Distinguishing between SU(5) and flipped SU(5)

We study in detail the d=6 operators for proton decay in the two possible matter unification scenarios based on SU(5) gauge symmetry. We investigate the way to distinguish between these two scenarios. The dependence of the branching ratios for the two body decays on the fermion mixing is presented in both cases. We point out the possibility to make a clear test of flipped SU(5) through the decay channel p \to \pi^+ \bar{\nu}, and the ratio \tau(p \to K^0 e^+_{\alpha}) / \tau(p \to \pi^0 e^+_{\alpha}).Comment: 10 pages, minor correction

Could we rotate proton decay away?

In this work we investigate the possibility to completely rotate away proton decay. We show that by choosing specific mass matrices for fermions it is possible to accomplish this in flipped SU(5).Comment: 6 pages, RevTeX4, minor correction

Fermion mixings vs d=6 proton decay

It is well known, although sometimes ignored, that not only the d=5 but also d=6 proton decay depends on fermion mixings. In general we study carefully the dependence of d=6 decay on fermion mixings using the effective operator approach. We find that without specifying a theory it is impossible to make clear predictions. Even in a given model, it is often not possible to determine all the physical parameters. We point out that it is possible to make a clear test of any grand unified theory with symmetric Yukawa couplings. We discuss in some detail realistic theories based on SU(5) and SO(10) gauge symmetry.Comment: 12 pages, two new references, a new equation, and few correction

Analysis of Couplings with Large Tensor Representations in SO(2N) and Proton Decay

We develop techniques for the analysis of SO(2N) invariant couplings which allow a full exhibition of the SU(N) invariant content of the spinor and tensor representations. The technique utilizes a basis consisting of a specific set of reducible SU(N) tensors in terms of which the SO(2N) invariant couplings have a simple expansion. The technique is specially useful for couplings involving large tensor representations. We exhibit the technique by performing a complete determination of the trilinear couplings in the superpotential for the case of SO(10) involving the 16 plet of matter, i.e., we give a full determination of the ${16-16-10_s}$, ${16-16-120_a}$ and ${16-16-\bar{126}_s}$ couplings. The possible role of large tensor representations in the generation of quark lepton textures is discussed. It is shown that the couplings involving $\bar{126}$ dimensional representation generate extra zeros in the Higgs triplet textures which can lead to an enhancement of the proton decay lifetime by a factor of $10^3$. These results also have implications for neutrino mass textures.Comment: 14 pages, Latex: With the corrections of an Erratum included in the text of pape

SUSY GUTs under Siege : Proton Decay

SO(10) supersymmetric grand unified theories [SUSY GUTs] provide a beautiful framework for physics beyond the standard model. Experimental measurements of the three gauge couplings are consistent with unification at a scale $M_G \sim 3 \times 10^{16}$ GeV. In addition predictive models for fermion masses and mixing angles have been found which fit the low energy data, including the recent data for neutrino oscillations. SO(10) boundary conditions can be tested via the spectrum of superparticles. The simplest models also predict neutron and proton decay rates. In this paper we discuss nucleon decay rates and obtain reasonable upper bounds. A clear picture of the allowed SUSY spectra as constrained by nucleon decay is presented.Comment: 13 page

Large Angle MSW Solution in Grand Unified Theories with SU(3) X U(1) Horizontal Symmetry

We construct a model with a SU(3) X U(1) horizontal symmetry in the context of Grand Unified Theories. In our models, the bi-maximal lepton mixing and suitable neutrino masses for the large angle MSW solution are obtained without any fine-tuning due to the spontaneously broken SU(3)_H symmetry. The three generations of quarks and leptons are unified as members of the SU(3)_H fundamental representation, and the U(1)_H charge gives the origin of the fermion mass hierarchy and mixing angles. We present two explicit examples of SU(5)_GUT and SO(10)_GUT models, in which the Yukawa structures are given successfully.Comment: 23 pages, 6 figures, references adde

Do experiments suggest a hierarchy problem?

The hierarchy problem of the scalar sector of the standard model is reformulated, emphasizing the role of experimental facts that may suggest the existence of a new physics large mass scale, for instance indications of the instability of the matter, or indications in favor of massive neutrinos. In the see-saw model for the neutrino masses a hierarchy problem arises if the mass of the right-handed neutrinos is larger than approximatively $10^7$ GeV: this problem, and its possible solutions, are discussed.Comment: revtex, 4 pages, 1 figur

Fermion masses, neutrino oscillations, and proton decay in the light of SuperKamiokande

Within the framework of unified gauge models, interactions responsible for neutrino masses can also provide mechanisms for nucleon instability. We discuss their implications concretely in the light of recent results on neutrino oscillation from the SuperKamiokande collaboration. We construct a predictive SO(10)-based framework that describes the masses and mixing of all quarks and leptons. An overconstrained global fit is obtained, that makes five successful predictions for quarks and charged leptons. The same description provides agreement with the SuperK results on atmospheric neutrinos and supports a small-angle MSW mechanism. We find that current limits on nucleon stability put significant stress on the framework. Further, a distinctive feature of the SO(10) model developed here is the likely prominence of the $\mu^+ K^0$ mode in addition to the $\bar{\nu} K^+$ mode of proton decay. Thus improved searches in these channels for proton decay will either turn up events, or force us outside this circle of ideas.Comment: 57 pages in RevTex, typos correcte

SUSY GUT Model Building

I discuss an evolution of SUSY GUT model building, starting with the construction of 4d GUTs, to orbifold GUTs and finally to orbifold GUTs within the heterotic string. This evolution is an attempt to obtain realistic string models, perhaps relevant for the LHC. This review is in memory of the sudden loss of Julius Wess, a leader in the field, who will be sorely missed.Comment: 24 pages, 14 figures, lectures given at PiTP 2008, Institute for Advanced Study, Princeton, to be published in the European Physical Journal

Violations of fundamental symmetries in atoms and tests of unification theories of elementary particles

High-precision measurements of violations of fundamental symmetries in atoms are a very effective means of testing the standard model of elementary particles and searching for new physics beyond it. Such studies complement measurements at high energies. We review the recent progress in atomic parity nonconservation and atomic electric dipole moments (time reversal symmetry violation), with a particular focus on the atomic theory required to interpret the measurements.Comment: 103 pages, 23 figures; submitted to Physics Reports; comments welcom