NEUTRAL-CURRENT IN THE HEAVY TOP-QUARK LIMIT AND THE RENORMALIZATION OF THE SINGLET AXIAL CURRENT

We use a simple renormalization group argument to demonstrate that the large top mass limit of the axial neutral current in QCD is unambiguo usly determined by the renormalization of the singlet axial current wh ich suggests a natural scheme independent normalization condition for the latter. By calculating the anomalous dimension of the singlet axia l current at three-loop level in the MSBAR scheme we derive the result for the top quark decoupling from the axial neutral current valid up to (and including) the terms of order alpha(s)2.