TTP15-05 Decoupling of heavy quarks at four loops and effective Higgs-fermion coupling

We compute the decoupling constant ζm relating light quark masses of effective nl-flavour QCD to (nl+1)-flavour QCD to four-loop order. Immediate applications are the evaluation of the MS charm quark mass with five active flavours and the bottom quark mass at the scale of the top quark or even at GUT scales. With the help of a low-energy theorem ζm can be used to obtain the effective coupling of a Higgs boson to light quarks with five-loop accuracy. We briefly discuss the influence on Γ(H → bb̄). PACS numbers: 12.38.-t, 12.38.Bx, 14.65.Dw, 14.65.Fy 1 Introduction an