arXiv subjects

K. S. Babu

Publications and source records attributed to K. S. Babu.

At least 199 records · Page 11Linked to original sources

arXiv1995

Indirect Neutrino Oscillations

We show how two different scales for oscillations between $e$ and $μ$ neutrinos, characterized by different mixing angles and effective mass scales, can arise in a simple and theoretically attractive framework. One scale characterizes direct oscillations, which can accommodate the MSW approach to the solar neutrino problem, whereas the other can be considered as arising indirectly, through virtual transitions involving the $τ$ neutrino with a mass $\sim$ 1 eV. This indirect transition allows the possibility of observable $\bar ν_μ\leftrightarrow \bar ν_e$ oscillations at accelerator and reactor energies. We discuss specifically the parameters suggested by a recent experiment at Los Alamos within this framework.

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arXiv1995

A Predictive SO(10) Scheme for Fermion Masses and Mixings

We present a predictive scheme for fermion masses and mixings inspired by supersymmetric SO(10) in which the gauge hierarchy problem is resolved without fine tuning the parameters. There are six predictions in the flavor sector, all consistent with the present data. The scheme reproduces the familiar asymptotic relations $m_b=m_τ$ and $m_dm_sm_b=m_e m_μm_τ$. A new expression for $V_{cb}$ is obtained in terms of the quark masses. The remaining predictions involve the quark mixing angles $V_{us}$ and $V_{ub}$, as well as the parameter tan$β$ which turns out to be close to $m_t/m_b$.

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arXiv1995

"An SO(10) Solution to the Puzzle of Quark and Lepton Masses"

It is shown that almost all features of the quark and lepton masses can be satisfactorily and simply explained without family symmetry, including the threefold mass hierarchy among the generations, and the relations $m_τ^0 = m_b^0$, $m_μ^0 = 3 m_s^0$, $m_e^0 = \frac{1}{3} m_d^0$, $m_u^0/m_t^0 << m_d^0/m_b^0$, $\tan θ_c = \sqrt{m_d^0/m_s^0}$, $V_{cb} << \sqrt{m_s^0/m_b^0}$, and $V_{ub} \sim V_{cb}V_{us}$. Various aspects of the group theory of $SO(10)$ play an essential role in explaining these relations. The form of the mass matrices, rather than being imposed arbitrarily, emerges naturally from a simple structure at the unification scale. This structure involves only vector, spinor and adjoint representations. There are distinctive and testable predictions for $\tan β$ and the neutrino mixing angles.

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arXiv1994

Mass Matrix Textures From Superstring Inspired SO(10) Models

We give a general prescription for deriving quark and lepton mass matrices with ``texture'' zeros in the framework of superstring inspired $SO(10)$ models. The key to our approach is a new way to naturally implement the doublet--triplet splitting which enables us to obtain symmetric quark and lepton mass matrices which have different structures in the up and the down quark sectors. We illustrate our method by deriving the Georgi--Jarlskog texture which has six predictions in the flavor sector, and then show how it generalizes to other symmetric texture models.

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arXiv1994

"A Hint From the Inter-Family Mass Hierarchy: Two Vector-Like Families in the TeV range"

Two vector-like families with masses of order 1 TeV, one of which is a doublet of $SU(2)_L$ and the other a doublet of $SU(2)_R$, have been predicted to exist in the context of a viable and economical SUSY composite model. One of the many attractive features of the model is an explanation of the inter-family mass-hierarchy for which the existence of the two vector-families is crucial. This paper is devoted to a detailed study of the expected masses, mixings and decay modes of the fermions belonging to the two vector-like families. The masses of the vector-like quarks are expected to lie in the range of 500 GeV to about 2.5 TeV, while those of the vector-like leptons are expected to be in the range of 200 GeV to 1 TeV. Their mass pattern and decay modes exhibit certain distinguishing features and characteristic signals. For example, when the LHC and, possibly a future version of the SSC are built, pair-production of the vector-like quarks would lead to systems such as ($b\overline{b}+4Z+W^+W^-$) and ($b\overline{b}+2Z+W^+W^-$), while an $e^-e^+$ linear collider (NLC) of suitable energy can produce appreciably a single neutral heavy lepton $N$ together with $ν_τ$, followed by the decay of $N$ into $(Z+ν_τ) \rightarrow (e^-e^+) + ν_τ$. This last signal may conceivably materialize even at LEP 200 if $N$ is lighter than about 190 GeV.

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arXiv1994

Supersymmetric SO(10) Simplified

In the context of supersymmetric $SO(10)$ grand unified models, it is shown that the gauge symmetry breaking as well as a natural doublet--triplet splitting can be achieved with a minimal Higgs system consisting of a single adjoint and a pair of vector and spinor multiplets. Such a Higgs spectrum has been shown to arise in the free fermionic formulation of superstrings. Since the symmetry breaking mechanism relies on non--renormalizable operators, some of the Higgs particles of the model turn out to have masses somewhat below the GUT scale. As a consequence, the unification scale is raised to about $2 \times 10^{17}~GeV$ and sin$^2θ_W$ is predicted to be slightly larger than the minimal SUSY--$SU(5)$ value. Including threshold uncertainties, which turn out to be surprisingly small in the model, we show that sin$^2θ_W$ prediction is consistent with experiments.

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arXiv1994

Axion Dissipation Through the Mixing of Goldstone Bosons

By coupling axions strongly to a hidden sector, the energy density in coherent axions may be converted to radiative degrees of freedom, alleviating the ``axion energy crisis''. The strong coupling is achieved by mixing the axion and some other Goldstone boson through their kinetic energy terms, in a manner reminiscent of paraphoton models. Even with the strong coupling it proves difficult to relax the axion energy density through particle absorption, due to the derivative nature of Goldstone boson couplings and the effect of back reactions on the evolution of the axion number density. However, the distribution of other particle species in the hidden sector will be driven from equilibrium by the axion field oscillations. Restoration of thermal equilibrium results in energy being transferred from the axions to massless particles, where it can redshift harmlessly without causing any cosmological problems.

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arXiv1994

An Automatic Invisible Axion In The SUSY Preon Model

It is shown that the recently proposed preon model which provides a unified origin of the diverse mass scale and an explanation of family replication as well as of inter-family mass-hierarchy, possesses a Peccei-Quinn symmetry whose spontaneous breaking leads to an automatic invisible axion. Existence of the PQ-symmetry is simply a consequence of supersymmetry and requirement of minimality in the field-content and interactions, which propose that the lagrangian should possess only those terms which are dictated by the gauge principle and no others. In addition to the axion, the model also generates two superlight Goldstone bosons and their superpartners which are cosmologically safe.

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arXiv1994

Natural Gauge Hierarchy in SO(10)

It is shown that a natural gauge hierarchy and doublet-triplet splitting can be achieved in SO(10) using the Dimopoulos-Wilczek mechanism. Artificial cancellations (fine-tuning) and arbitrary forms of the superpotential are avoided, the superpotential being the most general compatible with a symmetry. It is shown by example that the Dimopoulos-Wilczek mechanism can be protected against the effects of higher-dimension operators possibly induced by Planck-scale physics. Natural implementation of the mechanism leads to an automatic Peccei-Quinn symmetry. The same local symmetries that would protect the gauge hierarchy against Planck-scale effects tend to protect the axion also. It is shown how realistic quark and lepton masses might arise in this framework. It is also argued that ``weak suppression'' of proton decay can be implemented more economically than can ``strong suppression'', offering some grounds to hope (in the context of SO(10)) that proton decay could be seen at Superkamiokande.

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arXiv1994

Supersymmetric Models With Tan(beta) Close to Unity

Within the framework of supersymmetric grand unification, estimates of the $b$ quark mass based on the asymptotic relation $m_b \simeq m_τ$ single out the region with $\tanβ$ close to unity, particularly if $m_t(m_t) \stackrel{_<}{_\sim} 170\ GeV$. We explore the radiative breaking of the electroweak symmetry and the associated sparticle and higgs spectroscopy in models with $1 < \tanβ\stackrel{_<}{_\sim} 1.6$. The lightest scalar higgs is expected to have a mass below $100\ GeV$, while the remaining four higgs masses exceed $300\ GeV$. The lower bounds on some of the sparticle masses are within the range of LEP 200.

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arXiv1993

Constraints on left-right symmetric models from the process $b \rightarrow s γ$

In left-right symmetric models, large contributions to the decay amplitude $b \rightarrow s+ γ$ can arise from the mixing of the $W_L$ and $W_R$ gauge bosons as well as from the charged Higgs boson. These amplitudes are enhanced by the factor $\mt/\mb$ compared to the contributions in the standard model. We use the recent CLEO results on the radiative $B$ decay to place constraints on the $W_L-W_R$ mixing angle $ζ$ and the mass of the charged Higgs boson $m_{H^\pm}$. Significant departures from the standard model predictions occur when $|ζ| > 0.003$ and/or when $m_{H^\pm} < $ a few TeV.

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arXiv1993

Closing the Windows on Mev Tau Neutrinos

In this note, we analyze various constraints on the ``visible'' decay modes of a massive $τ$ neutrino, $ν_τ\rightarrowν^\prime\,γ$ and $ν_τ\rightarrowν^\prime\, e^+ e^-$, where $ν^\prime$ is a light neutrino. The BEBC beam dump experiment provides model-independent constraints on these modes. The lifetime for the $ν^\prime\, e^+e^-$ mode is constrained to be $τ_{ν^\prime\, e^+e^-} \ge 0.18~(m_{ν_τ}/MeV)~sec.$ We point out that the same experiment implies a similar constraint on the $ν^\prime\,γ$ mode. This results in a new upper limit on the transition magnetic moment of $ν_τ$, $μ_{\rm tran} \le 1.1 \times 10^{-9} (MeV/m_{ν_τ})^2 μ_B $. Furthermore, a limit on the electric charge of $ν_τ$ may be obtained, $Q_{ν_τ} \le 4 \times 10^{-4}e$. Combining these constraints with those arising from supernova observations and primordial nucleosynthesis calculations, we show that these ``visible'' decays cannot be the dominant decay modes of the $τ$ neutrino.

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arXiv1993

A Solution to the Small Phase Problem of Supersymmetry

It is a well--known problem that in supersymmetric models there are new CP--violating phases which, if unsuppressed, would give a neutron electric dipole moment $10^2$ to $10^3$ times the present experimental limit. Here we propose that these new phases are suppressed by CP invariance, which is broken spontaneously at a high scale and that this breaking shows up at low energies only through a universal phase of the gaugino masses. It is shown that this can well fit both $ε$ and $ε^\prime$ of the neutral Kaon system. The electric dipole moments of the neutron and the electron should be not much below present limits. A model incorporating these ideas in a very economical way is presented.

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arXiv1993

Renormalization of the Neutrino Mass Operator

A small neutrino Majorana mass can arise in the Standard Model as an effective dimension 5 operator. We calculate the renormalization of this operator in the minimal Standard Model and in its two-Higgs-doublet and supersymmetric extensions. Renormalization from the scale of lepton number violation (e.g., the Planck scale or a GUT scale) to the weak scale decreases the strength of this operator by an order of magnitude or more if the top quark and Higgs boson masses are large. Neutrino mixing angles also run with momentum. We show instances where small mixing at a high scale becomes large at the weak scale, and vice versa.

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arXiv1993

Spontaneous CP violation in the Supersymmetric Higgs Sector

Spontaneous CP--violation in the minimal supersymmetric standard model with a gauge singlet and a cubic superpotential is examined. Although the tree--level Higgs potential conserves CP, it is shown that with the inclusion of the one--loop top--quark radiative effects, CP may be broken spontaneously. The CP--violating minimum requires two neutral $(h_1,h_2)$ and one charged $(H^\pm)$ Higgs--bosons to be relatively light with $m_{h_1}+m_{h_2} \stackrel{_<}{_\sim} 100~GeV$ and $m_{H^\pm} \stackrel{_<}{_\sim} 110~GeV$. The electric dipole moment of the electron is in the observable range of $({1 \over 3}~{\rm to}~3) \times 10^{-27}$e-cm.

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arXiv1993

Natural Suppression of Higgsino-Mediated Proton Decay in Supersymmetric SO(10)

In supersymmetric Grand Unified Theories, proton decay mediated by the color--triplet higgsino is generally problematic and requires some fine--tuning of parameters. We present a mechanism which naturally suppresses such dimension 5 operators in the context of SUSY $SO(10)$. The mechanism, which implements natural doublet--triplet splitting using the adjoint higgs, converts these dimension 5 operators effectively into dimension 6. By explicitly computing the higgs spectrum and the resulting threshold uncertainties we show that the successful prediction of $\sin^2θ_W$ is maintained {\it as a prediction} in this scheme. It is argued that only a weak suppression of the higgsino mediated proton decay is achievable within SUSY $SU(5)$ without fine--tuning, in contrast to a strong suppression in SUSY $SO(10)$.

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arXiv1993

Leptonic Fritzsch Matrices and Neutrino Oscillations

It was recently shown by the authors that the Fritzsch ansatz for the quark mass matrices prescribed at the supersymmetric grand unified scale is compatible with a moderately heavy top quark ($m_t \simeq 120-150~GeV$). Here we extend the ansatz to incorporate the charged leptons and the neutrinos. It is found that the $ν_e-ν_μ$ mixing angle is small and consistent with the MSW solution of the solar neutrino puzzle. Furthermore, the model predicts observable $ν_μ-ν_τ$ oscillations with sin$^22θ_{μτ} \simeq 0.1$ and $ν_τ$ mass in the $(1-3)~eV$ range.

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arXiv1993

Planck Scale Symmetry Breaking and Majoron Physics

Majoron models provide neutrino masses via the spontaneous breaking of a global $U(1)$ symmetry. However, it may be argued that all global symmetries will be explicitly violated by gravitational effects. We show that it is possible to preserve most of the usual features of majoron models by invoking $U(1)_{B-L}$ to be a gauge symmetry and adding a second singlet scalar field. The majoron gets a small model dependent mass. The couplings of majorons to neutrinos may be of ordinary strength or may be made arbitrarily weak. We discuss the cosmological and astrophysical consequences of majoron models in the context of a model dependent majoron mass and neutrino coupling. For an appropriate choice of parameters majorons can play the role of dark matter.

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