Renormalizable Adjoint SU(5)

We investigate the possibility to find the simplest renormalizable grand unified theory based on the SU(5) gauge symmetry. We find that it is possible to generate all fermion masses with only two Higgs bosons, 5_H and 45_H. In this context the neutrino masses are generated through the type III and type I seesaw mechanisms. The predictions coming from the unification of gauge couplings and the stability of the proton are discussed in detail. In this theory the leptogenesis mechanism can be realized through the out of equilibrium decays of the fermions in the adjoint representation.Comment: 6 pages, minor corrections, to appear in Physics Letters

Predictions from type II see-saw mechanism in SU(5)

We propose a simple, testable, SU(5) model within the context of the type II neutrino see-saw mechanism. It is based on requiring renormalizability, the absence of any other matter fields besides those already present in the Standard Model and consistency with all experimental data. These "minimal" requirements, together with group-theoretical considerations, uniquely determine the model and lead to interesting implications. The model predicts correlation between a light SU(2) triplet boson responsible for the type II see-saw mechanism and observable proton decay signatures. It also allows for an enhanced production of doubly charged Higgs particles through the WW fusion process due to a built-in custodial symmetry. This could also have profound impact on the explicit realization of electroweak symmetry breaking. The model also predicts the existence of a light scalar that transforms as a colour octet and electroweak doublet, with interesting phenomenological consequences.Comment: 19 pages, 1 figure, references adde

On Unification and Nucleon Decay in Supersymmetric Grand Unified Theories Based on SU(5)

We investigate the unification constraints in the minimal sypersymmetric grand unified theories based on SU(5) gauge symmetry. The most general constraints on the spectrum of minimal supersymmetric SU(5) and flipped SU(5) are shown. The upper bound on the mass of the colored Higgs mediating proton decay is discussed in detail in the context of the minimal supersymmetric SU(5). In the case of the minimal SUSY SU(5) we show that if we stick to the strongest bound on the colored triplet mass coming from dimension five proton decay contributions there is no hope to test this model at future nucleon decay experiments through the dimension six operators. We find a lower bound on the partial proton decay lifetime for all relevant channels in the context of flipped SUSY SU(5). We conclude that flipped SUSY SU(5) might be in trouble if proton decay is found at the next generation of experiments with a lifetime below 10^{37} years.Comment: 17 pages, 6 figures, some corrections and references adde

Heavy and light scalar leptoquarks in proton decay

We list scalar leptoquarks that mediate proton decay via renormalizable couplings to the Standard Model fermions. We employ a general basis of baryon number violating operators to parameterize contributions of each leptoquark towards proton decay. This then sets the stage for investigation of bounds on the leptoquark couplings to fermions with respect to the most current Super Kamiokande results on proton stability. We quantify if, and when, it is necessary to have leptoquark masses close to a scale of grand unification in the realistic SU(5) and flipped SU(5) frameworks. The most and the least conservative lower bounds on the leptoquark masses are then presented. We furthermore single out a leptoquark without phenomenologically dangerous tree-level exchanges that might explain discrepancy of the forward-backward asymmetries in $t \bar t$ production observed at Tevatron, if relatively light. The same state could also play significant role in explaining muon anomalous magnetic moment. We identify contributions of this leptoquark to dimension-six operators, mediated through a box diagram, and tree-level dimension-nine operators, that would destabilize proton if sizable leptoquark and diquark couplings were to be simultaneously present.Comment: 26 pp, 2 figures, extensive expansion of Section V with new result

Minimal SO(10) splits supersymmetry

A good fit of the fermion masses and mixings has been found in the minimal renormalizable supersymmetric SO(10). This solution needs a strongly split supersymmetry breaking scenario with gauginos and higgsinos around 100 TeV, sfermions close to 10^14 GeV and a low GUT scale of around 6 10^15 GeV. We predict fast proton decays through SO(10) type of d=6 operators and the leptonic mixing angle theta_13 approximately 0.1.Comment: 26 pages, 3 figure