Triple-gauge couplings in LHC diboson production: a SMEFT view from every angle

TL;DR

This work provides a comprehensive NLO QCD SMEFT analysis of fully leptonic diboson production at the LHC, focusing on CP-even and CP-odd dimension-six triple-gauge-coupling operators. By comparing inclusive and fiducial setups, and by examining polarisation-sensitive observables, angular coefficients, and boost/asymmetry observables, it demonstrates how fiducial cuts and high-energy tails enhance SMEFT sensitivity, including interference resurrection for certain observables. The study performs detailed fits to ATLAS/CMS-like data, yielding improved constraints on the CP-even Wilson coefficient $c_W$ (and, with quadratic terms, on the CP-odd coefficient $c_{\widetilde{W}}$), while showing WW production offers weaker sensitivity due to helicity and scale effects. The results underscore the necessity of NLO EFT predictions and polarisation analyses for robust SMEFT constraints and lay the groundwork for future two-dimensional CP-even/CP-odd fits and expanded operator sets in global analyses.

Abstract

This study explores fully leptonic WZ and WW production at the LHC within the SMEFT framework at NLO in QCD, focusing on both CP-even and CP-odd triple-gauge-coupling dimension-six operators. We investigate the off-shell processes, contrasting our findings in inclusive setups with those in the presence of realistic fiducial selections. Alongside the conventional kinematic observables, we examine polarisation-sensitive observables and angular coefficients. Moreover, we assess potential SMEFT effects on asymmetry observables. Through a sensitivity analysis, we identify critical LHC observables that are particularly sensitive to SMEFT-induced modifications, thereby shedding light on potential avenues for new-physics searches in diboson production at the LHC.

Figures (15)

• Figure 1: Distributions in the reconstructed four-lepton invariant mass in W$^+$Z production at the LHC@13TeV, for the inclusive (left) and fiducial ATLAS setup (right): effect of the inclusion of the CP-even operator ${O}_{3{W}}$ with WCs set to $1{\rm TeV}^{-2}$. Main panels: absolute differential cross-sections at LO (dashed) and NLO QCD (solid), for the SM (red), for the sum of the SM and the SMEFT linear term (green), and also including the quadratic SMEFT term (blue). First/second inset: ratio of the linear/quadratic SMEFT term (at LO and NLO QCD) over the NLO QCD SM cross-section. Third inset: QCD K-factors (NLO QCD over LO).
• Figure 2: Distributions in the $\text{Z}\xspace$-boson transverse momentum in W$^+$Z production at the LHC@13TeV, for the inclusive (left) and fiducial ATLAS setup (right): effect of the inclusion of the CP-even operator ${O}_{3{W}}$ with WCs set to $1~{\rm TeV}^{-2}$. Same structure as \ref{['fig:m4lrec']}.
• Figure 3: Distributions in the azimuthal-angle separation between the positron and the antimuon in W$^+$Z production at the LHC@13TeV, for the inclusive (left) and fiducial ATLAS setup (right): effect of the inclusion of the CP-even operator ${O}_{3{W}}$, with Wilson coefficient set to $1{\rm TeV}^{-2}$. Same structure as \ref{['fig:m4lrec']}.
• Figure 4: Dependence of the angular coefficients $A_0,\,A_2,\,A_4,\,A_7$ of the W (top) and Z boson (bottom) in W$^+$Z production at the LHC@13TeV on the Wilson coefficient value for the CP-even operator ${O}_{3{W}}$ (left) and of the CP-odd one $O_{3\widetilde{W}}$ (right). The inclusive setup (see Eq. \ref{['eq:mllcut']}) is understood. The dashed and solid curves correspond respectively to the $A^{(1)}$ and $A^{(2)}$ SMEFT parametrisations of the coefficients defined in Eq. \ref{['eq:coef_par']}.
• Figure 5: Distributions in the azimuthal decay angle of the positron in the W-boson rest frame, for a longitudinal (0), left-handed (L) and right-handed (R) W boson, in W$^+$Z production at the LHC@13TeV. The results are shown for an inclusive setup at the MC-truth level (left) and for the ATLAS fiducial setup after neutrino reconstruction (right). Polarised-boson signals have been computed in the SM at NLO QCD accuracy with PowHeg-Box-ResChiesa:2020ttlPelliccioli:2023zpd.