Mediated Proton Decay Research Articles

Two-loop renormalization group analysis of supersymmetric SO(10) models with an intermediate scale

Two-loop evolutions of the gauge couplings in a class of intermediate-scale supersymmetric SO(10) models including the effect of third generation Yukawa couplings are studied. The unification scale, the intermediate scale and the value of the unification gauge coupling in these models are calculated and the gauge boson mediated proton decay rates are estimated. In some cases the predicted proton lifetime turns out to be on the borderline of experimental limit. The predictions of the top quark mass, the mass ratio m b ( m b )/ m τ ( m τ ) from the two-loop evolution of Yukawa couplings and the mass of the left-handed neutrino via a see-saw mechanism are summarized. The lower bounds on the ratio of the VEVs of the two low energy doublets (tanβ) from the requirement of the perturbative unitarity of the top quark Yukawa coupling up to the grand unification scale are also presented. All the predictions have been compared with those of the one-step unified theory.

Natural suppression of Higgsino-mediated proton decay in supersymmetric SO(10).

In supersymmetric Grand Unified Theories, proton decay mediated by the color--triplet higgsino is generally problematic and requires some fine--tuning of parameters. We present a mechanism which naturally suppresses such dimension 5 operators in the context of SUSY $SO(10)$. The mechanism, which implements natural doublet--triplet splitting using the adjoint higgs, converts these dimension 5 operators effectively into dimension 6. By explicitly computing the higgs spectrum and the resulting threshold uncertainties we show that the successful prediction of $\sin^2\theta_W$ is maintained {\it as a prediction} in this scheme. It is argued that only a weak suppression of the higgsino mediated proton decay is achievable within SUSY $SU(5)$ without fine--tuning, in contrast to a strong suppression in SUSY $SO(10)$.

A supersymmetric SU(4)XO(4) model

A simple N=1 supersymmetric model is described, based on the group G=SU(4)×SU(2) L×SU(2) R. It can in principle be derived from the four-dimensional fermionic superstring and has the following attractive features: there are no large Higgs representations, the coloured triplets which mediate proton decay become naturally heavy, while the mass relations m e= m d at M GUT are retained. The right-handed neutrinos acquire large masses whilst their left-handed partners remains essentially massless.

A supersymmetric gut

We propose a grand unified supersymmetric theory based on SU(5) with spontaneously broken supersymmetry. The theory (really a class of theories) is completely realistic. In particular, supersymmetry partners of ordinary fermions and bosons are heavy. The model requires one fine-tuning in order to render the color triplet partners of the Higgs fields (which mediate proton decay) superheavy. This fine-tuning is stable against radiative corrections. At the tree level, the model contains two scales, the unification scale, of order 10 16 GeV, and the supersymmetry breaking scale, of order 10 10 GeV. The breaking of SU(2) × U(1) invariance arises as a radiative effect. The lightest of the new particles implied by supersymmetry are expected to have masses of order tens of GeV.