Why the angular distribution of the top decay lepton is unchanged by anomalous $tbW$ coupling

TL;DR

This paper explains why the angular distribution of decay leptons from top quarks remains unaffected by anomalous $tbW$ couplings, emphasizing the role of top polarization.

Contribution

It provides a simple physical argument demonstrating the independence of lepton angular distribution from anomalous couplings at linear order.

Findings

Lepton angular distribution depends solely on top polarization.

Anomalous $tbW$ couplings do not alter the distribution at linear order.

The result simplifies analysis of top decay measurements.

Abstract

We give a simple physical argument to understand the observation that the angular distribution of the top decay lepton depends only on the polarisation of the top and is independent of any anomalous $tbW$ coupling to linear order.