Exclusive dielectron production in ultraperipheral Pb+Pb collisions at $\sqrt{s_{{}_{\text{NN}}}}=5.02$ TeV with ATLAS

Abstract

Exclusive production of dielectron pairs, $\gamma \gamma \to e^{+}{e}^{-}$, is studied using ${\mathcal{L}}_{\mathrm{int}}=1.72\mathrm{n}{\mathrm{b}}^{-1}$ of data from ultraperipheral collisions of lead nuclei at $\sqrt{s_{{}_{\text{NN}}}}=5.02$ TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon-photon interaction in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum, $p_{\text{T}}^e>2.5$ GeV, absolute electron pseudorapidity, $|{\eta }^e|<2.5$, invariant mass of the dielectron system, $m_{ee}>5$ GeV, and transverse momentum of the dielecton pair, $p_{\text{T}}^{ee}<2$ GeV. Differential cross-sections are measured as a function of $m_{ee}$, average $p_{\text{T}}^e$, absolute rapidity of the dielectron system, $|y_{ee}|$, and scattering angle in the dielectron rest frame, $|\cos {\theta }^{\ast }|$ in the inclusive sample, and also under the requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be $215\pm 1{\text{(stat.)}}_{-20}^{+23}\text{(syst.)}\pm 4\text{(lumi.)}\mu$b. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs Starlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences with these predictions which are more pronounced at high $|y_{ee}|$ and $|\cos {\theta }^{\ast }|$ values.

21 pages in total, 7 figures, 0 tables, submitted to JHEP. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2021-16

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• Glauber Modeling in High Energy Nuclear Collisions
• The Physics of Ultraperipheral Collisions at the LHC

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• Physics with high-luminosity proton-nucleus collisions at the LHC