Neutrinoless Double Beta Decay and Physics Beyond the Standard Model

The various mechanisms for neutrinoless double beta decay in gauge theories are reviewed and the present experimental data is used to set limits on physics scenarios beyond the standard model. The positive indications for nonzero neutrino masses in various experiments such as those involving solar, atmospheric and accelerator neutrinos are discussed and it is pointed out how some neutrino mass textures consistent with all data can be tested by the ongoing double beta decay experiments. Finally, the outlook for observable neutrinoless double beta decay signal in grand unified theories is discussed.Comment: 21 pages, latex; 10 figures available on request; Invited talk presented at the "International Workshop on Neutrinoless Double Beta Decay and relate topics", Trento, Italy; April, 1995; to appear in the proceedings to be published by World scientifi

Gauged Flavor, Supersymmetry and Grand Unification

I review a recent work on gauged flavor with left-right symmetry, where all masses and all Yukawa couplings owe their origin to spontaneous flavor symmetry breaking. This is suggested as a precursor to a full understanding of flavor of quarks and leptons. An essential ingredient of this approach is the existence of heavy vector-like fermions, which is the home of flavor, which subsequently gets transmitted to the familiar quarks and leptons via the seesaw mechanism. I then discuss implications of extending this idea to include supersymmetry and finally speculate on a possible grand unified model based on the gauge group $SU(5)_L\times SU(5)_R$ which provides a group theoretic origin for the vector-like fermions.Comment: Invited talk at the GUT 2012 workshop held in Kyoto, Japan in March, 201

TeV Scale Universal Seesaw, Vacuum Stability and Heavy Higgs

We discuss the issue of vacuum stability of standard model by embedding it within the TeV scale left-right universal seesaw model (called SLRM in the text). This model has only two coupling parameters $(\lambda_1, \lambda_2)$ in the Higgs potential and only two physical neutral Higgs bosons $(h, H)$. We explore the range of values for $(\lambda_1, \lambda_2)$ for which the light Higgs boson mass $M_h=126$ GeV and the vacuum is stable for all values of the Higgs fields. Combining with the further requirement that the scalar self couplings remain perturbative till typical GUT scales of order $10^{16}$ GeV, we find (i) an upper and lower limit on the second Higgs $(H)$ mass to be within the range: $0.4 \leq \frac{M_H}{v_R}\leq 0.7$, where the $v_R$ is the parity breaking scale and (ii) that the heavy vector-like top, bottom and $\tau$ partner fermions ($P_3, N_3, E_3$) mass have an upper bound $M_{P_3, N_3, E_3} \leq v_R$. We discuss some phenomenological aspects of the model pertaining to LHC.Comment: 21 pages, 7 figures, some typos corrected and references updated, accepted for publication in JHE

A Supersymmetric Solution to CP Problems

We analyze the minimal supersymmetric left-right model with non-re\-nor\-malizable interactions induced by higher scale physics and study its {\it CP} violating properties. We show that it: (i) solves the strong {\it CP} problem; (ii) predicts the neutron electric dipole moment well within experimental limits (thus solving the usual SUSY {\it CP} problem). In addition, it automatically conserves {\it R}-parity. The key points are that the parity symmetry forces the Yukawa couplings to be hermitean, while supersymmetry ensures that the scalar potential has a minimum with real higgs doublet vacuum expectation values. Gluino and B-L gaugino masses are automatically real. The observed {\it CP} violation in the kaon system comes, as in the Standard Model, from the Kobayashi-Maskawa-type phases. These solutions are valid for any value of the right-handed breaking scale $M_R$, as long as the effective theory below $M_R$ has only two Higgs doublets that couple fully to fermions. ({\it i.e.} the theory below $M_R$ is MSSM-like.) The potentially dangerous $SU(2)_L$ gaugino one-loop contributions to $\bar{\Theta}$ below $M_R$ can be avoided if the left-right symmetry originates from a unified theory in which the $SU(2)_{L,R}$ gaugino masses are real. As an example, we show how the left-right symmetry can be embedded in an SO(10) theory.Comment: 23 pages, plain Latex. A discussion of one-loop contributions to the gluino mass phase and some references added; misprints and a minor error in Appendix C corrected; notation improve

Leptogenesis with TeV Scale WRW_R

Successful leptogenesis within the conventional TeV-scale left-right implementation of type-I seesaw has been shown to require that the mass of the right-handed $W_R^\pm$ boson should have a lower bound much above the reach of the Large Hadron Collider. This bound arises from the necessity to suppress the washout of lepton asymmetry due to $W_R^\pm$-mediated $\Delta L\neq 0$ processes. We show that in an alternative quark seesaw realization of left-right symmetry, the above bound can be avoided. Lepton asymmetry in this model is generated not via the usual right-handed neutrino decay but rather via the decay of new heavy scalars producing an asymmetry in the $B-L$ carrying Higgs triplets responsible for type-II seesaw, whose subsequent decay leads to the lepton asymmetry. This result implies that any evidence for $W_R$ at the LHC 14 will point towards this alternative realization of left-right symmetry, which is also known to solve the strong CP problem.Comment: 8 page

Proton Decay and Flavor Violating Thresholds in SO(10) Models

Discovery of neutrino mass has put the spotlight on supersymmetric SO(10) as a natural candidate for grand unification of forces and matter. However, the suppression of proton decay is a major problem in any supersymmetric grand unified models. In this paper we show how to alleviate this problem by simple threshold effect which raises the colored Higgsino masses and the grand unification scale to \gtrsim 10^{17} GeV. There exist only four types of fields arising from different SO(10) representations which can generate this kind of threshold effects. Some of these fields also generate a sizable flavor violation in the quark sector compared to the lepton sector. The b-\tau unification can work in these types of models even for intermediate values of tan\beta.Comment: 5 pages, 1 figur

LHC Accessible Second Higgs Boson in the Left-Right Model

A second Higgs doublet arises naturally as a parity partner of the standard model (SM) Higgs, once SM is extended to its left-right symmetric version (LRSM) to understand the origin of parity violation in weak interactions as well as to accommodate small neutrino masses via the seesaw mechanism. The flavor changing neutral Higgs (FCNH) effects in the minimal version of this model (LRSM), however, push the second Higgs mass to more than 15 TeV making it inaccessible at the LHC. Furthermore since the second Higgs mass is directly linked to the $W_R$ mass, discovery of a "low" mass $W_R$ ($M_{W_R}\leq 5-6$ TeV) at the LHC would require values for some Higgs self couplings larger than one. In this paper we present an extension of LRSM by adding a vector-like $SU(2)_R$ quark doublet which weakens the FCNH constraints allowing the second Higgs mass to be near or below TeV and a third neutral Higgs below 3 TeV for a $W_R$ mass below 5 TeV. It is then possible to search for these heavier Higgs bosons at the LHC, without conflicting with FCNH constraints. A right handed $W_R$ mass in the few TeV range is quite natural in this class of models without having to resort to large scalar coupling parameters. The CKM mixings are intimately linked to the vector-like quark mixings with the known quarks, which is the main reason why the constraints on the second Higgs mass is relaxed. We present a detailed theoretical and phenomenological analysis of this extended LR model and point out some tests as well as its potential for discovery of a second Higgs at the LHC. Two additional features of the model are: (i) a 5/3 charged quark and (ii) a fermionic top partner with masses in the TeV range.Comment: 22 pages, 4 figures, lots of stuff moved to the appendices, errors and typos corrected, version to appear in PR