Helicity fractions of W bosons from top quark decays at next-to-next-to-leading order in QCD

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