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

TL;DR

This work provides the first dielectron measurement at midrapidity in central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV, focusing on $m_{ee}<3.5$ GeV/$c^2$ and $p_{T,ee}<8$ GeV/$c$ and a direct-photon study via virtual photons. It employs a data-driven hadronic cocktail, two heavy-flavor background models, and a topological $DCA_{ee}$ analysis to separate prompt and displaced dielectrons, comparing results to thermal radiation predictions from hadronic and partonic phases. The analysis observes a low-mass excess over the cocktail consistent with thermal radiation and finds evidence for HF suppression with an additional prompt component in the IMR, while identifying a direct-photon signal compatible with a multi-stage emission scenario. A direct-photon measurement at 5.02 TeV centrality is reported, with model predictions including prompt, pre-equilibrium, and viscous-hydrodynamic phases providing a reasonable description within uncertainties. The results advance the understanding of QGP radiation and heavy-flavor interactions in the quark-gluon plasma and showcase the potential of DCA-based source separation for future high-precision dielectron studies at the LHC.

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.

Figures (17)

• Figure 1: Relative acceptance correction factor $R$ as a function of $m_{\rm ee}$ and $p_{\rm T,ee}$ (left panel) and as a function of $m_{\rm ee}$ integrated over $p_{\rm T,ee}$ (right panel) in the 10% most central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeemV. Statistical uncertainties are represented by vertical bars.
• Figure 2: Left: raw mass distributions of same-sign and opposite-sign pairs from same events and mixed events in the 10% most central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeemV. Right panel: corresponding ratios of same-event and mixed-event same-sign distributions, as well as estimations of the full and uncorrelated combinatorial backgrounds. Statistical uncertainties are represented by vertical bars.
• Figure 3: Raw yield ($S$) overlaid with the opposite-sign pair distribution and the combinatorial background (left panel), as well as signal-over-background ratio (right panel), in the 10% most central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeemV.
• Figure 4: Corrected dielectron yields as a function of $S/B$ in two different mass intervals, i.e. 0.18-0.5 GeemV$/c^2$ (left panel) and 0.5-0.7 GeemV$/c^2$ (right panel), for $0.2 < p_{\rm T,e} < 10$ GeemV$/c$ and $|\eta_{\rm e}| < 0.8$ in the 10% most central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeemV. The red curves show the expected dependence assuming a bias of the estimated background of the order of $\pm 10^{-4}$ ($b = -10^{-4}$ in Eq. \ref{['sysb']}).
• Figure 5: Relative systematic uncertainty from the background uncertainty of $10^{-4}$ on the corrected dielectron yield in bins of $m_{\rm ee}$ and $p_{\rm T,ee}$ for $0.2 < p_{\rm T,e} < 10$ GeemV$/c$ and $|\eta_{\rm e}| < 0.8$ in the 10% most central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeemV.