On interference and non-interference in the SMEFT

TL;DR

This paper investigates how interference effects between SMEFT operators and the Standard Model are suppressed or missed in certain limits and approximations, emphasizing the importance of off-shell calculations for accurate phenomenology.

Contribution

It demonstrates that common approximations like the narrow width approximation can overlook interference effects, and provides explicit calculations to uncover these effects in SMEFT scattering processes.

Findings

Interference effects can be suppressed in the high-energy limit for certain helicity configurations.

On-shell approximations may miss interference terms present in the full phase space.

Off-shell calculations reveal interference effects overlooked by narrow width approximation.

Abstract

We discuss interference in the limit $\hat{m}_{W}^2/s \rightarrow 0$ in the Standard Model Effective Field Theory (SMEFT). Dimension six operators that contribute to $\bar{\psi} \psi \rightarrow \bar{\psi'}_1 \psi_2' \bar{\psi'}_3 \psi'_4$ scattering events can experience a suppression of interference effects with the Standard Model in this limit. This occurs for subsets of phase space in some helicity configurations. We show that approximating these scattering events by $2\rightarrow 2$ on-shell scattering results for intermediate unstable gauge bosons, and using the narrow width approximation, can miss interference terms present in the full phase space. Such interference terms can be uncovered using off-shell calculations as we explicitly show and calculate. We also study the commutation relation between the SMEFT expansion and the narrow width approximation, and discuss some phenomenological implications of these results.