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

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.