Natural R-Parity, \mu-term, and Fermion Mass Hierarchy From Discrete Gauge Symmetries

In the minimal supersymmetric Standard Model with seesaw neutrino masses we show how R-parity can emerge naturally as a discrete gauge symmetry. The same discrete symmetry explains the smallness of the \mu-term (the Higgsino mass parameter) via the Giudice--Masiero mechanism. The discrete gauge anomalies are cancelled by a discrete version of the Green--Schwarz mechanism. The simplest symmetry group is found to be Z_4 with a charge assignment that is compatible with grand unification. Several other Z_N gauge symmetries are found for N=10,12,18,36 etc, with some models employing discrete anomaly cancellation at higher Kac-Moody levels. Allowing for a flavor structure in Z_N, we show that the same gauge symmetry can also explain the observed hierarchy in the fermion masses and mixings.Comment: 22 pages, LaTe

TeV Scale Leptogenesis, theta_13 And Doubly Charged Particles At LHC

We explore a realistic supersymmetric SU(2)_L \times SU(2)_R \times U(1)_B-L model spontaneously broken at around 10^12 GeV. The presence of D and F-flat directions gives rise to TeV mass doubly charged particles which can be found at the LHC. We implement TeV scale leptogenesis and employing both type I and II seesaw, the three light neutrinos are partially degenerate with masses in the 0.02-0.1 eV range. The effective mass parameter for neutrinoless double beta decay is 0.03-0.05 eV. We also find the interesting relation tan 2 \theta_13 ~ [\Delta m^2_{\odot} / \Delta m^2_{atm}] [sin 2 \theta_12 /tan 2 \theta_23] <~ 0.02.Comment: 18 pages, 4 figures, revtex4. v2: minor changes, matches published versio

Naturally Light Neutrinos and Unification in Theories with Low Scale Quantum Gravity

Within low scale theories traditional see-saw and scalar triplet mechanisms, for neutrino mass suppression, do not work out anymore and for realistic model building some new ideas are needed. In this paper we suggest mechanism, different from existing ones, which provides natural suppression of the neutrino masses. The mechanism is realized through extended scalars of 4, 5 or 6 dimensional $SU(2)_L$ multiplets. Scenario, with fundamental mass scale $M_f$ in a $\sim 10^{3}$ TeV range, requires 4-plets guaranteeing neutrino masses $\stackrel{<}{\sim}1$ eV. For theories with $M_f= few \cdot 10$ TeV 5-plets should be involved, while in scenarios with $M_f=$ few TeV, 6-plets could be efficient. The considered mechanism could be successfully applied also for supersymmetric theories, building scenarios with various values of low $M_f$. Within considered models we also address the question of gauge coupling unification. For low scale unification, existence of compact extra dimensions turns out to be crucial. Due to additional scalar multiplets, some new examples of unification are found for both - non SUSY and SUSY cases. Within non SUSY scenarios introduced $SU(2)_L$ scalars take advantage and are important for successful unification.Comment: LaTex, 1 eps figure, discussions and references adde

Heavy top quark from Fritzsch mass matrices

It is shown, contrary to common belief, that the Fritzsch ansatz for the quark mass matrices admits a heavy top quark. With the ansatz prescribed at the supersymmetric grand unified (GUT) scale, one finds that the top quark may be as heavy as 145 GeV, provided that tan$\beta$ (the ratio of the vacuum expectation values of the two higgs doublets) $\gg 1$. Within a non-supersymmetric GUT framework with two (one) light higgs doublets, the corresponding approximate upper bound on the top mass is $120~ (90)$ GeV. Our results are based on a general one--loop renormalization group analysis of the quark masses and mixing angles and are readily applied to alternative mass matrix ans\"{a}tze.Comment: LaTeX, 14 figures (not included, available on request

Phenomenology of Pseudo Dirac Neutrinos

We formulate general conditions on $3\times 3$ neutrino mass matrices under which a degenerate pair of neutrinos at a high scale would split at low scale by radiative corrections involving only the standard model fields. This generalizes the original observations of Wolfenstein on pseudo Dirac neutrinos to three generations. A specific model involving partially broken discrete symmetry and solving the solar and atmospheric anomalies is proposed. The symmetry pattern of the model naturally generates two large angles one of which can account for the large angle MSW solution to the solar neutrino problem.Comment: 15 pages LATE

The Interplay between Neutrinos and Charged Leptons in the Minimal SU(3)_LxU(1)_N Gauge Model

In the minimal SU(3)_LxU(1)_N gauge model with a global L_e-L_mu-L_tau (=L') symmetry and a discrete Z_4 symmetry, it is found that the interplay between neutrinos and charged leptons contained in triplets of \psi^i=(\nu^i_L, \ell^i_L, \ell^{ci}_L) (i=1,2,3) naturally leads to the large mixing angle (LMA) MSW solution. The model includes two (anti)sextet Higgs scalars, S^(0) with L'=0 and S^(+) with L'=2, which, respectively, couple to \psi^1\psi^{2,3} for the electron mass and masses of atmospheric neutrinos and to \psi^{2,3}\psi^{2,3} for the \mu- and \tau-masses and one-loop radiative neutrino masses relevant to solar neutrinos. This mechanism is realized by utilizing an additional residual discrete symmetry supplied by explicitly broken L', which guarantees the absence of tree-level neutrino mass terms of the \psi^{2,3}\psi^{2,3}-type. Pure rotation effects due to the diagonalization of neutrino and charged-lepton mass matrices are estimated to yield \Delta m^2_\odot/\Delta m^2_{atm} \leq (m_e/m_\mu)^{3/2}=O(10^{-4}) but the radiative effects supersede the rotation effects to yield \Delta m^2_\odot/\Delta m^2_{atm}=O(10^{-2}) as the LMA solution.Comment: 16 pages, RevTeX, including 2 figures with typos and misprints corrected and with modifications in sections II-B and V, accepted by Nuclear Physics

Mixed Models with n>1 and Large Scale Structure constraints

Recent data on CBR anisotropies show a Doppler peak higher than expected in CDM cosmological models, if the spectral index $n=1$. However, CDM and LCDM models with n>1 can hardly be consistent with LSS data. Mixed models, instead, whose transfer function is naturally steeper because of free--streaming in the hot component, may become consistent with data if n>1, when Omega_h is large. This is confirmed by our detailed analysis, extended both to models with a hot component whose momentum space distribution had a thermal origin (like massive neutrinos), and to models with a non--cold component arising from heavier particle decay. In this work we systematically search models which fulfill all constraints which can be implemented at the linear level. We find that a stringent linear constraint arises from fitting the extra-power parameter Gamma. Other significant constraints arise comparing the expected abundances of galaxy clusters and high-z systems with observational data. Keeping to models with Gamma \geq 0.13, a suitable part of the space parameter still allows up to \sim 30% of hot component (it is worth outlining that our stringent criteria allow only models with 0.10 \mincir Omega_h \mincir 0.16, if n \leq 1). We also outline that models with such large non--cold component would ease the solution of the so--called baryon catastrophe in galaxy clusters.Comment: 28 pages + 9 figures, uses elsart.sty, to be published in New Astronom

A General Classification of Three-Neutrino Models and U_e3

A classification of models with three light neutrinos is given. This classification includes virtually all of the three-neutrino models proposed in the last few years, of which there are approximately one hundred. The essential ideas, attractive features, and characteristic problems of the different classes of model are discussed. The classification is based principally on how the large \nu_{\mu} - \nu_{\tau} mixing is obtained. A general discussion of the mixing parameter U_{e3} is then given, showing what values are to be expected for it in each type of model.Comment: 37 pages, LaTex. Several serious typos correcte

New Prediction For Leptonic Theta_13

An extension of the neutrino sector with two right handed singlet neutrinos responsible for Dirac neutrino masses is discussed. We show that this setup with flavor symmetry can give large solar and atmospheric neutrino mixings and suppressed Theta_13. The flavor symmetry U(1) x S_4 is shown to lead to Theta_23 = \pi/4 and a new predictive formula for the Theta_13(\simeq 0.015).Comment: 6 pages, LaTex. References added, to appear in Phys. Lett.

Scaling in the Neutrino Mass Matrix, mu-tau Symmetry and the See-Saw Mechanism

The scaling hypothesis postulates proportionality of two columns of the Majorana neutrino mass matrix in the flavor basis. This Ansatz was shown to lead to an inverted hierarchy and U_{e3} = 0. We discuss theoretical and phenomenological properties of this hypothesis. We show that (i) the neutrino mass matrix with scaling follows as a consequence of a generalized mu-tau symmetry imposed on the type-I see-saw model; (ii) there exists a unique texture for the Dirac mass matrix m_D which leads to scaling for arbitrary Majorana matrix M_R in the context of the type-I see-saw mechanism; (iii) unlike in the mu-tau symmetric case, a simple model with two right-handed neutrinos and scaling can lead to successful leptogenesis both with and without the inclusion of flavor effects.Comment: 18 pages, matches published versio