# Top quark decay at next-to-leading order in the Standard Model Effective Field Theory

## TL;DR

This paper calculates the top quark decay width and W-boson helicity fractions at next-to-leading order in the Standard Model Effective Field Theory, including all relevant operators, to improve bounds on new physics from current and future collider data.

## Contribution

It provides the first complete NLO calculation of top decay in SMEFT, including four-fermion and dipole operators, and discusses technical aspects like gamma_5 treatment for future studies.

## Key findings

- Operators appearing at NLO can be constrained by current data.
- NLO effects are crucial for accurate bounds on new physics.
- The calculation framework aids future higher-order SMEFT analyses.

## Abstract

We consider top quark decay in the Standard Model Effective Field Theory (SMEFT). We present a calculation of the total decay width and the $W$-boson helicity fractions at next-to-leading order (NLO) in SMEFT. Our result includes the complete set of contributing four-fermion operators in addition to QCD dipole operators and bottom-mass suppressed effects. We show that operators that first appear at NLO in the SMEFT can be bounded by the current data as well as future data from both a high-luminosity LHC and a potential $e^+e^-$ collider, demonstrating the importance of going beyond leading order when studying the SMEFT. We discuss technical aspects of our calculation that we believe will be useful in future higher-order studies of the SMEFT, in particular the treatment of $\gamma_5$ in loop diagrams.