arXiv subjects

R. N. Mohapatra

Publications and source records attributed to R. N. Mohapatra.

At least 127 records · Page 7Linked to original sources

arXiv2003

Minimal SUSY SO(10) model and predictions for neutrino mixings and leptonic CP violation

We discuss a minimal Supersymmetric SO(10) model where B-L symmetry is broken by a {\bf 126} dimensional Higgs multiplet which also contributes to fermion masses in conjunction with a {\bf 10} dimensional superfield. This minimal Higgs choice provides a partial unification of neutrino flavor structure with that of quarks and has been shown to predict all three neutrino mixing angles and the solar mass splitting in agreement with observations, provided one uses the type II seesaw formula for neutrino masses. In this paper we generalize this analysis to include arbitrary CP phases in couplings and vevs. We find that (i) the predictions for neutrino mixings are similar with $U_{e3}\simeq 0.18$ as before and other parameters in a somewhat bigger range and (ii) that to first order in the quark mixing parameter $λ$ (the Cabibbo angle), the leptonic mixing matrix is CP conserving. We also find that in the absence of any higher dimensional contributions to fermion masses, the CKM phase is different from that of the standard model implying that there must be new contributions to quark CP violation from the supersymmetry breaking sector. Inclusion of higher dimensional terms however allows the standard model CKM phase to be maintained.

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arXiv2003

High scale mixing unification and large neutrino mixing angles

Starting with the hypothesis that quark and lepton mixings are identical at or near the GUT scale, we show that the large solar and atmospheric neutrino mixing angles together with the small reactor angle $U_{e3}$ can be understood purely as a result of renormalization group evolution. The only requirements are that the three neutrinos must be quasi degenerate in mass and have same CP parity. It predicts that the common Majorana mass for the neutrinos must be larger than 0.1 eV making the idea testable in the currently planned or ongoing experiments searching for neutrinoless-double-beta decay.

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arXiv2003

Large Extra Dimensions and Decaying KK Recurrences

We suggest the possibility that in ADD type brane-world scenarios, the higher KK excitations of the graviton may decay to lower ones owing to a breakdown of the conservation of extra dimensional ``momenta'' and study its implications for astrophysics and cosmology. We give an explicit realization of this idea with a bulk scalar field $Φ$, whose nonzero KK modes acquire vacuum expectation values. This scenario helps to avoid constraints on large extra dimensions that come from gamma ray flux bounds in the direction of nearby supernovae as well as those coming from diffuse cosmological gamma ray background. It also relaxes the very stringent limits on reheat temperature of the universe in ADD models.

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arXiv2003

Understanding neutrino masses and mixings within the seesaw framework

Understanding the disparate mixing patterns between quarks and leptons is one of the major challenges in particle theory today. I discuss some of the ways to understand this difference within the seesaw framework using new symmetries of quarks and leptons. After a brief introduction to the various types of seesaw formulae, proposals for understanding large solar and atmospheric mixings for the three mass hierarchies i.e. normal, inverted and degenerate, are presented and their implications discussed.

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arXiv2003

Lepton Flavor Violation and Neutrino Mixings in a 3X2 Seesaw Model

We study the predictions for BR(mu-> e+gamma) and BR(tau-> e,mu+gamma) in a class of recently proposed horizontal SU(2) model that leads to a 3 X 2 seesaw model for neutrino masses. We consider two such models and obtain the correct bi-large mixing pattern for neutrinos. In these models, the effective low energy theory below the SU(2)_H scale is the MSSM. Assuming a supersymmetry breaking pattern as in the minimal SUGRA models, we find that consistent with present g-2, b->s+gamma and WMAP dark matter constraints on the parameters of the model, the BR(mu-> e+gamma) prediction is accessible to the proposed MEG experiment at PSI making this class of models testable in the near future.

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arXiv2003

Minimal SUSY SO(10), b-$τ$ unification and large neutrino mixings

We show that the assumption of type II seesaw mechanism for small neutrino masses coupled with $b-τ$ mass unification in a minimal SUSY SO(10) model leads not only to a natural understanding of large atmospheric mixing angle ($θ_{23}$) among neutrinos, as recently pointed out, but also to large solar angle ($θ_{12}$) and a small $θ_{13}\equiv U_{e3}$ as required to fit observations. This is therefore a minimal, completely realistic grand unified model for all low energy observations that naturally explains the diverse mixing patterns between the quark and leptons without any additional inputs such as extra global symmetries. The proposed long baseline neutrino experiments will provide a crucial test of this model since it predicts $U_{e3}\simeq 0.16$ for the allowed range of parameters.

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arXiv2002

Neutrino mass, proton decay and dark matter in TeV scale universal extra dimension models

We show how the problem of small neutrino masses and suppressed proton decay can be simultaneously resolved in 6-D universal extra dimension models (UED) with a low fundamental scale using extended gauge groups that contain the local $B-L$ symmetry. The extra space dimensions are compactified either on a $T^2/Z_2$ or $T^2/Z_2\times Z'_2$ orbifold depending on whether the full gauge group is $SU(2)_L\times U(1)_{I_{3R}}\times U(1)_{B-L}$ or $SU(2)_L\times SU(2)_{R}\times U(1)_{B-L}$. In both cases, neutrino masses are suppressed by an appropriate orbifold parity assignment for the standard model singlet neutrinos and the proton decay rate is suppressed due to a residual discrete symmetry left over from compactification. For lower values of the fundamental scale, a dominant decay mode of the neutron is $n\to 3 ν$. An interesting consequence of the model is a possible two component picture for dark matter of the universe.

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arXiv2002

ICTP Lectures on Theoretical Aspects of Neutrino Masses and Mixings

Recent neutrino oscillation experiments are yielding valuable information on the nature of neutrino masses and mixings even though we are far from a complete understanding of the new physics implied by them. In these lectures, I summarize the current theoretical status of neutrino mass physics.

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arXiv2002

Lepton Flavor Violation and the Origin of the Seesaw Mechanism

The right--handed neutrino mass matrix that is central to the understanding of small neutrino masses via the seesaw mechanism can arise either (i) from renormalizable operators or (ii) from nonrenormalizable or super-renormalizable operators, depending on the symmetries and the Higgs content of the theory beyond the Standard Model. In this paper, we study lepton flavor violating (LFV) effects in the first class of seesaw models wherein the ν_R Majorana masses arise from renormalizable Yukawa couplings involving a B-L = 2 Higgs field. We present detailed predictions for τ-> μ+ γand μ-> e + γbranching ratios in these models taking the current neutrino oscillation data into account. Focusing on minimal supergravity models, we find that for a large range of MSSM parameters suggested by the relic abundance of neutralino dark matter and that is consistent with Higgs boson mass and other constraints, these radiative decays are in the range accessible to planned experiments. We compare these predictions with lepton flavor violation in the second class of models arising entirely from the Dirac Yukawa couplings. We study the dependence of the ratio r \equiv B(μ-> e+γ)/B(τ->μ+γ) on the MSSM parameters and show that measurement of r can provide crucial insight into the origin of the seesaw mechanism.

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arXiv2002

Testing leptonic SU(2) horizontal symmetry using neutrino mixings

After some preliminary arguments suggesting that neutrino mixings with inverted mass pattern may be easier to understand within the framework of a local horizontal symmetry SU(2)_H acting on leptons, we construct a specific extension of the minimal supersymmetric standard model that implements the idea and analyze its predictions. We show that the horizontal symmetry leads to an experimentally testable relation between the neutrino parameters U_{e3} and the ratio of solar and atmospheric mass difference squared i.e. U^2_{e3} cos 2 theta_{odot} = (Delta m^2_{odot})/(2 Delta m^2_A) + O(U^4_{e3}, (m_e/m_mu)^2). Taking the solar neutrino parameters inferred from present data at 99.7% confidence level, the above relation leads to a lower bound on U_{e3} >= 0.08 and an allowed region in the U_{e3} and (Delta m^2_{odot})/(2 Delta m^2_A) space which can be tested in proposed long baseline experiments.

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arXiv2002

Bimaximal Neutrino Mixixing from a Local SU(2) Horizontal Symmetry

Freedom from global anomalies in the presence of a local SU(2)_H horizontal symmetry under which right handed charged leptons transform nontrivially requires that there be at least two right handed neutrinos with masses of order of the horizontal symmetry breaking scale. If a third right handed neutrino is introduced to satisfy quark lepton symmetry, it is unprotected by the horizontal symmetry, becomes superheavy with a Planck scale mass and decouples from lower energy physics. The resulting seesaw mechanism with two right handed neutrinos in combination with the horizontal symmetry leads naturally to a near bimaximal pattern for the neutrino mixing with an inverted mass hierarchy and is compatible with all data. It predicts a correlation between the solar mixing angle and U_{e3}, that is testable in the proposed long baseline experiments.

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arXiv2002

Left-right symmetry in 5D and neutrino mass in TeV scale gravity models

We construct a left-right symmetric model based on the gauge group $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ in five dimensions where both the gauge bosons and fermions reside in all five dimensions. The orbifold boundary conditions are used not only to break the gauge symmetry down to $SU(2)_L\times U(1)_Y\times U(1)_{Y'}$ but also to ``project'' the right handed neutrino out of the zero mode part of the spectrum, providing a new way to understand the small neutrino masses without adding (singlet) bulk neutrinos. This formulation of the left-right model has also two new features: (i) it avoids most existing phenomenological bounds on the scale of the right handed $W_R$ boson allowing for the possibility that the right handed gauge bosons could have masses under a TeV, and (ii) it predicts a stable lepton with mass of order of the inverse radius of the fifth dimension.

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arXiv2002

Testing neutrino mass matrices with approximate L_e-L_mu-L_tau symmetry

As neutrino experiments are starting to probe the detailed structure of the neutrino mass matrix, we present sumrules relating its matrix elements for a class of models with approximate $L_e-L_μ-L_τ$ symmetry and the observables in neutrino oscillation experiments. We show that regardless of how the above symmetry is broken (whether in the neutrino sector or the charged lepton sector), as long as the breaking terms are small, there is a lower bound on the solar neutrino mixing angle, $sin^22θ_{\odot}$, correlated with the solar mass difference square, $Δm^2_{\odot}$, or the mixing parameter, $U_{e3}$. We also discuss models where such patterns can arise.

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arXiv2002

Neutrino magnetic moment, large extra dimensions and high energy cosmic neutrino spectra

We point out that the presence of bulk neutrinos in models with large extra spatial dimensions can lead to observable flavour specific deformations in the spectra of extreme high energy cosmic neutrinos. These deformations are due to the spin precession of the high energy neutrinos in the background magnetic fields via electromagnetic interactions. Measurements with existing and proposed neutrino telescopes which are meant to detect high energy neutrinos can therefore provide a novel way to probe the size of extra hidden dimensions. We qualitatively illustrate the flavour suppression due to the Earth, Sun and intergalactic magnetic fields. An observable consequence of this precession could be an angular asymmetry for the extreme high energy neutrinos from the atmosphere and flavour specific deformations of the intergalactic neutrinos.

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arXiv2002

Big bang nucleosynthesis constraints on bulk neutrinos

We examine the constraints imposed by the requirement of successful big bang nucleosynthesis on models with one large extra dimension and a single bulk neutrino. We solve the Boltzman kinetic equations for the thermal distribution of bulk neutrino modes and evaluate their contribution to the energy density of the universe at the BBN epoch. This helps us to constrain the size of the extra dimensions and the mixing parameter between the active and bulk neutrinos.

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arXiv2002

Predictive Schemes for Bimaximal Neutrino Mixings

We present simple and predictive scenarios for neutrino masses which lead to a bimaximal pattern of mixings that apparently provides the best fit to the current solar and atmospheric neutrino data. It is shown that an approximate global L_e-L_μ-L_τsymmetry, broken only by Planck scale effects, can naturally explain the bimaximal mixing pattern. In one scenario, the solar neutrino oscillation parameters are induced entirely through radiative renormalization group effects. A one parameter model describing all neutrino data is presented, which predicts U_{e3} = \sqrt{m_e/m_τ} \simeq 0.017. A second two parameter scheme predicts U_{e3} to be vanishingly small. We also show how these scenarios can be extended to accommodate the LSND observations within a 3+1 neutrino oscillation scheme by including a mirror sector of particles and forces.

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arXiv2001

Mirror Matter as Self Interacting Dark Matter

It has been argued that the observed core density profile of galaxies is inconsistent with having a dark matter particle that is collisionless and alternative dark matter candidates which are self interacting may explain observations better. One new class of self interacting dark matter that has been proposed in the context mirror universe models of particle physics is the mirror hydrogen atom whose stability is guaranteed by the conservation of mirror baryon number. We show that the effective transport cross section for mirror hydrogen atoms, has the right order of magnitude for solving the ``cuspy'' halo problem. Furthermore, the suppression of dissipation effects for mirror atoms due to higher mirror mass scale prevents the mirror halo matter from collapsing into a disk strengthening the argument for mirror matter as galactic dark matter.

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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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