K. S. Babu

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 boson, 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[sup 2][theta][sub [ital W]] is maintained [ital as] [ital a] [ital 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).

New vector - scalar contributions to neutrinoless double beta decay and constraints on R-parity violation

We show that in the minimal supersymmetric standard model (MSSM), with {ital R}-parity breaking as well as in the left-right symmetric model, there are new observable contributions to neutrinoless double beta decay ({beta}{beta}{sub 0{nu}}) arising from hitherto overlooked diagrams involving the exchange of one {ital W} boson and one scalar boson. In particular, in the case of MSSM, the present experimental bounds on {beta}{beta}{sub 0{nu}} lifetime improves the limits on certain {ital R}-parity violating couplings by about 2 orders of magnitude. It is shown that similar diagrams also lead to enhanced rates for {mu}{sup {minus}}{r_arrow}{ital e}{sup +} conversion in nuclei, which are in the range accessible to ongoing experiments.

Mass matrix textures from superstring inspired SO(10) models.

We give a general prescription for deriving quark and lepton mass matrices with ``texture`` zeros in the framework of superstring inspired SO(10) models. The key to our approach is a new way to naturally implement the doublet-triplet splitting which enables us to obtain symmetric quark and lepton mass matrices which have different structures in the up and down quark sectors. We illustrate our method by deriving the Georgi-Jarlskog texture which has six predictions in the flavor sector, and then show how it generalizes to other symmetric texture models.

7 keV Scalar Dark Matter and the Anomalous Galactic X-ray Spectrum

We present a simple model for a 7 keV scalar dark matter particle which also explains the recently reported anomalous peak in the galactic X-ray spectrum at 3.55 keV in terms of its two photon decay. The model is arguably the simplest extension of the Standard Model, with the addition of a real scalar gauge singlet field subject to a reflection symmetry. This symmetry breaks spontaneously at an energy scale of a few MeV which triggers the decay of the dark matter particle into two photons. In this framework, the Higgs boson of the Standard Model is also the source of dark matter in the Universe. The model fits the relic dark matter abundance and the partial lifetime for two photon decay, while being consistent with constraints from domain wall formation and dark matter self-interactions. We show that all these features of the model are preserved in its natural embedding into a simple dark

Post-Sphaleron Baryogenesis

U(1)

Supersymmetric SO(10) simplified

gauge theory with a Higgs mechanism. The properties of the dark photon get determined in such a scenario. High precision cosmological measurements can potentially test these models, as there are residual effects from domain wall formation and non-negligible self-interactions of dark matter.

Constraints on left-right symmetric models from the process b→sγ

We present a new mechanism for generating the baryon asymmetry of the Universe directly in the decay of a singlet scalar field S(r) with a weak scale mass and a high dimensional baryon number-violating coupling. Unlike most currently popular models, this mechanism, which becomes effective after the electroweak phase transition, does not rely on the sphalerons for inducing a nonzero baryon number. CP asymmetry in S(r) decay arises through loop diagrams involving the exchange of W+/- gauge bosons and is suppressed by light quark masses, leading naturally to a value of eta(B) approximately 10(-10). The simplest realization of this idea which uses a six quark DeltaB=2 operator predicts colored scalars accessible to the CERN Large Hadron Collider and neutron-antineutron oscillation within reach of the next-generation experiments.

Proton decay and realistic models of quark and lepton masses

In the context of supersymmetric SO(10) grand unified models, it is shown that the gauge symmetry breaking as well as a natural doublet-triplet splitting can be achieved with a minimal Higgs system consisting of a single adjoint and a pair of vector and spinor multiplets. Such a Higgs spectrum has been shown to arise in the free fermionic formulation of superstrings. Since the symmetry-breaking mechanism relies on nonrenormalizable operators, some of the Higgs particles of the model turn out to have masses somewhat below the GUT scale. As a consequence, the unification scale is raised to about 2[times]10[sup 17] GeV and sin[sup 2][theta][sub [ital W]] is predicted to be slightly larger than the minimal SUSY SU(5) value. Including threshold uncertainties, which turn out to be surprisingly small in the model, we show that the sin[sup 2][theta][sub [ital W]] prediction is consistent with experiments.

An SO(10) solution to the puzzle of quark and lepton masses.

Abstract In left-right symmetric models, large contributions to the decay amplitude b→sγ can arise from the mixing of the WL and WR gauge bosons as well as from the charged Higgs boson. These amplitudes are enhanced by the factor m t m b compared to the contributions in the standard model. We use the recent CLEO results on the radiative B decay to place constraints on the WL−WR mixing angle ζ and the mass of the charged Higgs boson mH±. Significant departures from the standard model predictions occur when |ζ|≳0.003 and/or when mH±≲ a few TeV.

Solution to the small-phase problem of supersymmetry.

Abstract It is shown that in realistic SUSY GUT models of quark and lepton masses both the proton decay rate and branching ratios differ in general from those predicted in the minimal SU (5) supersymmetric model. The observation of proton decay, and in particular the branching ratio B[ (p → π + ν ) (p → K + ν ) ] , would thus allow decisive tests of these fermion mass schemes. It is shown that the charged lepton decay mode p → K 0 μ + arising through gluino dressing diagrams is significant and comparable to the neutrino modes in large tanβ models. Moreover, it is found that in certain classes of models the higgsino-mediated proton decay amplitudes are proportional to a model-dependent group-theoretical factor which in some cases can be quite small. There is thus a natural suppression mechanism which can explain without adjustment of parameters why in the context of SUSY GUTs proton decay has not yet been seen. The most interesting such class consists of SO (10) models in which the dominant flavor-symmetric contribution to the up-quark mass matrix comes from an effective operator of the form 16 i 16 j 10 H 45 H , where 〈 45 H 〉 points approximately in the I 3 R -direction. This class includes a recent model of quark and lepton masses proposed by the authors.