ’π) and pair transverse momentum (π T,πβ’π ) ranges π πβ’π < 3.5 GeV/π 2 and π T,πβ’π < 8 GeV/π. For 0.18 < π πβ’π < 0.5 GeV/π 2 the ratio of data to the cocktail of hadronic contributions amounts to 1.40 Β± 0.11β’(stat.) Β± 0.23 β’(syst.) Β± 0.16β’ (cocktail) and 1.42 Β± 0.11β’ (stat.) Β± 0.23 (syst.)$^{+0.24}_{β0.29}$ β’(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 π+β’πβ pairs from the hadronic and partonic phases. In the intermediate-mass range (1.2 < π πβ’π < 2.6 GeV/π 2 ), the pair transverse impact parameter of the π + β’π β pairs (DCAπβ’π, where βDCAβ denotes βdistance of closest approachβ) 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 π + β’π β pairs from $π\bar{π}$ and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV is reported via the study of virtual direct photons in the transverse momentum range 1 < π T < 5 GeV/π. 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.