GUT Model Hierarchies from Intersecting Branes

Abstract

By employing D6-branes intersecting at angles in D = 4 type I strings, we construct the first examples of three generation string GUT models (PS-A class), that contain at low energy exactly the standard model spectrum with no extra matter and/or extra gauge group factors. They are based on the group SU(4) C × SU(2) L × SU(2) R . The models are non-supersymmetric, even though SUSY is unbroken in the bulk. Baryon number is gauged and its anomalies are cancelled through a generalized Green-Schwarz mechanism. We also discuss models (PS-B class) which at low energy have the standard model augmented by an anomaly free U(1) symmetry and show that multibrane wrappings correspond to a trivial redefinition of the surviving global U(1) at low energies. There are no colour triplet couplings to mediate proton decay and proton is stable. The models are compatible with a low string scale of energy less that 650 GeV and are directly testable at present or future accelerators as they predict the existence of light left handed weak fermion doublets at energies between 90 and 246 GeV. The neutrinos get a mass through an unconventional see-saw mechanism. The mass relation m e = m d at the GUT scale is recovered. Imposing supersymmetry at particular intersections generates non-zero Majorana masses for right handed neutrinos as well providing the necessary singlets needed to break the surviving anomaly free U(1), thus suggesting a gauge symmetry breaking method that can be applied in general left-right symmetric models.

Proton stability in grand unified theories, in strings and in branes

“…An interesting class of models are those based on intersecting D branes [355][356][357][358][359] and attempts have been made to build semi-realistic models based on these [360][361][362][363][364][365], and issues of gauge coupling unification, soft breaking and possible applications to the real world have also been discussed [366][367][368] (For reviews see Ref. [369][370][371]).…”

Getting just the supersymmetric standard model at intersecting branes on theZ6orientifold

“…In such an approach, a model containing only the standard model matter in the chiral sector has been obtained [20] (for similar subsequent models see [21,22]). But shortly afterwards, it was realized that this model is unstable due to the uncanceled NS ÿ NS-tadpoles becoming manifest in the runaway behavior of the complex structure and dilaton moduli [23,24] (see also the remarks in [25]).…”

Constraining the string scale: from Planck to weak and back again

“…The first models containing just the SM were presented in [28]. Since then, a great deal of effort has gone into into the study of the consistency and stability [29] and phenomenological implications of intersecting brane models, from the construction of supersymmetric models [23], gauge symmetry breaking [30], GUT or realistic SM constructions [31,32] to cosmological implications [33]. In the following we will review some of these developments paying particular attention to their flavour structure [34,35,36] and its profound experimental implications.…”

“…Therefore we seem to be forced back to high string scales solely on experimental grounds. A great deal of effort has been dedicated to the study of realistic supersymmetric models (in order to protect the hierarchy of scales) with a high string scale, of the order of the Planck or GUT scales [23,31] and it seems likely that it is to these that we must now turn.…”