2023/08/31 by ALICE Collaboration
#FOS: Physical sciences #High Energy Physics - Experiment (hep-ex) #Nuclear Experiment (nucl-ex)
paper · doi:10.48550/arxiv.2308.16704
The first measurement of the e+e- pair production at midrapidity and low invariant mass in central Pb-Pb collisions at √sNN=5.02 TeV at the LHC is presented. The yield of e+e- pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass (m\rm ee) and pair transverse momentum (p\rm T,ee) ranges m\rm ee < 3.5 GeV/c2 and p\rm T,ee < 8 GeV/c. For 0.18 < m\rm ee < 0.5 GeV/c2 the ratio of data to the cocktail of hadronic contributions without ρ mesons amounts to 1.42 ± 0.12 (\rm stat.) ± 0.17 (\rm syst.) ± 0.12 (\rm cocktail) and 1.44 ± 0.12 (\rm stat.) ± 0.17 (\rm syst.) +0.17-0.21 (\rm 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\rm ee < 2.6 GeV/c2), the pair transverse impact parameter of the e+e- pairs (DCA\rm ee) 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 \rm cc and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at √sNN=5.02 TeV is reported via the study of virtual direct photons in the transverse momentum range 1 < p\rm T < 5 GeV/c. A model including prompt photons, as well as photons from the pre-equilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties.