Unitarity bounds and sum rules in the SMEFT

TL;DR

This paper reevaluates perturbative unitarity bounds in the SMEFT using spinor-helicity methods, revealing that theoretical constraints can surpass experimental bounds at multi-TeV energies, especially for four-fermion operators.

Contribution

It introduces a new formalism for deriving unitarity bounds in SMEFT and demonstrates their strength compared to experimental limits, particularly with sum rules for four-fermion operators.

Findings

Unitarity bounds can be stronger than experimental bounds above a few TeV.

Spinor-helicity techniques enable a comprehensive reassessment of bounds.

Sum rules further tighten constraints on four-fermion operators.

Abstract

We present a comprehensive reassessment of perturbative unitarity bounds in the dimension-six Standard Model Effective Field Theory, exploiting a new formalism based on spinor-helicity techniques to derive partial-wave unitarity bounds for generic $N \to M$ scattering amplitudes. We find that, in several cases, these theoretical constraints are already competitive with, or even stronger than, the corresponding experimental bounds for energy scales above a few TeV. This is especially the case for four-fermion operators under realistic flavor assumptions, where unitarity bounds can be further strengthened by exploiting sum rules.