arXiv subjects

Anjan S. Joshipura

Publications and source records attributed to Anjan S. Joshipura.

At least 73 records · Page 4Linked to original sources

arXiv1999

Neutrino Anomalies in an Extended Zee Model

We discuss an extended $SU(2)\times U(1)$ model which naturally leads to mass scales and mixing angles relevant for understanding both the solar and atmospheric neutrino anomalies. No right-handed neutrinos are introduced in the model.The model uses a softly broken $L_e-L_μ-L_τ$ symmetry. Neutrino masses arise only at the loop level. The one-loop neutrino masses which arise as in the Zee model solve the atmospheric neutrino anomaly while breaking of $L_e-L_μ-L_τ$ generates at two-loop order a mass splitting needed for the vacuum solution of the solar neutrino problem. A somewhat different model is possible which accommodates the large-angle MSW resolution of the solar neutrino problem.

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arXiv1999

Constraining Leptogenesis from Laboratory Experiments

The presently available information on neutrino oscillations can be used to tightly constrain the light neutrino mass matrix $m_\n$ under the assumption that the three known neutrinos provide explanation of the solar and atmospheric anomalies. We exploit left right symmetry and the seesaw model to establish direct correlation between this constrained $m_\n$ and the mass matrix $M_R$ for the right handed (RH) neutrinos. Using this correlation, one could directly relate the lepton asymmetry $\e$ generated in the decay of the lightest RH neutrino to experimentally determined mixing angles of the light neutrinos. It is found that the parameters required to understand neutrino anomalies also give rise to the $\e$ in the range required to reproduce the correct baryon asymmetry in the leptogenesis scenario. Specifically, the small angle MSW or the the vacuum oscillation explanation of the solar anomaly can lead to correct $\e$ for maximal value of the CP violating phases. In contrast, the large angle MSW solution needs some suppression in CP violating phases in order to generate the right $\e$.

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arXiv1999

Sneutrino Vacuum Expectation Values and Neutrino Anomalies Through Trilinear R-parity Violation

Neutrino mass spectrum is reanalyzed in supersymmetric models with explicit trilinear $R$ violation. Models in this category are argued to provide simultaneous solution to the solar and atmospheric neutrino anomalies. It is shown specifically that large mixing and hierarchical masses needed for the vacuum solution of neutrino anomalies arise naturally in these models without requiring any additional symmetries or hierarchies among the trilinear couplings.

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arXiv1998

Vacuum Solutions of Neutrino Anomalies Through a Softly Broken U(1) symmetry

We discuss an extended SU(2)XU(1) model which naturally leads to mass scales and mixing angles relevant for understanding both the solar and atmospheric neutrino anomalies in terms of the vacuum oscillations of the three known neutrinos. The model uses a softly broken L_e-L_μ-L_τ symmetry and contains a heavy scale M_H\sim 10^{15} GeV. The L_e-L_μ-L_τ symmetric neutrino masses solve the atmospheric neutrino anomaly while breaking of L_e-L_μ-L_τ generates highly suppressed radiative mass scale Δ_S\sim 10^{-10} eV^2 needed for the vacuum solution of the solar neutrino problem. All the neutrino masses in the model are inversely related to M_H, thus providing seesaw-type of masses without invoking any heavy right-handed neutrinos. Possible embedding of the model into an SU(5) grand unified theory is discussed.

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arXiv1998

Gravitationally violated U(1) symmetry and neutrino anomalies

The current searches for neutrino oscillations seem to suggest an approximate $L_e-L_\m-L_τ$ flavor symmetry. This symmetry implies a pair of degenerate neutrinos with mass $m_0$ and large leptonic mixing. We explore the possibility that gravitational interactions break this global symmetry. The Planck scale suppressed breaking of the $L_e-L_\m-L_τ$ symmetry is shown to lead to the right amount of splitting among the degenerate neutrinos needed in order to solve the solar neutrino problem. The common mass $m_0$ of the pair can be identified with the atmospheric neutrino scale. A concrete model is proposed in which smallness of $m_0$ and hierarchy in the solar and atmospheric neutrino scales get linked to hierarchies in the weak, grand unification and the Planck scales.

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arXiv1998

Gauge mediated supersymmetry breaking and neutrino anomalies

Supersymmetric standard model with softly broken lepton symmetry provides a suitable framework to accommodate the solar and atmospheric neutrino anomalies. This model contains a natural explanation for large mixing and hierarchal masses without fine tuning of the parameters. Neutrino spectrum is particularly constrained in the minimal messenger model (MMM) of gauge mediated SUSY breaking, since all SUSY breaking effects are controlled in MMM by a single parameter. We study the structure of neutrino masses and mixing both in MMM and in simple extensions of it in the context of solar and atmospheric neutrino anomalies.

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arXiv1998

Rescuing Grand Unification Scenario for Neutrino Mass

The see-saw mechanism for neutrino masses based on the Grand Unification leads to the mass of the heaviest neutrino ($\approx ν_τ$) in the range $(2 - 3)\cdot 10^{-3}$ eV and hence to a solution of the solar neutrino problem through the $ν_e-ν_\t$ conversion. We suggest the existence of a light singlet fermion $S$ which mixes predominantly with muon neutrino through the mixing mass $m_{μs} \sim O(1)$ eV. The introduction of such a singlet allows one (i) to solve the atmospheric neutrino problem via the $ν_μ \leftrightarrow S$ oscillations, (ii) to explain the LSND result and (iii) to get two component hot dark matter in the Universe. The phenomenology of this scenario is considered and the origin of the singlet as well as of the scale $m_{\m s}$ is discussed.

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arXiv1998

R Parity Violation: Constraints and Implications

The constraints on trilinear R parity violating couplings $λ'_{ijk}$ following from ({\em i}) the neutrino mass resulting due to the induced vacuum expectation value for the sneutrino and (ii) the charm squark interpretation for the HERA anomalous events are discussed in this talk.}

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arXiv1997

On the charm-squark interpretation of the HERA events

The charm squark resonance in the $e^+ q$ channel provides one of the plausible interpretations of the reported anomaly at HERA. We show that the relevant $R$ violating coupling $λ'_{121}$ is required to be large, typically around 0.1 in a large class of supergravity based models including the minimal one with the universal boundary condition at the GUT scale. Existing constraints on this coupling are reanalysed in this light and it is argued that a large $λ'_{121}$ may be feasible but would require fine tuned cancelations.

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arXiv1997

Neutrino Mass Constraints on R violation and HERA anomaly

R parity violating trilinear couplings $λ'_{1jk}$ of the minimal supersymmetric standard model (MSSM) are constrained from the limit on the electron neutrino mass. Strong limits on these couplings follow from the earlier neglected contribution due to sneutrino vacuum expectation values. The limits on most of the $λ'_{1jk}$ couplings derived here are stronger than the existing ones for a wide range in parameters of MSSM. These limits strongly constrain the interpretation of recent HERA results in $e^+ p$ scattering in terms of production of squarks through $R$ violating couplings. In particular, the interpretation in terms of $\tilde{t}_L$ production off strange quark as suggested recently is not viable for wide ranges in MSSM parameters.

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arXiv1996

Gravitational Violation of R Parity and its Cosmological Signatures

The discrete R-parity ($R_P$) usually imposed on the Supersymmetric (SUSY) models is expected to be broken at least gravitationally. If the neutralino is a dark matter particle its decay channels into positrons, antiprotons and neutrinos are severely constrained from astrophysical observations. These constraints are shown to be violated even for Planck-mass-suppressed dimension-five interactions arising from gravitational effects. We perform a general analysis of gravitationally induced $R_P$ violation and identify two plausible and astrophysically consistent scenarios for achieving the required suppression.

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arXiv1996

Automatic CP Invariance and Flavor Symmetry

The approximate conservation of $CP$ can be naturally understood if it arises as an automatic symmetry of the renormalizable Lagrangian. We present a specific realistic example with this feature. In this example, the global Peccei-Quinn symmetry and gauge symmetries of the model make the renormalizable Lagrangian $CP$ invariant but allow non zero hierarchical masses and mixing among the three generations. The left-right and a horizontal $U(1)_H$ symmetry is imposed to achieve this. The non-renormalizable interactions invariant under these symmetries violate $CP$ whose magnitude can be in the experimentally required range if $U(1)_H$ is broken at very high, typically, near the grand unification scale.

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arXiv1995

Novel Scalar Boson Decays in SUSY with Broken R-Parity

R parity violation can induce mixing of the supersymmetric Higgs bosons with the sneutrinos at the tree level. We study the effect of this mixing on the decays of Higgs scalars as well as sneutrinos in an effective model where the violation of R parity is included in the minimal supersymmetric model through bilinear lepton number violating superpotential terms. We show that a small violation of R parity can lead to a sizeable branching ratio for the supersymmetric Higgs boson decay mode $H \to χ\ell$ (where $χ$ denotes an electroweak gaugino and $\ell$ is either a tau neutrino or a tau lepton). Relevant constraints on R parity violation as well as those coming from SUSY particle searches still allow the decay $H \to χ\ell$ to compete with the conventional decay $H\ra b \bar{b}$, at least for some ranges of parameters of the model. Moreover, the tau sneutrino will have dominant R parity violating decays to standard model fermions $b \bar{b}$, $τ^+ τ^-$ or to the invisible mode $ν\barν$ whenever the phase space for R parity conserving channels is closed.

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arXiv1995

Models of Light Singlet Fermion and Neutrino Phenomenology

We suggest that a singlet fermion $S$ exists beyond the standard see-saw structure. It mixes with light neutrinos via interactions with the right-handed neutrino components, so that $\n_e \to S$ conversion solves the solar neutrino problem. Supersymmetry endowed with R-symmetry is shown to give a natural framework for existence, mass scale and mixing ($\sin^22þ_{es} \sim (0.1-1.5)\cdot 10^{-2}$) of such a fermion. Models with an approximate horizontal symmetry are constructed, which embed the fermion $S$ and explain simultaneously solar, atmospheric, hot dark matter problems as well as may predict the oscillation $\bar{\n}_\m \to \bar{\n}_e$ in the region of sensitivity of KARMEN and LSND experiments.

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arXiv1995

Beyond Standard Model: Report of Working Group II

Working group II at WHEPP3 concentrated on issues related to the supersymmetric standard model as well as SUSY GUTS and neutrino properties. The projects identified by various working groups as well as progress made in them since WHEPP3 are briefly reviewed.

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arXiv1994

Reconciling Cold Dark Matter with COBE/IRAS Plus Solar and Atmospheric Neutrino Data

We present a model where an unstable MeV Majorana tau \neu can naturally reconcile the cold dark matter model (CDM) with cosmological observations of large and small scale density fluctuations and, simultaneously, with data on solar and atmospheric neutrinos. The solar \neu deficit is explained through long wavelength, so-called {\sl just-so} oscillations involving conversions of \ne into both \nm and a sterile species \ns, while atmospheric \neu data are explained through \nm to \ne conversions. Future long baseline \neu oscillation experiments, as well as some reactor experiments will test this hypothesis. The model is based on the spontaneous violation of a global lepton number symmetry at the weak scale. This symmetry plays a key role in generating the cosmologically required decay of the \nt with lifetime $τ_{ν_τ} \sim 10^2 - 10^4$ seconds, as well as the masses and oscillations of the three light \neus \ne, \nm and \ns required in order to account for solar and atmospheric \neu data. It also leads to the invisibly decaying higgs signature that can be searched at LEP and future particle colliders.

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arXiv1994

Just so oscillations in supersymmetric standard model

We analyze the spectrum and mixing among neutrinos in the minimal supersymmetric standard model with explicit breaking of R parity. It is shown that ({\em i}) the mixing among neutrinos is naturally large and ({\em ii}) the non zero neutrino mass is constrained to be $\leq 10^{-5}$ eV from arguments based on baryogenesis. Thus vacuum oscillations of neutrinos in this model may offer a solution of the solar neutrino problem. The allowed space of the supersymmetric parameters consistent with this solution is determined.

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arXiv1994

Limits on Associated Production of Visibly and Invisibly Decaying Higgs Bosons from Z Decays

Many extensions of the standard electroweak model Higgs sector suggest that the main Higgs decay channel is "invisible", for example, $h \to J J$ where $J$ denotes the majoron, a weakly interacting pseudoscalar Goldstone boson associated to the spontaneous violation of lepton number. In many of these models the Higgs boson may also be produced in association to a massive pseudoscalar boson (HA), in addition to the standard Bjorken mechanism (HZ). We describe a general strategy to determine limits from LEP data on the masses and couplings of such Higgs bosons, using the existing data on acoplanar dijet events as well as data on four and six $b$ jet event topologies. For the sake of illustration, we present constraints that can be obtained for the ALEPH data.

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