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→bb¯) .