arXiv subjects

K. S. Babu

Publications and source records attributed to K. S. Babu.

At least 163 records · Page 9Linked to original sources

arXiv2001

L_e + L_μ- L_τ- L_s Symmetry and a Mixed 2+2 Scenario for Neutrino Oscillations

Recent results from SuperKamiokande and SNO experiments have set severe constraints on possible mixings of a light sterile neutrino, ν_s, with the three active species required for a simultaneous explanation of the solar, atmospheric and LSND neutrino oscillation data. A consistent scheme has emerged from a global analysis of the data wherein two of the neutrinos are nearly degenerate with a mass of order 1 eV, that mix significantly with the two lighter states. We present realizations of such a mixed 2+2 oscillation scenario based on L_e + L_μ- L_τ- L_s symmetry (L_i stands for the ith lepton number). Breaking of of this lepton number symmetry by a small mass term for ν_s leads to the required large mixings for both the atmospheric and the solar neutrino oscillations. Sum rules for the neutrino oscillation parameters are derived within this scheme, and are shown to be consistent with present data. These models predict U_{e3} = 0.02-0.03, which can serve as a test of this idea. We also present gauge models based on mirror extensions of the Standard Model that naturally lead to a light sterile neutrino with the required mixing pattern.

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arXiv2001

Observable Neutron-Antineutron Oscillations in Seesaw Models of Neutrino Mass

We show that in a large class of supersymmetric models with spontaneously broken B-L symmetry, neutron--antineutron oscillations occur at an observable level even though the scale of B-L breaking is very high, v_{B-L} ~ 2 x 10^{16} GeV, as suggested by gauge coupling unification and neutrino masses. We illustrate this phenomenon in the context of a recently proposed class of seesaw models that solves the strong CP problem and the SUSY phase problem using parity symmetry. We obtain an upper limit on N-\bar{N} oscillation time in these models, τ_{N-\bar{N}} < 10^{9} -10^{10} sec. This suggests that a modest improvement in the current limit on τ_{N-\bar{N}} of 0.86 x 10^8 sec will either lead to the discovery of N-\bar{N} oscillations, or will considerably restrict the allowed parameter space of an interesting class of neutrino mass models.

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arXiv2001

Solving the Strong CP and the SUSY Phase Problems with Parity Symmetry

We propose a simultaneous solution to the strong CP problem and the SUSY phase problem based on parity symmetry realized when the supersymmetric standard model is embedded into a left-right symmetric framework at a scale near 2 x 10^{16} GeV, as suggested by neutrino masses and gauge coupling unification. In this class of models, owing to parity, SUSY contributions to the muon anomalous magnetic moment can be naturally large without conflicting with the EDM of the electron and the neutron. The strong CP violation parameter θis zero at the tree level, also due to parity (P), but is induced due to P-violating effects below the unification scale. We estimate the induced θto be < 10^{-16}, if we adopt a constrained supersymmetric spectrum with universal scalar masses. In the more general SUSY breaking scenario, after imposing flavor changing constraints, we find theta ~ (10^{-8}-10^{-10}), which is compatible with, but not much below the present limit on neutron EDM. We also argue that potential non-perturbative corrections to θfrom quantum gravitational effects are not excessive in these models.

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arXiv2001

Classification of Effective Neutrino Mass Operators

We present a classification of SU(3) x SU(2) x U(1) gauge invariant ΔL = 2 (L being lepton number) effective operators relevant for generating small Majorana neutrino masses. Operators of dimension up to 11 have been included in our analysis. This approach enables us to systematically identify interesting neutrino mass models. It is shown that many of the well-known models fall into this classification. In addition, a number of new models are proposed and their neutrino phenomenology is outlined. Of particular interest is a large class of models in which neutrinoless double beta decays arise at a lower order compared to the neutrino mass, making these decays accessible to the current round of experiments.

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arXiv2001

Solving the mu problem in gauge mediated supersymmetry breaking models with flavor symmetry

We suggest a solution to the μproblem of gauge mediated supersymmtery breaking models based on flavor symmetries. In this scenario the μterm arises through the vacuum expectation value of a singlet scalar field which is suppressed by a flavor symmetry factor relative to the scale of dynamical SUSY breaking. The same flavor symmetry also ensures that the soft SUSY breaking parameter Bμis not much larger than μ^2, a necessary condition for the stability of electroweak symmetry breaking. Explicit examples where Bμ~ μ^2 and Bμ<< μ^2 are presented. The latter case provides a natural solution to the supersymmetric CP problem. We show that the same flavor symmetry that suppresses the μand the Bμparameters can also play a role in explaining the fermion mass and mixing hierarchy puzzle.

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arXiv2000

The scaling of lepton dipole moments with lepton mass

The dipole moments of the leptons and quarks are matrices in flavor space, which can potentially reveal as much about the flavor structure of the theory as do the mass matrices. The off-diagonal elements of the dipole matrices lead to flavor-changing decays such as μ-> e γ, while the imaginary parts of the diagonal elements give rise to electric dipole moments. We analyze the scaling of the leptonic dipole moments with the lepton masses in theories beyond the standard model. While in many models the dipole moments scale roughly as lepton mass, it is shown that simple models exist in which the dipoles scale as the cube of the mass or in other ways. An explicit example with cubic scaling is presented, which is motivated on independent grounds from large angle neutrino oscillation data. Our results have great significance for the observability of the electric dipole moments of e, μand τ, and the rare decays μ-> e γ, and τ-> μγand will be tested in several forthcoming experiments.

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arXiv2000

Natural Mass Generation for the Sterile Neutrino

We point out that there is a serious cosmological problem in the supersymmetric standard model if a sterile neutrino is responsible for the solar neutrino oscillation, and propose a possible solution to this problem. We show that our solution induces naturally a mass of order 10^{-4} eV for the sterile neutrino, which is deeply related to the mechanism of supersymmetry breaking.

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arXiv2000

Enhanced electric dipole moment of the muon in the presence of large neutrino mixing

The electric dipole moment of the muon (d_mu) is evaluated in supersymmetric models with nonzero neutrino masses and large neutrino mixing arising from the seesaw mechanism. It is found that if the seesaw mechanism is embedded in the framework of a left-right symmetric gauge structure, the interactions responsible for the right-handed neutrino Majorana masses lead to an enhancement in d_mu. We find d_mu as large as 5. 10^{-23} ecm with a correlated value of (g-2) = 13. 10^{-10}, even for low values of tan beta. This should provide a strong motivation for improving the edm of the muon to the level of 10^{-24} ecm as has recently been proposed.

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arXiv2000

A mass relation for neutrinos

A generalization of the well-known Georgi-Jarlskog relation m_μ/m_τ = 3 (m_s/m_b) to neutrinos is found in the context of SO(10). This new relation is m_{ν_μ}/m_{ν_τ} = 16 (m_c/m_t), which is consistent with present data, assuming the small-angle MSW solution to the solar neutrino problem.

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arXiv1999

Higgs-Mediated $B^0 -> μ^+ μ^-$ in Minimal Supersymmetry

In this letter we demonstrate a new source for large flavor-changing neutral currents within the minimal supersymmetric standard model. At moderate to large tan(beta), it is no longer possible to diagonalize the masses of the quarks in the same basis as their Yukawa couplings. This generates large flavor-violating couplings of the form $\bar b_R d_L H$ and $\bar b_R s_L H$ where H is any of the three neutral, physical Higgs bosons. These new couplings lead to rare processes in the B system such as $B^0 -> μ^+ μ^-$ decay and B-Bbar mixing. We show that the latter is anomalously suppressed, while the former is in the experimentally interesting range. Current limits on $B^0 -> μ^+ μ^-$ already provide nontrivial constraints on models of moderate to large tan(beta), with an observable signal possible at Run II of the Tevatron if m_A < 400-600 GeV, extending to the TeV range if a proposed Run III of 30/fb were to occur.

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arXiv1999

Natural fermion mass hierarchy and new signals for the Higgs boson

We suggest a novel approach towards resolving the fermion mass hierarchy problem within the framework of the Standard Model. It is shown that the observed masses and mixings can be explained with order one couplings using successive higher dimensional operators involving the SM Higgs doublet field. This scenario predicts flavor-dependent enhancement in the the Higgs boson coupling to the fermions (by a factor of 3 to the b-quark and τand by a factor of 5 to μrelative to the SM). It also predicts flavor changing \bar{t}ch^0 interaction with a strength comparable to that of \bar{b}bh^0. This opens up a new discovery channel for the Higgs boson at the upgraded Tevatron and the LHC through t -> ch^0 or h^0 -> \bar{t}c + \bar{c}t. Additional tests of the framework include D^0-\bar{D^0} mixing which is predicted to be near the current experimental limit and a host of new phenomena associated with flavor physics at the TeV scale.

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arXiv1999

Supersymmetry, local horizontal unification, and a solution to the flavor puzzle

Supersymmetric gauge models with local horizontal symmetries are known to generate large flavor changing neutral current effects induced by supersymmetry breaking D-terms. We show how the presence of a U(1) gauge symmetry solves this problem. We then construct a realistic gauge model with SU(2)_H x U(1)_H as the local horizontal symmetry and suggest that the U(1)_H factor may be identified with the anomalous U(1) induced by string compactification. This model explains the observed hierarchies among the quark masses and mixing angles, accommodates naturally the solar and atmospheric neutrino data, and provides simultaneously a solution to the supersymmetric flavor problem. The model can be excluded if the rare decay μ--> e γis not observed in the current round of experiments.

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arXiv1999

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 $μ^+ K^0$ mode in addition to the $\barν K^+$ 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.

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arXiv1999

Seesaw-constrained MSSM, Solution to the SUSY CP Problem and a Supersymmetric Explanation of $ε'/ε$

We analyze CP violation in the supersymmetric standard model (MSSM) embedded minimally into a left-right symmetric gauge structure with the seesaw mechanism for neutrino masses. With the plausible assumption of universal scalar masses it is shown that CP violation in the hadron sector of the MSSM is described by a single phase residing in the supersymmetry breaking Lagrangian. This improves the CP properties of the MSSM by providing a natural solution to the SUSY CP problem. Furthermore, $ε'/ε$ vanishes in this model above the seesaw scale; extrapolation to the weak scale then leads to a prediction in agreement with the NA31 and the recent KTeV observations. The electric dipole moment of the neutron is naturally suppressed, we estimate its magnitude to be about $4 \times 10^{-29}$ ecm. Additional predictions include a tightly constrained super particle spectrum and vanishingly small CP asymmetries in the $B$ meson system.

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arXiv1999

Up-Down Unification, Neutrino Masses and Rare Lepton Decays

In a recent paper, we showed that tree level up-down unification of fermion Yukawa couplings is a natural consequence of a large class of supersymmetric models. They can lead to viable quark masses and mixings for moderately large values of $\tanβ$ with interesting and testable predictions for CP violation in the hadronic sector. In this letter, we extend our discussion to the leptonic sector focusing on one particular class of these models, the supersymmetric left-right model with the seesaw mechanism for neutrino masses. We show that fitting the solar and the atmospheric neutrino data considerably restricts the Majorana-Yukawa couplings of the leptons in this model and leads to predictions for the decay $τ\to μ+γ$, which is found to be accessible to the next generation of rare decay searches. We also show that the resulting parameter space of the model is consistent with the requirements of generating adequate baryon asymmetry through lepton-number violating decays of the right-handed neutrino.

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arXiv1998

Partial Yukawa unification and a supersymmetric origin of flavor mixing

In a large class of supersymmetric SO(10) and left-right models, requiring that the effective theory below the scale of $SU(2)_R$ breaking be the MSSM implies partial Yukawa unification with $Y_u=Y_d$ and $Y_e=Y_{ν^D}$. The same result also emerges in models with a horizontal $SU(3)_H$ symmetry. As a result, at the tree level, these models lead to vanishing quark mixing angles. We show that the correct mixing pattern can be generated in these models at the loop level from the flavor structure associated with the supersymmetry breaking terms. We generalize the constraints on supersymmetric parameters from flavor changing neutral current (FCNC) processes to include several squark mass insertions and confirm the consistency of the scheme. The expectations of this scheme for CP violating observables in the $B$ meson system are quiet different from the KM model, so it can be tested at the B factories.

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arXiv1998

Signatures of Supersymmetry and Yukawa Unification in Higgs Decays

We show that the branching ratio R_{b/tau}=BR(h^0->bb)/BR(h^0->tau tau) of the Higgs boson h^0 may usefully differentiate between the Higgs sectors of the Minimal Supersymmetric Standard Model (MSSM) and non-supersymmetric models such as the Standard Model or its two Higgs doublet extensions. Although at tree level R_{b/tau} is the same in all these models, only in the MSSM can it receive a large radiative correction, for moderate to large values of the parameter tan(beta). Such large corrections are motivated in supersymmetric unified schemes wherein the Yukawa couplings of the b-quark and the tau-lepton are equal at the unification scale; otherwise the b-quark mass prediction is too large by 15-30% for most of parameter space. Thus accurate measurements of the Higgs branching ratios can probe physics at the unification scale. The branching ratio of h^0 into charm quarks, as well as of the other Higgs bosons (H^0,A^0) into bb, tau tau, and cc can provide additional information about the supersymmetric nature of the Higgs sector.

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arXiv1998

CP Violation, Higgs Couplings, and Supersymmetry

Supersymmetric extensions of the standard model generically contain additional sources of CP violation. We discuss how at one loop a potentially large CP violating coupling of the lightest Higgs, h^0, to leptons is induced in the minimal supersymmetric standard model (MSSM). The CP violating couplings of h^0 in extensions of the MSSM, such as the next-to-minimal supersymmetric standard model (NMSSM) are also considered. We indicate how this CP violation might be observed; in particular a polarization-dependent production asymmetry, in the context of a muon collider, provides a means to access this coupling cleanly. In the MSSM, existing limits on the electric dipole moment (EDM) of the electron, coupled with standard universality assumptions, severly constrains any such signal. Nevertheless, extensions of the MSSM, such as the NMSSM, allow CP-violating signals as large as 100%.

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