Stuart Raby

Fermion masses and induction of the weak scale by supergravity

Abstract We discuss some problems which arise in a low-energy theory of strong and electroweak interactions when the weak scale is induced via supergravity. The main problem concerns the origin of quark and lepton masses. If the fermions obtain their masses in the conventional way via Yukawa couplings, then the low-energy supergap (mass difference between supersymmetric partners) is comparable to the mass of the lightest massive fermion. One solution would require that at least the light generations receive their mass indirectly via radiative corrections. Alternatively it might be that the SU(2) × U(1) breaking is induced radiatively.

Cosmological problems for the polonyi potential

We study the cosmological implications of N = 1 supergravity with the Polonyi potential. We find that for typical values of the gravitino mass (102-103 GeV) the universe goes through a late period of reheating (i.e., from a temperature of about 10 -7 MeV to 10 -2 MeV). Any baryon-to-photon ratio is thus diluted by an unacceptable 15 orders of magnitude, with no hope of regeneration.

New flavor violations in supergravity models

Abstract The supersymmetric standard model has mild flavor violations, especially if the top quark is not very heavy. A new supersymmetric mechanism for flavor violation is introduced. It will occur in almost all unified schemes. This mechanism can greatly affect the chiral symmetry structure of the theory. The down-quark mass can be generated by weak-scale radiative corrections even if it is zero at tree level. The tree-level SU(5) mass relation m e / m ω = m d / m s is no longer problematic, and an explanation of m u / m d is given. The value of the down-quark mass can be linked to flavor-changing processes of B mesons: maximal B d 0 − B d 0 mixing; and branching ratios of 1 10 for B → n π and 10 −3 for B → (hard γ + X) induced by the two-body b decays b → dg and b → d γ .

A Mini-landscape of exact MSSM spectra in heterotic orbifolds

We explore a “fertile patch” of the heterotic landscape based on a 6-II orbifold with SO(10) and E6 local GUT structures. We search for models allowing for the exact MSSM spectrum. Our result is that of order 100 out of a total 3 × 10 4 inequivalent models satisfy this requirement.

Tumbling Gauge Theories

We consider a class of gauge theories in which chiral fermion fields are coupled to gauge fields. If the fermion content does not form a real representation of the gauge group, these theories may undergo dynamical symmetry breaking. In all but the simplest cases the breakdown appears to occur in a sequence of steps leading to a hierarchy of mass scales. This may be related to the hierarchal character of quark and lepton masses.

Searching for realistic 4d string models with a Pati-Salam symmetry: Orbifold grand unified theories from heterotic string compactification on a Z(6) orbifold

Motivated by orbifold grand unified theories, we construct a class of three-family Pati-Salam models in a Z(6) Abelian symmetric orbifold with two discrete Wilson lines. These models have marked differences from previously-constructed three-family models in prime-order orbifolds. In the limit where one of the six compactified dimensions (which lies in a Z(2) sub-orbifold) is large compared to the string length scale, our models reproduce the supersymmetry and gauge symmetry breaking pattern of 5d orbifold grand unified theories on an S-1/Z(2) orbicircle. We find a horizontal 2 divided by 1 splitting in the chiral matter spectra-2 families of matter are localized on the Z(2) orbifold fixed points. and 1 family propagates in the 5d bulk-and identify them as the first-two and third families. Remarkably. the first two families enjoy a non-Abelian dihedral D-4 family symmetry. due to the geometric setup of the compactified space. In all our models there are always some color triplets. i.e.. (6.1.1) representations of the Pati-Salam group, survive orbifold projections. They could be utilized to spontaneously break the Pati-Salam symmetry to that of the Standard Model. One model. with a 5d E-6 symmetry. may give rise to interesting low energy phenomenology. We study gauge coupling unification. allowed Yukawa couplings and some of their phenomenological consequences. The E-6 model has a renormalizable Yukawa coupling only for the third family. It predicts a gauge-Yukawa unification relation at the 5d compactification scale, and is capable of generating reasonable quark/lepton masses and mixings. Potential problems are also addressed, they may point to the direction for refining our models

PROTON DECAY IN SUPERSYMMETRIC MODELS

Abstract We discuss proton decay in supersymmetric theories. We find that it is possible to obtain rates which are comparable with those of standard SU(5). In the presence of a discrete symmetry which occurs in an SU(5) supersymmetric unified model we obtain a definite prediction for the dominant decay mode, i.e. p → K+−νμ and n → K0−νμ.

Stringy origin of non-Abelian discrete flavor symmetries

Abstract We study the origin of non-Abelian discrete flavor symmetries in superstring theory. We classify all possible non-Abelian discrete flavor symmetries which can appear in heterotic orbifold models. These symmetries include D 4 and Δ ( 54 ) . We find that the symmetries of the couplings are always larger than the symmetries of the compact space. This is because they are a consequence of the geometry of the orbifold combined with the space group selection rules of the string. We also study possible breaking patterns. Our analysis yields a simple geometric understanding of the realization of non-Abelian flavor symmetries.

Experimental signatures of low energy gauge-mediated supersymmetry breaking.

The experimental signatures for gauge mediated supersymmetry breaking are presented. The phenomenology associated with this class of models is distinctive since the gravitino is naturally the LSP. The next lightest supersymmetric particle (NLSP) can be a gaugino, Higgsino, or right handed slepton. Decay of the NLSP to its partner plus the LSP proceeds through the Goldstino component of the gravitino. For a significant range of parameters this decay can take place within the detector, and can be measured as a displaced vertex or kink in a charged particle track. In the case that the NLSP is mostly gaugino, we identify the discovery modes as

Light Composite Fermions

e^+e^- \rightarrow \gamma \gamma + \Emiss