(Non)decoupling of the Higgs triplet effects

We consider the electroweak theory with an additional Higgs triplet at one loop, using the hybrid renormalization scheme based on alpha(EM), G(F) and M-Z as input observables. We show that in this scheme loop corrections can in a natural way be split into a standard model part and corrections due to new physics. The latter, however, do not decouple in the limit of an infinite triplet mass parameter, if the triplet trilinear coupling to the SM Higgs doublets grows with the triplet mass. For electroweak observables computed at one loop this effect can be attributed to the radiative generation in this limit of a nonvanishing vacuum expectation value of the triplet. We also point out that whenever tree level expressions for the electroweak observables depend on vacuum expectation values of scalar fields other than the standard model Higgs doublet, a tadpole contribution to the oblique parameter T should in principle be included. The origin of nondecoupling is discussed also on the basis of symmetry principles in a simple scalar field theory.