Higgs masses in the standard,multi-Higgs and supersymmetric models

Theoretical constraints and limits on the masses of Higgs scalars in the standard electroweak model, in electroweak models with additional Higgs doublets and in various supersymmetric models are presented. In the standard model, the lower limit on the Higgs mass, based on vacuum stability arguments, is reviewed in detail, as are “upper limits” based on perturbative constraints. In most grand unified and all supersymmetric models, however, at least two doublets are needed. The masses of the various Higgs scalars in the two-doublet model are discussed and constraints on their masses are found, including the generalization of the above limits. The results are then generalized to models with more than two doublets. Finally, recent attempts at constructing models with low-energy supersymmetry are reviewed and it is shown that in many models, fairly stringent tree-level mass relations among the Higgs scalars can be found. These relations are interesting in that they do not refer to the supersymmetric partners of ordinary particles, and they are most restrictive in models in which the supersymmetry is explicitly broken, i.e., via arbitrary mass terms.     

Prospects for superstring models with two vacuum expectation values for standard model singlets

The spontaneous breaking of rank-six intermediate groups to the standard model in superstring phenomenology is examined. It requires the existence of two vacuum expectation values, one for each standard model singlet in the 27 chiral representation of E₆. We use a simplified model based on the group SU(3)_c×SU(2)_L×SU(2)_R×U(1)_L×U(1)_R. It is found that the well-known phenomenological constraints such as proton decay, vanishing neutrino masses and flavour-changing neutral currents allow for the possibility of sizeable Yukawa couplings for the two standard model singlets which can originate low energy negative masses. The proof of this radiative breaking relies, however, on several parameters whose magnitude is unpredicted, but is facilitated if the mass of the exotic quark is large (∼300 GeV).     

Gauge-invariant inflaton in the minimal supersymmetric standard model

We argue that all the necessary ingredients for successful inflation are present in the flat directions of the Minimally Supersymmetric Standard Model. We show that out of many gauge-invariant combinations of squarks, sleptons, and Higgs bosons, there are two directions, LLe and udd, which are promising candidates for the inflaton. The model predicts more than 10(3) e-foldings, with an inflationary scale of H(inf) approximately O(1-10) GeV, provides a tilted spectrum with an amplitude of delta(H) approximately 10(-5) and a negligible tensor perturbation. The temperature of the thermalized plasma could be as low as T(rh) approximately O(1-10) TeV. Parts of the inflaton potential can be determined independently of cosmology by future particle physics experiments.     

LEP Higgs boson searches beyond the standard model and minimum supersymmetric standard model

Ever since the center-of-mass energy was increased in 1995 above the Zℴ resonance, the four LEP experiments (ALEPH, DELPHI, OPAL and L3) have renewed their effort to search for the Higgs boson. Data taking ended in the year 2000 with about 130 pb⁻¹ of data collected per experiment above 206 GeV ine ⁺ e ⁻¹ collisions but the data analysis is still very active. Most recently, the wealth of theoretical models and predictions has stimulated new analyses and model interpretations which go beyond the standard model and minimal supersymmetric standard model. These include the searches for charged Higgs bosons, models with two Higgs field doublets, searches for ‘fermiophobic’ Higgs decay, invisible Higgs boson decays, decay-mode independent searches, and limits on Yukawa and anomalous Higgs couplings. I review the searches done by the four LEP experiments and present the LEP combined results when they exist.     

Nonlinear supersymmetric standard model and its phenomenology

We consider the Higgs sector of a nonlinear supersymmetric standard model. Analytic solutions for some useful mass relations are derived. We investigate physical consequences of the model for LEP 1, LEP 2, and 500, 1000 resp. 2000 GeV LCs, and show that the neutral scalar Higgs bosons will most probably enable LC 500, 1000, 2000 to test the model conclusively, whereas at LEP 1, 2 only a part of the parameter space can be explored. As for the two pseudoscalar Higgs bosons the lighter one cannot be detected at any of the considered colliders, whereas the heavier one could possibly be detected with about “10 percent probability” at LC 1000 and LC 2000.     

Charged current universality in the minimal supersymmetric standard model

We compute the complete one-loop contributions to low-energy charged current weak interaction observables in the minimal supersymmetric standard model (MSSM). We obtain the constraints on the MSSM parameter space which arise when precision low-energy charged current data are analyzed in tandem with measurements of the muon anomaly. While the data allow the presence of at least one light neutralino, they also imply a pattern of mass splittings among first and second generation sleptons and squarks which contradicts predictions of widely used models for supersymmetry-breaking mediation.     

Neutrino charge radius in the minimal supersymmetric standard model

As recently suggested by Degrassi, Marciano, and Sirlin, the electromagnetic form factor of the neutrino extracted from low energy elastic scattering cross sections ofv e and can be used to define the neutrino charge radius in the Standard Model in a gauge-invariant way. The complete one-loop contributions to the form factor consist not only the induced vertex but also the Z vertex corrections and theW andZ propagators arise from the counterterm diagrams, and theWW andZZ box diagrams. We show that the neutrino charge radius is a finite and gauge independent quantity in the Minimal Supersymmetric Standard Model. We consider only the oblique corrections to the neutrino charge radius in our numerical work, because the vertex and box contributions are small. The dependence of the oblique corrections on the supersymmetric parameters in each of the three sectors: Higgs, scalar matter and gaugino sectors are studied by taking into account of the current experimental bounds from LEP.     

Electric dipole moments in the generic supersymmetric standard model

The generic supersymmetric standard model is a model built from a supersymmetrized standard model field spectrum and the gauge symmetries only. The popular minimal supersymmetric standard model differs from the generic version in having R parity imposed by hand. We review an efficient formulation of the model and some of the recently obtained interesting phenomenological features, focusing on one-loop contributions to fermion electric dipole moments.