New physics in t-> b W decay at next-to-leading order in QCD

TL;DR

This paper investigates how non-standard effective operators affect top quark decay at next-to-leading order in QCD, highlighting the enhanced impact of the dipole operator on W helicity fractions and deriving bounds from experimental data.

Contribution

It provides the first detailed NLO QCD analysis of non-standard tbW operators' effects on top decay, especially on W helicity fractions, and compares bounds from different experimental measurements.

Findings

Dipole operator O_{LR} significantly enhances transverse-plus W helicity fraction F_+ at NLO.

Current measurements of F_L impose the most stringent bounds on O_{LR}.

The bounds from F_L are stronger than those from B -> X_s gamma decay.

Abstract

We consider contributions of non-standard tbW effective operators to the decay of an unpolarized top quark into a bottom quark and a W gauge boson at next-to-leading order in QCD. We find that the dipole operator O_{LR} contribution to the transverse-plus W helicity fraction F_+ is significantly enhanced compared to the leading order result at non-vanishing bottom quark mass. Nonetheless, presently the most sensitive observable to direct O_{LR} contributions is the longitudinal W helicity fraction F_L. In particular, the most recent CDF measurement of F_L already provides the most stringent upper bound on O_{LR} contributions, even when compared with indirect bounds from the rare decay B -> X_s gamma.