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

TL;DR

This paper reports the first measurement of dielectron production in central Pb-Pb collisions at 5.02 TeV, revealing excess yields consistent with thermal radiation and providing insights into medium effects and direct photon production.

Contribution

It presents the first dielectron and direct photon measurements at this energy, using novel techniques like impact parameter analysis to distinguish sources and compare with theoretical models.

Findings

Dielectron yield exceeds hadronic decay expectations, indicating thermal contributions.

Impact parameter analysis suggests suppression of heavy-flavor decay pairs and possible thermal sources.

First direct-photon measurement aligns with models including prompt and thermal photons.

Abstract

The first measurement of the e$^+$e$^-$ pair production at midrapidity and low invariant mass in central Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ 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$/c^2$ and $p_{\rm T,ee} < 8$ GeV$/c$. For $0.18 < m_{\rm ee} < 0.5$ GeV$/c^2$ the ratio of data to the cocktail of hadronic contributions amounts to $1.40 \pm 0.11 \ ({\rm stat.}) \pm 0.23 \ ({\rm syst.}) \pm 0.16 \ ({\rm cocktail})$ and $1.42 \pm 0.11 \ ({\rm stat.}) \pm 0.23 \ ({\rm syst.}) ^{+0.24}_{-0.29} \ ({\rm cocktail})$, 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$/c^2$), 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 ${\rm c\overline{c}}$ and an additional prompt component. Finally, the first direct-photon measurement in the 10\% most central Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ 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.