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

Exclusive production of dielectron pairs, $\gamma \gamma \to e^{+}{e}^{-}$, is studied using ${\mathcal{L}}_{\mathrm{i}\mathrm{n}\mathrm{t}}=1.72\mathrm{n}{\mathrm{b}}^{-1}$ 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, $\mathrm{\mid }{\eta }^e\mathrm{\mid }<2.5$, 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, $\mathrm{\mid }{y}_{ee}\mathrm{\mid }$, and scattering angle in the dielectron rest frame, $\mathrm{\mid }\cos {\theta }^{\ast }\mathrm{\mid }$ 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 $\mathrm{\mid }{y}_{ee}\mathrm{\mid }$ and $\mathrm{\mid }\cos {\theta }^{\ast }\mathrm{\mid }$ values.