Effect of a fourth fermion generation on the mt upper bound

Abstract

Abstract We discuss the possibility that the upper bound (mt)sup on the top mass may be relaxed due to the presence of a fourth generation of fermions. If the additional neutrino is a Dirac particle we find that, for arbitrary values of the masses of the extra quarks and leptons and of their mixings with the fermions of the third generation, (mt)sup can be increased by at most 7 GeV. We then consider the scenario in which, aside from the Dirac mass, the right-handed neutrino of the fourth generation is endowed with a Majorana mass of O (G μ − 1 2 ) . In this case we find a qualitatively different result: the contributions to Δr (Δp) are not mathematically bounded from above (below), so that there can be significant contributions of sign opposite to the top effect. Restricting the Dirac mass entry to ⩽300 GeV, we find that (mt)sup can be increased by at most 31 GeV.