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

TL;DR

This paper reports on the measurement of exclusive dielectron production in ultraperipheral lead-lead collisions at 5.02 TeV using the ATLAS detector, confirming QED predictions and revealing some systematic differences at high rapidity and angles.

Contribution

First measurement of exclusive dielectron production in ultraperipheral Pb+Pb collisions at LHC energies with detailed differential cross-sections.

Findings

Total fiducial cross-section measured as 215 μb

Good overall agreement with QED Monte Carlo predictions

Systematic differences observed at high rapidity and angles

Abstract

Exclusive production of dielectron pairs, $\gamma\gamma\rightarrow e^+e^-$, is studied using $\mathcal{L}_\mathrm{int}=1.72\; \mathrm{nb^{-1}}$ 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, $p_{\textrm{T}}^{e} > 2.5$ 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, $p_{\textrm{T}}^{ee} < 2$ GeV. Differential cross-sections are measured as a function of $m_{ee}$, average $p_{\textrm{T}}^{e}$, 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 $215 \pm 1 \text{(stat.)} ^{+23}_{-20} \text{(syst.)} \pm 4 \text{(lumi.)}\; \mu$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 $|\cos\theta^*|$ values.