Tree-level Flavor Changing Neutral Currents Research Articles

Phenomenology and cosmology of extra generations of Higgs bosons.

If the Higgs sector (like the fermion sector) in a supersymmetric model consists of several generations, then a basis in which only one generation gets vacuum values can be chosen. The other generations have been largely ignored, but can be interesting. For example, in the minimal model, if tree-level flavor-changing neutral currents are eliminated by some symmetry, then the lightest of these scalars is stable and the second lightest is only slightly more massive. The lightest is a superb dark-matter candidate, and if light enough, gives an unusual and detectectable signature at the SLAC Linear Collider or the CERN {ital e}{sup +}{ital e}{sup {minus}} collider LEP.

The flipped SU(5)×U(1) string model revamped

We present a refined version of our three-generation flipped ,SU(5)×U(1) string model with the following properties. • -The complete massless spectrum is derived and shown to be free of all gauge and mixed anomalies apart from a single anomalous U(1). • -The imaginary part of the dilaton supermultiplet is eaten by the anomalous U(1) gauge boson, and the corresponding D-term is cancelled by large VEVs for singlet fields that break surplus U(1) gauge factors, leaving a supersymmetric vacuum with an SU(5)×U(1) visible gauge group and an SO(10)×SO(6) hidden gauge group. • -There are sufficient Higgs multiplets to break the visible gauge symmetry down to the standard model in an essentially unique way. • -All trilinear superpotential couplings have been calculated and there are in particular some giving m t , m b , m τ ≠0. • -A renormalization group analysis shows that m t <190 GeV and m b ≅3 m τ . • -Light Higgs doublets are split automatically from heavy Higgs triplets, leaving no residual dimension-five operators for baryon decay, and the baryon lifetime τ B ∼ 2×10 34±2 yr. • -There are no tree-level flavour-changing neutral currents, but μ→ eγ may occur at a detectable level: B( μ→ eγ) ∼ 10 −11 −10 −14.

Left-right symmetry and finite one-loop Dirac neutrino mass

We present a modified version of the left-right symmetric model that includes a charged singlet scalar boson where the neutrino is massless at the tree level and acquires a smal and finite Dirac mass at the one-loop level. In the case if three families of fermions, it predicts m ve=0 and the other two neutrinos are massive with v τ lighter than v v . The model also predicts large magnetic moments and v μ → v e+ ψ decay rates. The new feature of the model is the inclusion of singlet charged heavy fermions which mix with the light quarks and leptons, leading to the existence of tree-level flavor-changing neutral currents, which can provide tests of the model.

Simple discrete symmetries in phenomenologically viable E6 superstring models.

We analyze the simplest discrete symmetries of the superpotential of superstring-inspired E/sub 6/ models necessary to simultaneously suppress (i) proton decay, (ii) neutrino masses, and (iii) tree-level flavor-changing neutral currents in E/sub 6/ superstring models without any intermediate scale. We show that there are two and only two different Z/sub 2/ symmetries that incorporate (i)--(iii). These symmetries put restrictions on the decays of exotic particles; in particular, the exotic quarks always decay semileptonically. A Z/sub 3/-symmetric model satisfying (i)--(iii) and with ..mu -->..e..gamma.. forbidden is also constructed.

Phenomenologically viable models from superstrings?

The phenomenology of a class of models based on superstring theories is discussed. If the supersymmetry breaking mechanism generates masses for scalars which are much larger than those for gauginos, then all models with only the weak and Planck scales are ruled out, provided a discrete symmetry eliminates tree-level flavor changing neutral currents. Some restrictions on models with an intermediate scale are also given.