Diboson production in the SMEFT at dimension-8

TL;DR

This paper assesses the impact of dimension-8 SMEFT operators on fully leptonic $WZ$ and $WW$ production at the LHC, focusing on operators with maximal energy growth in qq-initiated channels. It contrasts linear dimension-8 contributions with the squared dimension-6 terms, and tests whether a dimension-6 EFT interpretation remains robust when marginalising over dimension-8 effects, finding dim-8 contributions become subdominant only for new-physics scales $\Lambda\gtrsim 3~\text{TeV}$. The analysis combines NLO QCD SM and dim-6 predictions with LO dim-8 results across inclusive and differential observables, including polarization-sensitive angular coefficients, and uses current LHC data as well as HL-LHC projections. It identifies that in $WZ$ the linear dim-8 terms are generally subdominant to dim-6 squares, while in $WW$ the dim-8 operator $\mathcal{O}_{10}$ can be competitive, highlighting degeneracies between dim-8 coefficients and the need to include dim-8 effects for UV-agnostic EFT interpretations and more reliable constraints. The work also emphasizes the necessity of higher-order corrections for dim-8 EFT, and provides guidance on which observables robustly constrain dimension-6 parameters when dim-8 is neglected.

Abstract

We present a comprehensive analysis of dimension-8 and dimension-6 effects in fully leptonic $WZ$ and $WW$ production at the Large Hadron Collider (LHC) within the Standard Model (SM) Effective Field Theory (SMEFT). We focus on dimension-8 operators with maximal energy growth in the quark-(anti)quark-initiated production channel and assess their impact differentially through a variety of observables, including polarisation-sensitive ones. Leveraging existing data from measurements at the LHC, we perform fits to quantify the sensitivity of current and future data to dimension-8 effects and evaluate their interplay with squared dimension-6 contributions. By marginalising over the dimension-8 operators we examine the robustness of a dimension-6 SMEFT analysis in diboson production. We find that dimension-8 effects become subdominant only for new-physics scales above 3 TeV.

Figures (13)

• Figure 1: Distributions of the invariant mass of the four-lepton system, $m_{4l}$, (left) and the transverse momentum of the $Z$ boson, $p_T^{Z}$, (right) for $WZ$ production in the ATLAS fiducial setup. Predictions shown include the SM as well as linear and quadratic contributions of the dimension-6 SMEFT operator $\mathcal{O}_W$ computed at NLO in QCD, along with the linear dimension-8 contributions ($\mathcal{O}_{4}, \mathcal{O}_{6}, \mathcal{O}_{12}, \mathcal{O}_{13}$) at $\mathscr{O}(\Lambda^{-4})$ computed at LO. The inset displays the ratio of the EFT contributions to the SM. Some coefficients are scaled for visibility. Dashed lines indicate negative contributions.
• Figure 2: Same as \ref{['fig:wz_fid_m4l_ptz']} but for the azimuthal angle of $\mu$ (left), and the azimuthal separation between $e$ and $\mu$ (right).
• Figure 3: Same as \ref{['fig:wz_fid_m4l_ptz']} but for the polar angle of $e$ (left), and the rapidity separation between $Z$ and the lepton $l$ from the $W$ decay, $l_{W}$, (right).
• Figure 4: Differential cross sections in the azimuthal angle of $e$, including contributions from CP-odd dimension-6 and dimension-8 operators. Both panels correspond to the inclusive setup: the left plot uses truth-level information for the missing transverse momentum, whilst the right plot is based on reconstructed information.
• Figure 5: Distributions of the dilepton invariant mass, $m_{e\mu}$, in $WW$ production in the ATLAS fiducial (left) and the fully inclusive setup (right). The structure of the figure is similar to that of \ref{['fig:wz_fid_m4l_ptz']}.