UC Berkeley

The first measurement of the pair production at midrapidity and low invariant mass in central Pb-Pb collisions at at the Large Hadron Collider is presented. The yield of pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass ( ) and pair transverse momentum ( ) ranges and . For the ratio of data to the cocktail of hadronic contributions amounts to and , 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 pairs from the hadronic and partonic phases. In the intermediate-mass range ( ), the pair transverse impact parameter of the pairs ( , 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 pairs from and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at is reported via the study of virtual direct photons in the transverse momentum range . 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.