Observation of $W^{+}W^{-}γ$ production in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector and constraints on anomalous quartic gauge-boson couplings

TL;DR

This ATLAS study reports the first observation of $W^{+}W^{-}\gamma$ triboson production in $pp$ collisions at $\sqrt{s}=13$ TeV, using 140 fb$^{-1}$ of data. The analysis employs an $e^{\pm}\mu^{\mp}\nu\bar{\nu}\gamma$ final state and a multivariate approach to extract the signal, achieving a significance of $5.9\sigma$ and a fiducial cross-section of $6.2\pm0.8\,(stat)\pm0.6\,(syst)$ fb, in agreement with the SM prediction of $6.1^{+1.0}_{-0.7}$ fb. A comprehensive EFT interpretation is performed, constraining 13 dimension-8 operators that affect anomalous quartic gauge couplings, using high-$p_T$ photon events and unitarity clipping to derive 95% CL limits on $f_i/\Lambda^4$. The results provide the first WWγ-channel constraints on aQGCs and complement existing multiboson measurements, refining our understanding of EW gauge interactions and potential BSM effects at high energies.

Abstract

This Letter reports the observation of $W^{+}W^{-}γ$ triboson production in 140 fb$^{-1}$ of data collected by the ATLAS detector from proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV at the LHC. Events with an opposite-charge $eμ$ pair, a high transverse-momentum photon, and significant missing transverse momentum are considered. The observed (expected) significance of the signal is 5.9 (6.0) standard deviations. The measured fiducial cross-section, defined for the $W^{+}W^{-}γ\to e^{\pm}μ^{\mp}ν\barνγ$ final state is 6.2 $\pm$ 0.8 (stat.) $\pm$ 0.6 (sys.) fb, in good agreement with the Standard Model prediction of 6.1$^{+1.0}_{-0.7}$ fb. Constraints on the Wilson coefficients of 13 dimension-8 operators describing physics beyond the Standard Model through anomalous quartic gauge-boson couplings are derived using the effective field theory framework.

Figures (6)

• Figure 1: Representative leading-order Feynman diagrams for the $\PW\PW\gamma$ production process with a triple or quartic gauge-boson coupling vertex shown in red: (a) diagram featuring a QGC vertex (b,c) diagrams with a TGC vertex, and (d) a diagram showing photon radiation from an initial-state quark. Additional contributions arise from other diagrams not shown here.
• Figure 2: Comparison between data and prediction ("Pred.") for the four most discriminating BDT input variables after the fit to data ("Post-Fit") under the signal-plus-background hypothesis, shown in the SR. The variables displayed are (a) $m_\text{T}(\ell_{1}, \MET)$, (b) $\Delta R(jj)$, (c) $m(\ell\ell)$, and (d) $\pT(j_{2})$. The uncertainty band includes both the statistical and systematic uncertainties as obtained from the fit. The rightmost bin includes overflow events. The lower panels show the ratio of data to prediction. Open markers indicate data points lying outside the vertical range of the plot.
• Figure 3: Comparison between data and the post-fit prediction ("Pred."), under the signal-plus-background hypothesis, from the distributions of the boosted decision tree output ($O_{\text{BDT}}$) in the (a) $\PZ\gamma$ CR1, (b) $\PZ\gamma$ CR2, (c) $t\bar{t}\gamma$ CR, (d) $e\rightarrow\gamma$ CR, and (e) $e^{\pm}\mu^{\mp}\nu\bar{\nu}\gamma$ SR. The uncertainty band includes both the statistical and systematic uncertainties as obtained from the fit. The lower panels show the ratio of data to prediction. The open markers indicate data points lying outside the vertical range of the lower panel in (d).
• Figure 4: The 95% confidence interval of the $f_\mathrm{M3}/\Lambda^4$ Wilson coefficient as a function of the energy threshold used in the clipping method to restore unitarity. The blue dash-dotted lines indicate the unitarity bound. The black solid lines show the observed upper and lower limits, while the red dashed lines show the expected upper and lower limits.
• Figure 5: Comparison between data and prediction ("Pred.") for the jet multiplicity distribution after the fit to data ("Post-Fit") under the signal-plus-background hypothesis, shown in the SR. The uncertainty band includes both statistical and systematic uncertainties as obtained from the fit. The rightmost bin includes overflow events.