arXiv subjects

R. N. Mohapatra

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

At least 91 records · Page 5Linked to original sources

arXiv2006

Observable Neutron-Anti-Neutron Oscillation, Baryogenesis and High Scale Seesaw

Seesaw mechanism has been a dominant paradigm in the discussion of neutrino masses. I discuss how this idea can be tested via a baryon number violating process such as $N-\bar{N}$ oscillation. Since the expected seesaw scale is high and the $N-\bar{N}$ amplitude goes like $M^{-5}_{R}$, one might think that this process is not observable in realistic seesaw models for neutrino masses. In this talk I show that in supersymmetric models, the above conclusion is circumvented leading to an enhanced and observable rate for $N-\bar{N}$ oscillation. I also discuss a new mechanism for baryogenesis in generic models for neutron-anti-neutron oscillation and show how the requirement of adequate baryogenesis can put an upper limit on the neutron-anti-neutron oscillation time.

hep-ph↗

arXiv2006

S4 Flavor Symmetry and Fermion Masses: Towards a Grand Unified theory of Flavor

Pursuing a bottom-up approach to explore which flavor symmetry could serve as an explanation of the observed fermion masses and mixings, we discuss an extension of the standard model (SM) where the flavor structure for both quarks and leptons is determined by a spontaneously broken S4 and the requirement that its particle content is embeddable simultaneously into the conventional SO(10) grand unified theory (GUT) and a continuous flavor symmetry G_f like SO(3)_f or SU(3)_f. We explicitly provide the Yukawa and the Higgs sector of the model and show its viability in two numerical examples which arise as small deviations from rank one matrices. In the first case, the corresponding mass matrix is democratic and in the second one only its 2-3 block is non-vanishing. We demonstrate that the Higgs potential allows for the appropriate vacuum expectation value (VEV) configurations in both cases, if CP is conserved. For the first case, the chosen Yukawa couplings can be made natural by invoking an auxiliary Z2 symmetry. The numerical study we perform shows that the best-fit values for the lepton mixing angles theta_12 and theta_23 can be accommodated for normal neutrino mass hierarchy. The results for the quark mixing angles turn out to be too small. Furthermore the CP-violating phase delta can only be reproduced correctly in one of the examples. The small mixing angle values are likely to be brought into the experimentally allowed ranges by including radiative corrections. Interestingly, due to the S4 symmetry the mass matrix of the right-handed neutrinos is proportional to the unit matrix.

hep-ph↗

arXiv2006

$S_3$ Symmetry and Tri-bimaximal Mixing

The near maximal value for the atmospheric neutrino mixing angle together with the fact that the solar mixing angle satisfies the relation $\sin^2θ_\odot\simeq {1/3}$ is the basis for the so called tri-bimaximal mixing (tbm) when $θ_{13}=0$. In this note, we explore the possibility that tri-bimaximal mixing is an indication of a softly broken higher leptonic symmetry $S_3$, the permutation of three lepton families that embeds the $μ-τ$ exchange symmetry of leptons.

hep-ph↗

arXiv2006

Leptogenesis in Realistic SO(10) Models

We study the origin of baryonic matter via leptogenesis in realistic SO(10) models, in particular, in a new lopsided mass matrix model introduced recently by three of the authors. By introducing simple CP-violating phases in the mass matrix of the right-handed neutrinos, the model generates sufficient baryon asymmetry without fine-tuning. We compare this result with other realistic SO(10) models.

hep-ph↗

arXiv2006

Grand unification of $μ-τ$ Symmetry

Near maximal neutrino mixing needed to understand atmospheric neutrino data can be interpreted to be a consequence of an interchange symmetry between the muon and tau neutrinos in the neutrino mass matrix in the flavor basis. This idea can be tested by a measurement of the neutrino mixing parameter $θ_{13}$ and looking for its correlation with $θ_{23}-π/4$. We present a supersymmetric SU(5) grand unified model for quarks and leptons which obeys this exchange symmetry and is a realistic model that can fit all observations. GUT embedding shifts $θ_{13}$ from its zero $μ-τ$ symmetric value to a nonzero value keeping it under an upper limit.

hep-ph↗

arXiv2005

Theory of Neutrinos: A White Paper

During 2004, four divisions of the American Physical Society commissioned a study of neutrino physics to take stock of where the field is at the moment and where it is going in the near and far future. Several working groups looked at various aspects of this vast field. The summary was published as a main report entitled ``The Neutrino Matrix'' accompanied by short 50 page versions of the report of each working group. Theoretical research in this field has been quite extensive and touches many areas and the short 50 page report provided only a brief summary and overview of few of the important points. The theory discussion group felt that it may be of value to the community to publish the entire study as a white paper and the result is the current article. After a brief overview of the present knowledge of neutrino masses and mixing and some popular ways to probe the new physics implied by recent data, the white paper summarizes what can be learned about physics beyond the Standard Model from the various proposed neutrino experiments. It also comments on the impact of the experiments on our understanding of the origin of the matter-antimatter asymmetry of the Universe and the basic nature of neutrino interactions as well as the existence of possible additional neutrinos. Extensive references to original literature are provided.

hep-ph↗

arXiv2005

Predicting the Seesaw Scale in a Minimal Bottom-Up Extension of MSSM

We analyze a minimal bottom-up seesaw scenario where we require the theory to satisfy three phenomenological conditions: (i)it is supersymmetric; (ii) it has a local B-L symmetry as part of the SU(2)_L x SU(2)_R x U(1)_{B-L} gauge theory to implement the seesaw mechanism and (iii) B-L symmetry breaking is such that it leaves R-parity unbroken giving a naturally stable dark matter. We show that in such a theory, one can predict the seesaw scale for neutrino masses to be M_R ~ \sqrt{M_{SUSY} M_{Pl}} ~ 10^{11} GeV. We show that the ground state with this property is a stable minimum and is lower than possible electric charge violating minimum in this theory. Such models in their generic version are known to predict the existence of a light doubly charged Higgs boson and Higgsino which can be searched for in collider experiments. We give expressions for their masses in this minimal version. We then indicate how one can get different expectation values for the MSSM Higgs doublets in the theory required to have realistic quark masses.

hep-ph↗

arXiv2005

An SO(10) GUT Model With Lopsided Mass Matrix and Neutrino Mixing Angle theta_13

An alternative supersymmetric SO(10) grand unification model with lopsided fermion mass matrices is introduced. It generates a large solar-neutrino-mixing angle through the neutrinos' Dirac mass matrix constrained by the SO(10) group structure, avoiding the fine-tuning required in the Majorana mass matrix of right-handed neutrinos. The model fits well the known data on masses and mixings of quarks and leptons, and predicts a sizable lepton mixing $\sin^22θ_{13}\simeq 0.074$, which is significantly larger than that of the original lopsided model.

hep-ph↗

arXiv2005

Observable Neutron Antineutron Oscillation in High Scale Seesaw Models

We discuss a realistic high scale ($v_{BL}\sim 10^{12}$ GeV) supersymmetric seesaw model based on the gauge group $SU(2)_L\times SU(2)_R\times SU(4)_c$ where neutron-anti-neutron oscillation can occur at an observable rate. This is contrary to the naive dimensional arguments which say that $τ_{N-\bar{N}}\propto v_{BL}^5$ and is therefore unobservable if the seesaw scale $v_{BL}\geq 10^{5}$ GeV. Two reasons for the enhanced rate in our model are: (i) accidental global symmetries which keep some of the diquark Higgs masses at the weak scale and (ii) a new supersymmetric contribution from a lower dimensional operator. The net result is that $τ_{N-\bar{N}}\propto v_{BL}^2 v^3_{wk}$ rather than $v^5_{BL}$, causing the enhancement. The model can also explain the origin of matter via leptogenesis and is testable at LHC where the light diquark states can be produced.

hep-ph↗

arXiv2005

Broken mu-tau Symmetry and Leptonic CP Violation

We propose that leptonic CP violation arises from a breaking of the mu-tau exchange symmetry in the neutrino mass matrix which in turn is indicated by the near maximal atmospheric neutrino mixing and the near zero θ_{13}. We find that for the case of a normal hierarchy, present data already restricts the way CP violation may appear in the neutrino mass matrix and there is an interesting correlation between the mixing angle θ_{13}, the solar mixing angle θ_{12} and the Dirac CP phase. In the inverted hierarchy only θ_{13} and the Dirac phase are linked. We also discuss the impact of this kind of CP violation on the deviation of the atmospheric mixing from its maximal value. Moreover, if corrections to mu-tau symmetry arise from the charged lepton sector, where mu-tau symmetry is known to be broken anyway, we find interesting connections between the CP-even and -odd terms as well. Our predictions are testable in the proposed long baseline neutrino experiments.

hep-ph↗

arXiv2005

Some Implications of Neutron Mirror Neutron Oscillation

We comment on a recently discussed possibility of oscillations between neutrons and degenerate mirror neutrons in the context of mirror models for particles and forces. It has been noted by Bento and Berezhiani that if these oscillations occurred at a rate of $τ^{-1}_{NN'}\sim sec^{-1}$, it would help explain putative super GKZ cosmic ray events provided the temperature of the mirror radiation is $\sim 0.3-0.4$ times that of familiar cosmic microwave background radiation. We discuss how such oscillation time scales can be realized in mirror models and find that the simplest nonsupersymmetric model for this idea requires the existence of a low mass (30-3000 GeV) color triplet scalar or vector boson. A supersymmetric model, where this constraint can be avoided is severely constrained by the requirement of maintaining a cooler mirror sector. We also find that the reheat temperature after inflation in generic models that give fast $n-n'$ oscillation be less than about 300 GeV in order to maintain the required relative coolness of the mirror sector.

hep-ph↗

arXiv2005

Neutrino Mixing Predictions of a Minimal SO(10) Model with Suppressed Proton Decay

During the past year, a minimal renormalizable supersymmetric SO(10) model has been proposed with the following properties: it predicts a naturally stable dark matter and neutrino mixing angles theta_atm and theta_13 while at the same time accommodating CKM CP violation among quarks with no SUSY CP problem. Suppression of proton decay for all allowed values of tan beta strongly restricts the flavor structure of the model making it predictive for other processes as well. We discuss the following predictions of the model in this paper, e.g. down-type quark masses, and neutrino oscillation parameters, U_e3, delta_MNSP, which will be tested by long baseline experiments such as T2K and subsequent experiments using the neutrino beam from JPARC. We also calculate lepton flavor violation and the lepton asymmetry of the Universe in this model.

hep-ph↗

arXiv2005

Seesaw Right Handed Neutrino as the Sterile Neutrino for LSND

We show that a double seesaw framework for neutrino masses with $μ-τ$ exchange symmetry can lead to one of the righthanded seesaw partners of the light neutrinos being massless. This can play the role of a light sterile neutrino, giving a $3+1$ model that explains the LSND results. We get a very economical scheme, which makes it possible to predict the full $4\times 4$ neutrino mass matrix if CP is conserved. Once CP violation is included, effect of the LSND mass range sterile neutrino is to eliminate the lower bound on neutrinoless double beta decay rate which exists for the three neutrino case with inverted mass hierarchy. The same strategy can also be used to generate a natural $3+2$ model for LSND, which is also equally predictive for the CP conserving case in the limit of exact $μ-τ$ symmetry.

hep-ph↗

arXiv2005

Radiative Magnification of Neutrino Mixings in Split Supersymmetry

Radiative corrections to neutrino mixings in seesaw models depend on the nature of new physics between the weak and the GUT-seesaw scales and can be taken into account using the renormalization group equations. This new physics effect becomes particularly important for models with quasi-degenerate neutrino masses where small neutrino mixings at the seesaw scale can get magnified by radiative renormalization effects alone to match observations. This mechanism of radiative magnification which provides a simple understanding of why lepton mixings are so different from quark mixings was demonstrated by us for the standard supersymmetry scenario where the particle spectrum becomes supersymmetric above the weak scale. In this paper, we examine this phenomenon in split supersymmetry scenarios and find that the mechanism works also for this scenario provided the SUSY scale is at least 2-3 orders below the GUT-seesaw scale and one has larger values of ${\tanβ}$.

hep-ph↗

arXiv2005

Can a measurement of theta-13 tell us about quark-lepton unification ?

We argue that a high precision measurement of the neutrino mixing parameter $θ_{13}$, within a three neutrino seesaw framework can throw important light on the question of whether the quarks and leptons unify into a single matter at high scales. Based on a number of examples, we conclude that a value of $θ_{13} \leq {\frac{Δm^2_{\odot}}{Δm^2_A}}\simeq 0.04$ would require at the minimum a pure leptonic interchange symmetry between $μ$ and $τ$ generations for its natural theoretical understanding and will disfavor a quark-lepton unification type theory such as those based on $SU(4)_c$ or SO(10) whereas a bigger value would leave open the possibility of quark lepton unification.

hep-ph↗

arXiv2005

Leptogenesis, $μ-τ$ Symmetry and $θ_{13}$

We show that in theories where neutrino masses arise from type I seesaw formula with three right handed neutrinos and where large atmospheric mixing angle owes its origin to an approximate leptonic $μ-τ$ interchange symmetry, the primordial lepton asymmetry of the Universe, $ε_l$ can be expressed in a simple form in terms of low energy neutrino oscillation parameters as $ε_l = (a Δm^2_\odot+ b Δm^2_A θ^2_{13})$, where $a$ and $b$ are parameters characterizing high scale physics and are each of order $\leq 10^{-2} $ eV$^{-2}$. We also find that for the case of two right handed neutrinos, $ε_l \propto θ^2_{13}$ as a result of which, the observed value of baryon to photon ratio implies a lower limit on $θ_{13}$. For specific choices of the CP phase $δ$ we find $θ_{13}$ is predicted to be between $0.10-0.15$.

hep-ph↗

arXiv2005

Threshold Effects on Quasi-degenerate Neutrinos with High-scale Mixing Unification

We consider threshold effects on neutrino masses and mixings in a recently proposed model for understanding large solar and atmospheric mixing angles using radiative magnification for the case of quasi-degenerate neutrinos. We show that the magnitude of the threshold effects is sufficient to bring concordance between the predictions of this model and latest data from ${\rm KamLAND}$ and ${\rm SNO}$ on observations of neutrino oscillations.

hep-ph↗

arXiv2005

Photon as a Vector Goldstone Boson: Nonlinear $σ$ Model for QED

We show that QED in the Coulomb gauge can be considered as a low energy linear approximation of a non-linear $σ$-type model where the photon emerges as a vector Goldstone boson related to the spontaneous breakdown of Lorentz symmetry down to its spatial rotation subgroup at some high scale $M$. Starting with a general massive vector field theory one naturally arrives at this model if the pure spin-1 value for the vector field $A_μ(x)$ provided by the Lorentz condition $\partial_μA_μ(x)=0$ is required. The model coincides with conventional QED in the Coulomb gauge in the limit of M going to infinity and generates a very particular form for the Lorentz and CPT symmetry breaking terms, which are suppressed by powers of $M$.

hep-th↗