Higgs Boson Mass as the Discriminator of Electroweak Models

In the standard model (SM), vacuum stability implies a lower limit on the Higgs boson mass. In supersymmetric (SUSY) extensions, there is an upper bound on the lightest Higgs boson mass. We show that for a top quark mass $m_t\lesssim 165$ GeV, a gap exists between the SM and both the minimal (MSSM) and next-to-minimal SUSY model [(M $+$ 1)SSM] bounds. Thus, if the new $m_t$ measurement by the Collider Detector at Fermilab remains valid, a first measurement of the Higgs boson mass will exclude either the SM or the simplest SUSY Higgs sectors. We further discuss SUSY grand unified models, other extensions of the SM, and the potential for discovery of a light Higgs boson.