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R. N. Mohapatra

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

At least 109 records · Page 6Linked to original sources
arXiv2004

Leptogenesis and $μ-τ$ symmetry

If an exact $μ\leftrightarrow τ$ symmetry is the explanation of the maximal atmospheric neutrino mixing angle, it has interesting implications for origin of matter via leptogenesis in models where small neutrino masses arise via the seesaw mechanism. For seesaw models with two right handed neutrinos $(N_μ, N_τ)$, lepton asymmetry vanishes in the exact $μ\leftrightarrow τ$ symmetric limit, even though there are nonvanishing Majorana phases in the neutrino mixing matrix. On the other hand, for three right handed neutrino models, lepton asymmetry nonzero and is given directly by the solar mass difference square. We also find an upper bound on the lightest neutrino mass.

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arXiv2004

Seesaw Mechanism and Its Implications

The seesaw mechanism for neutrino masses is introduced and some of its different realizations and applications are discussed. It is pointed out how they can be used to understand the bi-large mixing patterns among neutrinos in combination with assumptions about high scale physics such as grand unification or quasi-degeneracy of neutrino masses.

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arXiv2004

Theory of Neutrinos

After a brief overview of the present knowledge of neutrino masses and mixing, we summarize what can be learned about physics beyond the standard model from the various proposed neutrino experiments. We also comment on the impact of the experiments on our understanding of the origin of the matter-antimatter asymmetry of the Universe as well as what can be learned from some experiments outside the domain of neutrinos.

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arXiv2004

Suppressing Proton Decay in the Minimal SO(10) Model

We show that in a class of minimal supersymmetric SO(10) models which have been found to be quite successful in predicting neutrino mixings, all proton decay modes can be suppressed by a particular choice of Yukawa textures. This suppression works for contributions from both left and right operators for nucleon decay and for arbitrary \tanβ. The required texture not only fits all lepton and quark masses as well as CKM parameters but it also predicts neutrino mixing parameter U_e3 and Dirac CP phase \sin|δ_MNS| to be 0.07-0.09 and 0.3-0.7 respectively. We also discuss the relation between the GUT symmetry breaking parameters for the origin of these textures.

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arXiv2004

Neutrino Mass and Grand Unification

Seesaw mechanism appears to be the simplest and most appealing way to understand small neutrino masses observed in recent experiments. It introduces three right handed neutrinos with heavy masses to the standard model, with at least one mass required by data to be close to the scale of conventional grand unified theories. This may be a hint that the new physics scale implied by neutrino masses and grand unification of forces are one and the same. Taking this point of view seriously, I explore different ways to resolve the puzzle of large neutrino mixings in grand unified theories such as SO(10) and models based on its subgroup $SU(2)_L\times SU(2)_R\times SU(4)_c$.

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arXiv2004

Possible Gauge Theoretic Origin for Quark-Lepton Complementarity

Similarity between the weak interaction properties of quarks and leptons has led to suggestions that the origin of lepton mixing angles may be related to those of quarks. In this paper, we present a gauge model based on $SU(2)_L \times SU(2)_R\times SU(4)_c$ group that leads to a new form for the quark lepton complementarity which predicts the solar neutrino mixing angle in terms of the Cabibbo angle for the case of inverted mass hierarchy for neutrinos. We also indicate how these ideas can be implemented in an $E_6$ inspired trinification $SU(3)_C \times SU(3)_L \times SU(3)_R$ model, which is more closely allied to string theory by the AdS/CFT correspondence.

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arXiv2004

Physics of Neutrino Mass

Recent discoveries in the field of neutrino oscillations have provided a unique window into physics beyond the standard model. In this lecture, I summarize how well we understand the various observations, what they tell us about the nature of new physics and what we are likely to learn as some of the planned experiments are carried out.

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arXiv2004

Theta-13 as a Probe of Mu-Tau symmetry for Leptons

Many experiments are being planned to measure the neutrino mixing parameter $θ_{13}$ using reactor as well as accelerator neutrino beams. In this note, the theoretical significance of a high precision measurement of this parameter is discussed. It is emphasized that it will provide crucial information about different ways to understand the origin of large atmospheric neutrino mixing and move us closer towards determining the neutrino mass matrix. For instance if exact $μ\leftrightarrow τ$ symmetry in the neutrino mass matrix is assumed to be the reason for maximal $ν_μ-ν_τ$ mixing, one gets $θ_{13}=0$. Whether $θ_{13}\simeq \sqrt{Δm^2_{\odot}/Δm^2_A}$ or $θ_{13}\simeq Δm^2_{\odot}/Δm^2_A$ can provide information about the way the $μ\leftrightarrow τ$ symmetry breaking manifests in the case of normal hierarchy. We also discuss the same question for inverted hierarchy as well as possible gauge theories with this symmetry.

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arXiv2004

Neutrino Masses and Mixings in a Predictive SO(10) Model with CKM CP Violation

It has recently been shown that a minimal SO(10) model with a single {\bf 10} and single {\bf 126} Higgs field breaking B-L symmetry predicts large solar and atmospheric mixings in agreement with observations if it is assumed that the neutrino mass obeys the type II seesaw formula. No additional symmetries need to be assumed for this purpose. Understanding CP violation in the renormalizable version of the model however requires a significant non-CKM source. In this paper we show that if we extend the model by the inclusion of a heavy {\bf 120} dimensional Higgs field, then it can accomodate CKM CP violation while remaining predictive in the neutrino sector. Among the predictions are:(i) solar mixing angle in the observed range; (ii)θ_{13} in the range of 0.1 to 0.26; (iii) the Dirac phase close to maximal for the central value of the solar mixing angle.

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arXiv2004

Working Group Report: Neutrino and Astroparticle Physics

This is the report of neutrino and astroparticle physics working group at WHEPP-8. We present the discussions carried out during the workshop on selected topics in the above fields and also indicate progress made subsequently. The neutrino physics subgroup studied the possibilites of constraining neutrino masses, mixing and CPT violation in lepton sector from future experiments. Neutrino mass models in the context of abelian horizontal symmetries, warped extra dimensions and in presence of triplet Higgs were studied. Effect of threshold corrections on radiative magnification of mixing angles was investigated. The astroparticle physics subgroup focused on how various particle physics inputs affect the CMBR fluctuation spectrum, and on brane cosmology. This report also contains an introduction on how to use the publicly available code CMBFAST to calculate the CMBR fluctuations.

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arXiv2004

SO(10) Symmetry Breaking and Type II Seesaw

A minimal SO(10) model with {\bf 126} Higgs field breaking B-L symmetry has been shown recently to predict large solar and atmospheric mixings in agreement with observations if it is assumed that the neutrino mass follows from the triplet dominated type II seesaw formula. No additional symmetries need to be assumed for this purpose. We discuss the conditions on the way SO(10) symmetry breaks down to MSSM and the Higgs multiplets in the model, required for the triplet dominated type II seesaw formula to hold. We find that (i) SO(10) must break to a nonminimal SU(5) before breaking to the standard model; (ii) $B-L$ symmetry must break at the time of SO(10) breaking and (iii) constraints of unification seem to require that the minimal model must have a {\bf 54} dimensional Higgs field together with a {\bf 210} and {\bf 126} to break the GUT symmetry.

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arXiv2004

Avoiding BBN Constraints on Mirror Models for Sterile Neutrinos

We point out that in models that explain the LSND result for neutrino oscillation using the mirror neutrinos, the big bang nucleosynthesis constraint can be avoided by using the late time phase transition that only helps to mix the active and the sterile neutrinos. We discuss the astrophysical as well as cosmological implications of this proposal.

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arXiv2004

Reactor Searches for Neutrino Magnetic Moment as a Probe of Extra Dimensions

We present calculations of the magnetic moment contribution to neutrino electron scattering in large extra dimension brane-bulk models (LED) with three bulk neutrinos. We discuss the cases with two and three large extra dimensions of sizes $R$. The calculations are done using reactor flux from Uranium, $^{235}U$ as the neutrino source. We find that if the electron neutrino mass is chosen to be in the range of one eV, the differential cross section for $\barν-e$ scattering for low electron recoil energy can be of the same order as the presently explored values in reactor experiments. Furthermore the spectral shape for the LED models is different from the four dimensional case. Future higher precision reactor experiments can therefore be used to provide new constraints on a class of large extra dimension theories.

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arXiv2004

Neutrino mass, dark matter and inflation

We show that spontaneous breaking of global $B-L$ symmetry responsible for small neutrino masses via the seesaw mechanism provides a unified picture of hybrid inflation and dark matter if the scale of $B-L$ breaking is close to the GUT scale. The majoron which acquires a small mass in the milli-eV range due to Planck scale breaking of $B-L$ is the dark matter candidate. The coupling of the majoron field to the neutrino induces a small violation of CPT and Lorentz invariance at the present epoch. We discuss some of the phenomenological and cosmological implications of the model.

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arXiv2004

CKM CP Violation in a Minimal SO(10) Model for Neutrinos and Its Implications

A minimal supersymmetric SO(10) model with one {\bf 10} and one {\bf 126} Higgs superfield has recently been shown to predict all neutrino mixings as well as the solar mass difference squared in agreement with observations. Two assumptions critical to the predictivity and success of the model are that: (i) the superpotential includes only renormalizable terms, thereby limiting the number of free parameters and (ii) the triplet term in the type II seesaw formula for neutrino mass dominates, leading to the sum rule M_nu = c(M_d - M_e) that is responsible for large mixings. However, CKM CP phase is constrained to be in the second or third quadrant requiring a significant non-CKM component to CP violation to explain observations. We revisit this issue using type I seesaw formula for neutrino masses and obtain the following results: (i) we show that the above sumrule responsible for large mixing angles can also emerge in type I seesaw models; the detailed predictions are however not compatible with present data for any choice of CP phases. (ii) We then show that addition of a nonrenormalizable term restores compatibility with neutrino data and CKM CP violation both in type I and type II cases. We further find that (iii) the MSSM parameter tan beta > 30 in the type I model and (iv) lepton flavor violation and lepton electric dipole moments which are accessible to proposed experiments in both type I and type II models. We also discuss the unification of the gauge couplings in type I model which requires an intermediate scale.

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arXiv2003

Proton Decay in a Minimal SUSY SO(10) Model for Neutrino Mixings

A minimal renormalizable SUSY SO(10) model with B-L symmetry broken by {\bf 126} Higgs field has recently been shown to predict all neutrino mixings and the ratio $Δm^2_{\odot}/Δm^2_A$ in agreement with observations. Unlike models where B-L is broken by {\bf 16} Higgs, this model guarantees automatic R-parity conservation and hence a stable dark matter as well as the absence of dim=4 baryon violating operator without any additional symmetry assumptions. In this paper, we discuss the predictions of the model for proton decay induced at the GUT scale. We scan over the parameter space of the model allowed by neutrino data and find upper bounds on the partial lifetime for the modes $τ(n\to π^0\barν)=~2τ(p\to π^+\barν)\leq (5.7-13)\times 10^{32}$ yrs and $τ(n\to K^0\barν)\leq 2.97\times 10^{33}$ yrs for the average squark mass of a TeV and wino mass of 200 GeV, when the parameters satisfy the present lower limits on $τ(p\to K^+\barν)$ mode. These results can be used to test the model.

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arXiv2003

Radion Cosmology in Theories with Universal Extra Dimensions

We discuss cosmology of models with universal extra dimensions, where the Standard Model degrees of freedom live in a $4+n$ dimensional brane, with $n$ compact and small extra spatial dimensions. In these models, the simplest way to obtain the conventional 4-dimensional Planck scale starting with a low string scale is to have also some larger extra dimensions, where only gravity propagates. In such theories, dimensional reduction generically leads to at least two radion fields, one associated with the total volume of the extra spatial dimensions, and the other with the ratio of the sizes of small and large extra dimensions. In this paper, we discuss the impact of the radion fields on cosmology. We emphasize various aspects of radion physics such as radion coupling to the Standard Model fields, bare and dressed radion masses during inflation, dynamical stabilization of radions during and after inflation, radion decay life time and its late dominance in thermal history of the Universe as well as its quantum fluctuations during inflation. We argue that models where the radion plays the role of an inflaton or the inflaton is a brane scalar field, run into problems. We then present a successful inflation model with bulk scalar fields that seems to have all the desired properties. We also briefly discuss the possibility of radion as a cold dark matter candidate.

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