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. 
S. Fajfer, J. Drobnak and J. F. Kamenik
hep-ph