Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)




Marc Sher


Physics that might lie beyond the standard model is discussed, with special emphasis on two possible extensions of the standard model: multi-Higgs extension, and the fourth generation leptons.;In multi-Higgs-boson extensions of the standard model, tree-level flavor-changing neutral currents exist naturally. It is often believed that the presence of the tree-level FCNC in this model is fatal, unless a discrete symmetry is added, since it requires the exchanged scalar to be extremely heavy. This follows, however, from the assumption that the flavor-changing coupling is quite large. Using a more natural value for the flavor-changing coupling, much smaller bounds were obtained. Unlike previous calculations, we calculate the bounds on the flavor-changing couplings of an additional scalar for the processes involving the third generation fields. We find that in the quark sector the strongest bound comes from {dollar}B - \=B{dollar} mixing and in the lepton sector, surprisingly, from {dollar}\mu \to e\gamma{dollar}. It is then noted that flavor-changing coupling in the quark sector are related to those in the lepton sector in many grand unified theories. Comparing flavor-changing B decays with rare {dollar}\tau{dollar} decays, we find that rare B decays provide the strongest bounds and the most promising decay models are {dollar}B \to K\mu\tau{dollar} and {dollar}B\sb{lcub}s{rcub} \to \mu\tau{dollar}.;If a fourth generation of leptons exists, both the neutrino and the charged lepton must have masses in excess of 45 GeV. It is certainly possible that the neutrino will be heavier than, or comparable in mass to, the charged lepton. In this case, the charged lepton can only decay through mixing with lighter generations, and might thus be extremely long-lived. First, we investigate the implications of very long-lived charged leptons for cosmology and astrophysics. We calculate the bounds on the mass and lifetime of long-lived charged particles which arise from terrestrial and cosmology. The strongest bounds come from the requirement that the decay products not unacceptably distort the cosmic microwave background. Phenomenological implications are also briefly discussed. Second, we study the production cross-sections and signatures for the SSC and LHC. Four models are considered. Two are fourth-generation extensions of the standard model in which the right-handed heavy leptons are either isosinglets or in an isodoublet; the other two are motivated by the aspon model of CP violation. The detection methods for these heavy leptons are also discussed.



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