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

Exclusive production of dielectron pairs, $\gamma\gamma\rightarrow e^+e^-$, is studied using $L=1.72$ nb$^{-1}$ of data from ultraperipheral collisions of lead nuclei at $\sqrt{s_\mathrm{NN}}$ = 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, $p_{T}^e > 2.5$ 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, $p_T^{ee}< 2$ GeV. Differential cross-sections are measured as a function of $m_{ee}$, average $p_T^e$ absolute rapidity of the dielectron system, $|y_{ee}|$ and scattering angle in the dielectron rest frame, $|\cos{\theta^*}|$, 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 $\pm 1$ (stat.) $^{+23}_{-20}$ (syst.) $\pm 4$ (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 $|\cos{\theta^*}|$ values.