Observation of $W\gamma\gamma$ triboson production in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

TL;DR

This paper reports the first observation of triboson production involving a W boson and two photons in proton-proton collisions at 13 TeV, confirming Standard Model predictions with high significance.

Contribution

It provides the first experimental observation of $W o ext{lepton} u$ with two photons at the LHC, using the full Run 2 dataset and measuring the cross section.

Findings

Significance of 5.6 standard deviations for the process

Measured cross section consistent with Standard Model

First observation of $W o ext{lepton} u$ with two photons

Abstract

This letter reports the observation of $W(\ell\nu)\gamma\gamma$ production in proton-proton collisions. This measurement uses the full Run 2 sample of events recorded at a center-of-mass energy of $\sqrt{s} = 13$ TeV by the ATLAS detector at the LHC, corresponding to an integrated luminosity of 140 fb$^{-1}$. Events with a leptonically-decaying $W$ boson and at least two photons are considered. The background-only hypothesis is rejected with an observed and expected significance of $5.6$ standard deviations. The inclusive fiducial production cross section of $W(e\nu)\gamma\gamma$ and $W(\mu\nu)\gamma\gamma$ events is measured to be $\sigma_{\mathrm{fid}} = 13.8 \pm 1.1 (\mathrm{stat}) \substack{+2.1 \\ -2.0} (\mathrm{syst}) \pm 0.1 (\mathrm{lumi})$ fb, in agreement with the Standard Model prediction.