Helicity fractions of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>W</mml:mi></mml:math>bosons from top quark decays at next-to-next-to-leading order in QCD

Abstract

Decay rates of unpolarized top quarks into longitudinally and transversally polarized $W$ bosons are calculated to second order in the strong coupling constant ${\ensuremath{\alpha}}_{s}$. Including the finite bottom quark mass and electroweak effects, the standard model predictions for the $W$-boson helicity fractions are ${\mathcal{F}}_{L}=0.687(5)$, ${\mathcal{F}}_{+}=0.0017(1)$, and ${\mathcal{F}}_{\ensuremath{-}}=0.311(5)$.