Negative delta rho with four families in the standard model.

Abstract

The possible consequences of heavy, fourth generation fermion, ``vector-meson'' bound states, formed by the exchange of the Higgs boson, are discussed. In particular, it is suggested that such bound states could make negative contributions to the \ensuremath{\rho} parameter, thereby loosening the perturbative constraint upon the mass of the top quark and/or the masses, mass splittings, and other characteristics of a potential fourth generation of heavy fermions. Phenomenological implications such as ``monojet'' events, coming from the decay of a heavy neutral lepton in Z\ensuremath{\rightarrow}N\ifmmode\bar\else\textasciimacron\fi{}${\ensuremath{\nu}}_{\mathrm{\ensuremath{\tau}}}$+N\ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{\ensuremath{\tau}}}$, as well as their correlations to the flavor-changing decay Z\ensuremath{\rightarrow}b\ifmmode\bar\else\textasciimacron\fi{}s+bs\ifmmode\bar\else\textasciimacron\fi{} are also discussed. The analysis also extends to the standard model with two Higgs doublets.