Electroweak Restoration: SMEFT and HEFT

TL;DR

This paper investigates electroweak restoration at high energies using SMEFT and HEFT frameworks, deriving amplitude ratios and identifying key observables to distinguish between linear and non-linear electroweak symmetry realizations.

Contribution

It provides a comprehensive analysis of electroweak restoration in SMEFT and HEFT, deriving amplitude ratios and proposing experimental observables for future collider tests.

Findings

Ratios of high-energy longitudinal di-boson amplitudes approach one in SM and SMEFT.

The cross section ratio of WZ to Wh is a promising observable to probe EW restoration.

Projected HL-LHC sensitivities can distinguish linear from non-linear EW symmetry realizations.

Abstract

Colliders continue to push our understanding of electroweak (EW) interactions to ever higher energies. At high energies, many observables in the broken EW theory are expected to approach the unbroken theory. This is the electroweak restoration regime, where longitudinal gauge boson production corresponds to Goldstone boson production. As such, in this paper we investigate electroweak restoration in the context of linear and non-linear realizations of the EW symmetry in longitudinal di-boson production: $f\bar{f}'\rightarrow V_LV'_L$ and $f\bar{f}'\rightarrow V_Lh$, where $V_L,V'_L$ are longitudinal gauge bosons, $h$ is the Higgs boson, and $f,f'$ are SM fermions. For the linear beyond the SM (BSM) theory, we use the Standard Model Effective Theory (SMEFT), and for the non-linear BSM theory, we use Higgs Effective Field Theory (HEFT). We give a general discussion of these amplitudes and cross sections in the SM, SMEFT, and HEFT. Using the Goldstone boson equivalence theorem, we derive a set of ratios among high energy $V_LV'_L$ and $V_Lh$ amplitudes that are expected to approach one in the SM and SMEFT. Through our explicit calculations, we show that these ratios do indeed approach one in the SM and dimension-6 SMEFT, but not necessarily HEFT. Beyond the amplitudes, both theoretically and experimentally, we identify the cross section ratio of $W^\pm_LZ_L$ and $W^\pm_L h$ as a particularly promising observable to probe EW restoration and distinguish HEFT and SMEFT. Using current LHC measurements as well as projections for $W^\pm_L h$ measurements, we project HL-LHC sensitivities to probing the linear vs. non-linear realizations of the EW symmetry.