Dielectron production at midrapidity at low transverse momentum in peripheral and semi-peripheral Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

The first measurement of the ${\rm e}^{+}{\rm e}^{-}$ pair production at low lepton pair transverse momentum ($p_{\rm T,ee}$) and low invariant mass ($m_{\rm ee}$) in non-central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity ($|\eta_{\rm e}| <~ 0.8$) as a function of invariant mass ($0.4 \leq m_{\rm ee} <~ 2.7$ GeV/$c^2$) in the 50$-$70% and 70$-$90% centrality classes for $p_{\rm T,ee} <~ 0.1$ GeV/$c$, and as a function of $p_{\rm T,ee}$ in three $m_{\rm ee}$ intervals in the most peripheral Pb$-$Pb collisions. Below a $p_{\rm T,ee}$ of 0.1 GeV/$c$, a clear excess of ${\rm e}^{+}{\rm e}^{-}$ pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The $m_{\rm ee}$ excess spectra are reproduced, within uncertainties, by different predictions of the photon$-$photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the $p_{\rm T,ee}$ spectra. The measured $\sqrt{\langle p_{\rm T,ee}^{2} \rangle}$ of the excess $p_{\rm T,ee}$ spectrum in peripheral Pb$-$Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region.

 

Submitted to: JHEP
e-Print: arXiv:2204.11732 | PDF | inSPIRE
CERN-EP-2022-068
Figure group

Figure 1

Left panel: raw pT,ee-differential yield (S) in peripheral (70–90%) Pb–Pb collisions at √sNN = 5.02 TeV for 0.7 ≤ mee < 1.1 GeV/c2 overlaid with the opposite charge-sign distribution (OS) and the same charge-sign spectrum multiplied by the acceptance correction factor Racc (B). Right panel: signal over background as a function of pT,ee in peripheral (70–90%) Pb–Pb collisions at √sNN = 5.02 TeV for 0.7 ≤ mee < 1.1 GeV/c

Figure 2

Dielectron $m_{\rm ee}$-differential yields in semi-peripheral (50--70\%) and peripheral (70--90\%) Pb--Pb collisions at \fivenn, compared with the expected e$^{+}$e$^{-}$ contributions from known hadronic decays. The error bars and boxes represent the statistical and systematic uncertainties of the data, respectively, whereas the bands show the uncertainties of the hadronic cocktail. Arrows indicate upper limits at 90\% confidence level.

Figure 3

Excess dielectron $m_{\rm ee}$-differential yields after subtraction of the cocktail of known hadronic decay contributions in semi-peripheral (left) and peripheral (right) Pb--Pb collisions at \fivenn, compared with calculations for coherent two-photon production of \ee pairs folded with the detector resolution\,. For details see the text. The error bars and boxes represent the statistical and systematic uncertainties of the data, respectively. Arrows indicate upper limits at 90\% confidence level.

Figure 4

Dielectron pT,ee-differential yields in peripheral (70–90%) Pb–Pb collisions at √sNN = 5.02 TeV for three different mee ranges, i.e. 0.4 ≤ mee < 0.7 GeV/c2 (left), 0.7 ≤ mee < 1.1 GeV/c2 (middle), and 1.1 ≤ mee < 2.7 GeV/c2 (right), compared with the expected e+e− contributions from known hadronic decays and calculations for coherent two-photon production of dielectrons folded with the detector resolution [21, 24, 28, 36, 84]. For details see the text. The error bars and boxes represent the statistical and systematic uncertainties of the data, respectively, whereas the bands show the uncertainties of the hadronic cocktail. Arrows indicate upper limits at 90% confidence level.

Figure 5

Left: Excess dielectron p2T,ee-differential yields after subtraction of the cocktail of known hadronic decay contributions in peripheral (70–90%) Pb–Pb collisions at √sNN = 5.02 TeV for different mee ranges, i.e. 0.4 ≤ mee < 0.7 GeV/c2, 0.7 ≤ mee < 1.1 GeV/c2 and 1.1 ≤ mee < 2.7 GeV/c2, compared with calculations for coherent photon–photon production of dielectrons folded with the detector resolution [21, 24, 28, 36, 84]. Right: Excess dielectron p2T,ee-differential yields after subtraction of the cocktail of known hadronic decay contributions in peripheral Pb–Pb (70–90%), Au–Au (60–80%) and U–U (60–80%) collisions at √sNN = 5.02, 0.2 and 0.193 TeV [12], respectively, in a similar mee range. The error bars and boxes represent the statistical and systematic uncertainties of the data, respectively. Arrows indicate upper limits at 90% confidence level