QCD effects and search for new physics in t -->b W

TL;DR

This paper analyzes the impact of new physics operators on top quark decay to W bosons, highlighting the enhanced dipole contributions at NLO and the sensitivity of the longitudinal W helicity fraction to new physics.

Contribution

It provides a comprehensive NLO analysis of effective operators affecting top decay and compares flavor constraints with collider measurements.

Findings

Dipole operator enhances transverse-plus W helicity fraction F+ at NLO.

Longitudinal W helicity fraction FL is most sensitive to new physics.

Flavor observables impose stronger constraints than collider data.

Abstract

The most general set of new physics effective operators contributing to the decay of an unpolarized top quark into a bottom quark and a W gauge boson are considered at next-to-leading order in QCD. We find that the dipole operator contribution to the transverse-plus W helicity fraction F+ is enhanced compared to the leading order result at non-vanishing bottom quark mass. Nonetheless, presently the observable most sensitive to new physics contributions is the longitudinal W helicity fraction FL. We also investigate constraints on t W b couplings coming from precision flavor observables, in particular from Delta B=1 and Delta B=2 transitions of Bq mesons. We find these to be mostly superior to present direct constraints coming from top decay and single top production measurements at the LHC and Tevatron.