Gauge symmetry breaking by boundary conditions on a manifold is known to be equivalent to Wilson-line breaking through a background gauge field, and is therefore spontaneous. These equivalent pictures are related by a nonperiodic gauge transformation. However, we find that boundary condition gauge symmetry breaking on orbifolds is explicit; there is no gauge where all the breaking can be attrib...

A 6-dimensional grand unified theory with the compact space having the topology of a real projective plane, i.e., a 2-sphere with opposite points identified, is considered. The space is locally flat except for two conical singularities where the curvature is concentrated. One supersymmetry is preserved in the effective 4d theory. The unified gauge symmetry, for example SU(5), is broken only by ...

It is shown that all possible gravitational, gauge and other interactions experienced by particles in ordinary d-dimensions (one-time) can be described in the language of two-time physics in a spacetime with d + 2 dimensions. This is obtained by generalizing the worldline formulation of two-time physics by including background fields. A given two-time model, with a fixed set of background field...

In this letter we study the low-energy phenomenology and cosmological implications of supersymmetric grand unified theory with asymmetric flavor structure, which is suggested by the recent observation of fermion masses and mixing angles. The predictions of the scenario are rather dependent on a Yukawa parameter fixed by group-theoretical argument. A reduced value of the parameter gives a resolu...

Requiring that supersymmetric SU(5) and SU(6) grand unifications in the heterotic string theory must have 3 chiral families, adjoint (or higher representation) Higgs fields in the grand unified gauge group, and a non-abelian hidden sector, we construct such string models within the framework of free conformal field theory and asymmetric orbifolds. Within this framework, we construct all such st...

It is demonstrated how the hierarchy between the gauge coupling unification scale of minimal supersymmetry and the Planck (or string) scale, which resembles in order of magnitude a loop factor, can actually be explained as such in supergravity-coupled supersymmetry. A gauge and global singlet field acquires a linear potential term due to its one-loop supergravity interactions and slides to the ...

It is shown that the recently proposed preon model which provides a unified origin of the diverse mass scales and an explanation of family replication as well as of inter–family mass–hierarchy, naturally possesses a Peccei–Quinn (PQ) symmetry whose spontaneous breaking leads to an automatic invisible axion. Existence of the PQ–symmetry is simply a consequence of supersymmetry and the requiremen...

We present two 3-family SU(5) grand unified models in the heterotic string theory. One model has 3 chiral families and 9 pairs of 5+ 5 Higgs fields, and an asymptotically-free SU(2) ⊗ SU(2) hidden sector, where the two SU(2)s have different matter contents. The other model has 6 left-handed and 3 righthanded 10s, 12 left-handed and 9 right-handed 5s, and an asymptotically-free SU(3) hidden sect...

A glossary overview of the phenomenological studies of quasi–realistic free fermionic heterotic string models is presented. I elaborate on the correspondence of these models with Z2 × Z2 orbifolds. Introduction The standard model of particle physics passes all experimental observations with flying colors. The gauge charges of the standard model matter states are strongly suggestive of the embed...