Measurement and interpretation of inclusive $Wγ$ production in proton-proton collisions at $\sqrt{s}=13$ TeV using the ATLAS detector

Abstract

Differential cross-section measurements are presented for the production of a $W$ boson in association with a photon. The analysis is performed using proton--proton collision data collected by the ATLAS experiment at $\sqrt{s}= 13$ TeV, corresponding to an integrated luminosity of 140~fb$^{-1}$. The differential cross sections are measured in the $Wγ\rightarrow \ell νγ$ decay channel ($\ell=e,μ$) as a function of 16 observables. Collectively, these observables probe the kinematic properties of the $Wγ$ system, the radiation amplitude zero effect predicted for the $Wγ$ final state, the polarisation of the $W$ boson, the charge conjugation and parity structure of the $WWγ$ triple gauge coupling, and the parton distribution functions of the proton. The data are corrected for the effects of detector inefficiency and resolution and are sufficiently precise that they can be used to distinguish between different state-of-the-art theoretical predictions provided by SHERPA, MADGRAPH5_aMC@NLO, and GENEVA. The differential cross sections are used to search for anomalous weak-boson self-interactions induced by dimension-six operators within an effective field theory. For CP-odd operators, dedicated detector-corrected observables based on the outputs of neural networks are found to be particularly sensitive to the interference between the Standard Model and dimension-six scattering amplitudes. Constraints are placed on the Wilson coefficients of the $\mathcal {o}_{W}$, $\mathcal{o}_{HWB}$, $\mathcal{o}_{\tilde{W}}$ and $\mathcal{o}_{H{\tilde{W}B}}$ operators in the effective field theory. The sensitivity to the $\mathcal{o}_{H{\tilde{W}B}}$ operator is improved by a factor of 2.5 compared to previous measurements in other final states.

Figures (14)

• Figure 1: $W\gamma$ production in the $s$-channel (left) and $t$-channel (right). The $s$-channel scattering amplitude contains the $WW\gamma$ triple gauge coupling.
• Figure 2: The reference frame used to measure the $W\rightarrow \ell\nu$ decay angles, $\theta_f$, and $\phi_f$, is constructed as follows. First, the measured missing transverse momentum is taken as a proxy for the neutrino transverse momentum; the neutrino longitudinal momentum is determined, with a two-fold ambiguity, by assuming a $W\rightarrow \ell\nu$ decay and imposing a $W$ boson mass constraint on the $\ell\nu\gamma$ system. One of the two solutions is chosen at random. Second, the $z$ axis is taken to be the $W$ boson direction in the centre-of-mass frame of the $\ell\nu\gamma$ system. A vector, $\hat{\textbf{ r}}$, is defined in the centre-of-mass frame that defines the boost direction to the lab frame. The $y$ axis is defined by $\hat{\textbf{ y}} = \hat{\textbf{ z}} \times \hat{\textbf{ r}}$ and the $x$ axis is defined as $\hat{\textbf{ x}} = \hat{\textbf{ y}} \times \hat{\textbf{ z}}$. The azimuthal decay angle, $\phi_f$, is taken to be the azimuthal angle of the negative helicity fermion ($f_{-}$) in this special frame. The polar decay angle, $\theta_f$, is defined in the rest frame of the $W$ boson relative to the $W$ boost direction.
• Figure 3: Predicted and observed yields as a function of (a) $p_\textrm{T}^\gamma$, (b) $\eta_\gamma$, (c) $m_{\ell \gamma}$ and (d) $\Delta\eta_{\ell\gamma}$. The data are represented as black points and the associated error bars represent the statistical uncertainty. Background processes with prompt leptons and photons, such as $Z\gamma$ production and the fully leptonic decays of $t\bar{t}\gamma$ production, are estimated by using simulations and labelled as 'Prompt.' Backgrounds arising from non-prompt leptons ($j\rightarrow \ell$), non-prompt photons ($j\rightarrow \gamma$), electrons-faking-photons ($e\rightarrow \gamma$) and pile-up photons are estimated using data-driven techniques as outlined in Section \ref{['sec:backgrounds']}. The total uncertainty on the combined signal and background prediction is shown as a grey band (the calculation of these uncertainties is outlined in Section \ref{['sec:systematics']}).
• Figure 4: Predicted and observed yields as a function of $O_{\mathrm{NN}}$ for (a) the $\mathcal{O}_{H{\tilde{W}B}}$ operator and for (b) the $\mathcal{O}_{\tilde{W}}$ operator. The data are represented as black points and the associated error bars represent the statistical uncertainty. Background processes with prompt leptons and photons are estimated by using simulations and labelled as 'Prompt'. Backgrounds arising from non-prompt leptons ($j\rightarrow \ell$), non-prompt photons ($j\rightarrow \gamma$), electrons-faking-photons ($e\rightarrow \gamma$) and pile-up photons are estimated using data-driven techniques. The total uncertainty on the combined signal and background prediction is shown as a grey band.
• Figure 5: Relative uncertainties on the differential cross-sections as a function of (a) $p_\textrm{T}^\gamma$, (b) $\eta_\gamma$, (c) $\Delta\eta_{\ell\gamma}$ and (d) $O_{\mathrm{NN}}$. Sources of uncertainty are grouped together for clarity. The total systematic uncertainty is shown as the quadrature sum of each source of uncertainty.