arXiv subjects

Qaisar Shafi

Publications and source records attributed to Qaisar Shafi.

At least 235 records · Page 13Linked to original sources

arXiv2001

Neutrino oscillations and rare processes in models with a small extra dimension

We discuss Dirac neutrino masses and mixings in a scenario where both the standard model fermions and right handed neutrinos are bulk fields in a non-factorizable geometry in five dimensions. We show how the atmospheric and solar neutrino anomalies can be satisfactorily resolved, and in particular how bimaximal mixing is realized. We also consider rare processes such as neutron-antineutron oscillations and mu --> e + gamma, which may occur at an observable rate.

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arXiv2001

Axionic Domain Wall and Warped Geometry

We discuss how a three-brane with an associated non-factorizable (warped) geometry can emerge from a five dimensional theory of gravity coupled to a complex scalar field. The system possesses a discrete $Z_2$ symmetry, whose spontaneous breaking yields an 'axionic' three-brane and a warped metric. Analytic solutions for the wall profile and warp factor are presented. The Kaluza-Klein decomposition and some related issues are also discussed.

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arXiv2001

Bi-maximal Neutrino Mixing And Anomalous Flavor ${\cal U}(1)$

We describe how bi-maximal neutrino mixing can be realized in realistic models based on MSSM and SUSY GUTs such as SU(5) and SO(10). A crucial role is played by an anomalous ${\cal U}(1)$ flavor symmetry, which also helps understand the observed charged fermion mass hierarchies and the magnitudes of the CKM matrix elements. While in MSSM a variety of solutions for the solar neutrino puzzle are possible, SUSY SU(5) and SO(10) only permit the large mixing angle MSW solution. Models in which the ${\cal U}(1)$ symmetry also mediates SUSY breaking allow both the large mixing angle and the low MSW solutions. We also present renormalization group studies for the neutrino mass matrix, generated through the ${\cal U}(1)$ flavor symmetry. Our analysis shows that renormalization does not change the desirable picture of bi-maximal mixing.

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arXiv2000

Fermion Masses, Mixings and Proton Decay in a Randall-Sundrum Model

We consider a Randall-Sundrum model in which the Standard Model fermions and gauge bosons correspond to bulk fields. We show how the observed charged fermion masses and CKM mixings can be explained, without introducing hierarchical Yukawa couplings. We then study the impact on the mass scales associated with non-renormalizable operators responsible for proton decay, neutrino masses, and flavor changing neutral currents. Although mass scales as high as 10^{11}- 10^{12} GeV are in principle possible, dimensionless couplings of order 10^{-8} are still needed to adequately suppress proton decay. Large neutrino mixings seem to require new physics beyond the Standard Model.

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arXiv2000

Higgs Mechanism and Bulk Gauge Boson Masses in the Randall-Sundrum Model

Assuming the breaking of gauge symmetries by the Higgs mechanism, we consider the associated bulk gauge boson masses in the Randall-Sundrum background. With the Higgs field confined on the TeV-brane, the W and Z boson masses can naturally be an order of magnitude smaller than their Kaluza-Klein excitation masses. Current electroweak precision data requires the lowest excited state to lie above about 30 TeV, with fermions on the TeV-brane. This bound is reduced to about 10 TeV if the fermions reside sufficiently close to the Planck-brane. Thus, some tuning of parameters is needed. We also discuss the bulk Higgs case, where the bounds are an order of magnitude smaller.

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arXiv2000

Anomalous Flavor U(1): Predictive Texture For Bi-maximal Neutrino Mixing

We present a scenario which naturally provides bi-maximal neutrino mixings for a simultaneous explanation of the recent atmospheric and solar neutrino data. A crucial role is played by an anomalous flavor ${\cal U}(1)$ symmetry, which is also important for a natural understanding of charged fermion mass hierarchies and magnitudes of the CKM matrix elements. Within MSSM the solar neutrino problem can be resolved either through vacuum oscillations or the large mixing angle MSW solution. In supersymmetric GUTs such as SU(5) and SO(10) the MSW solution is realized. If the flavor ${\cal U}(1)$ also mediates supersymmetry breaking, the vacuum solution in MSSM is eliminated, and only the large mixing angle MSW solution survives.

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arXiv1999

Flavor Problem, Proton Decay And Neutrino Oscillations In SUSY Models With Anomalous ${\cal U}(1)$

We discuss how realistic supersymmetric models can be constructed by employing an anomalous ${\cal U}(1)$ flavor symmetry which also mediates supersymmetry breaking. A judicious choice of ${\cal U}(1)$ charges enables the first two squark families to be sufficiently heavy ($\stackrel{>}{_\sim}10$ TeV), so that flavor changing neutral currents as well as dimension five nucleon decay are adequately suppressed. Using the SU(5) example, the charged fermion mass hierarchies, magnitudes of the CKM matrix elements, as well as the observed neutrino oscillations are simultaneously accommodated. We estimate the proton lifetime to be $τ_p\sim 10^3\cdot τ_p[{\rm minimal} SU(5)]$, with the decay mode $p\to Kμ$ being comparable to $p\to Kν_{μ, τ}$.

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arXiv1999

Bi-maximal Neutrino Mixings And Proton Decay In SO(10) With Anomalous Flavor ${\cal U}(1)$

By supplementing supersymmetric SO(10) with an anomalous ${\cal U}(1)$ flavor symmetry and additional `matter' superfields carrying suitable ${\cal U}(1)$ charges, we explain the charged fermion mass hierarchies, the magnitudes of the CKM matrix elements, as well as the solar and atmospheric neutrino data. We stress bi-maximal vacuum neutrino mixings, and indicate how the small or large mixing angle MSW solution can be incorporated. The ${\cal U}(1)$ symmetry also implies that $τ_{p\to Kν}[SO(10)]\sim (10-100)\cdot τ_{p\to Kν}[{\rm minimal}~SU(5)]$.

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arXiv1999

Supersymmetric CP violating Phases and the LSP relic density and detection rates

For varying values of $\tanβ$, we study the effect of CP violating phases from the soft supersymmetry breaking terms in string-inspired models on the relic abundance and detection rates of the lightest neutralino (LSP). We find that the phases have no significant effect on the LSP relic abundance but can have a substantial impact on the detection rates.

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arXiv1999

Proton Decay, Neutrino Oscillations And Other Consequences From Supersymmetric SU(6) With Pseudo-Goldstone Higgs

We suggest a new mechanism for naturally suppressing dimension five baryon number violating in supersymmetric SU(5+N) (N=0, 1,...) GUTs. The mechanism is realized through suppression of qqT type couplings, and is implemented by introducing new `matter' multiplets belonging to symmetric representations of SU(5+N). Together with the suppression of nucleon decay, these multiplets also enable one to avoid the unwanted asymptotic mass relations $m_s=m_μ$, $\frac{m_d}{m_s}=\frac{m_e}{m_μ}$. As an example, we consider a SU(6) model with pseudo-Goldstone Higgs. By supplementing the model with an anomalous ${\cal U}(1)$ flavor symmetry, we also obtain a simple `all-order' solution of the gauge hierarchy problem and natural explanation of charged fermion mass hierarchies and values of the CKM matrix elements. The proton life time $τ_p\sim 10^{2}τ_p^{SU(5)}$~yr. is compatible with experiments, with the dominant decay being $p\to Kν_{μ, τ}$. Thanks to the SU(6) symmetry, successful unification of the gauge couplings can be retained, and the value of the strong coupling $α_s(M_Z)$ can be reduced to $\simeq 0.12$. Finally, we show how to accommodate the solar and atmospheric neutrino data through the bi-maximal neutrino mixing scenario, with maximal vacuum $ν_e-ν_{μ, τ}$ and large angle $ν_μ-ν_τ$ oscillations.

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arXiv1999

R-Parity Violation: Origin of $μ$-Term and Other Consequences

We propose a new mechanism in which the generation of the supersymmetric $μ$-term as well charged lepton masses is closely tied to R-parity violation involving heavy vector-like families . A realistic example based on $SU(3)_c\times SU(2)_L\times U(1)_Y$, supplemented by the symmetry ${\cal R}\times {\cal U}(1)$, is presented, where ${\cal R}$ (${\cal U}(1)$) denotes a continuous $R$ (flavor) symmetry. In addition to the $μ$-term, the charged fermion mass hierarchies and mixings, as well as baryon number conservation are also nicely explained. Bilinear R-parity violating coupling involving the first generation gives rise to neutrino mass relevant for the small angle $ν_e-ν_s$ MSW oscillations, where $ν_s $ denotes a sterile state. The atmospheric neutrino puzzle is resolved via maximal mixing angle $ν_{μ}-ν_τ$ oscillations. The decay of the lightest neutralino (LSP) and leptogenesis are briefly discussed.

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arXiv1999

Low energy consequences from supersymmetric models with left-right symmetry

We consider several low energy consequences arising from a class of supersymmetric models based on the gauge groups $SU(2)_L\times SU(2)_R \times U(1)_{B-L}$ and $SU(4)_C\times SU(2)_L \times SU(2)_R$ in which the gauge hierarchy and $μ$ problems have been resolved. There are important constraints on the MSSM parameters $\tan β(\simeq m_t/m_b)$, $B$ and $μ$, and we discuss how they are reconciled with radiative electroweak breaking. We also consider the ensuing sparticle and Higgs spectroscopy, as well as the decays $b\to s γ$ and $μ\to e γ$. The latter process may be amenable to experimental tests through an order of magnitude increase in sensitivity.

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arXiv1999

An Improved Supersymmetric SU(5)

By supplementing minimal supersymmetric SU(5) (MSSU(5)) with a flavor ${\cal U}(1)$ symmetry and two pairs of $\bar{15}+15$ `matter' supermultiplets, we present an improved model which explains the charged fermion mass hierarchies and the magnitudes of the CKM matrix elements, while avoiding the undesirable asymptotic mass relations $m_s=m_μ, m_d/m_s = m_e/m_μ$. The strong coupling $α_s (M_Z)$ is predicted to be approximately 0.115, and the proton lifetime is estimated to be about five times larger than the MSSU(5) value. The atmospheric and solar neutrino puzzles are respectively resolved via maximal $ν_μ-ν_τ$ and small mixing angle $ν_e-ν_s$ MSW oscillations, where $ν_s$ denotes a sterile neutrino. The ${\cal U}(1)$ symmetry ensures not only a light $ν_s$ but also automatic `matter' parity.

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arXiv1999

Phenomenology with Supersymmetric Flipped SU(6)

The supersymmetric flipped $SU(6)\times U(1)$ gauge symmetry can arise through compactification of the ten dimensional $E_8\times E_8$ superstring theory. We show how realistic phenomenology can emerge from this theory by supplementing it with the symmetry ${\cal R}\times {\cal U}(1)$, where ${\cal R}$ denotes a discrete `R'-symmetry. The well-known doublet-triplet splitting problem is resolved to `all orders' via the pseudo-Goldstone mechanism, and the GUT scale arises from an interplay of the Planck and supersymmetry breaking scales. The symmetry ${\cal R}\times {\cal U}(1)$ is also important for understanding the fermion mass hierarchies as well as the magnitudes of the CKM matrix elements. Furthermore, the well known MSSM parameter $\tan β$ is estimated to be of order unity, while the proton lifetime ($τ_p\sim 10^2τ_{pSU(5)}$) is consistent with observations. Depending on some parameters, $p\to Kμ^+ $ can be the dominant decay mode. Finally, the observed solar and atmospheric neutrino `anomalies' require us to introduce a `sterile' neutrino state. Remarkably, the ${\cal R}\times {\cal U}(1)$ symmetry protects it from becoming heavy, so that maximal angle $ν_μ$ oscillations into a sterile state can explain the atmospheric anomaly, while the solar neutrino puzzle is resolved via the small angle $ν_e-ν_τ$ MSW oscillations. The existence of some ($\sim 15 - 20% $ of critical energy density) neutrino hot dark matter is also predicted.

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arXiv1999

Neutrino Mixings and Fermion Masses in Supersymmetric SU(5)

We consider neutrino mixings in supersymmetric SU(5) supplemented by a U(1) flavor symmetry. In particular, we show how bi-maximal neutrino mixings can be realized. Two scenarios for implementing the small mixing angle MSW solution, with one involving a sterile state $ν_s$ and maximal $ν_μ-ν_s$ mixing to resolve the atmospheric anomaly, are also discussed. Finally, a new mechanism for eliminating the asymptotic relations $m^{(0)}_μ =m^{(0)}_s$ and $m^{(0)}_e=m^{(0)}_d$, while retaining $m^{(0)}_τ=m^{(0)}_b$, is presented. It employs `matter' multiplets in the $15+\bar{15}$ of SU(5), and is consistent with the various oscillation scenarios, as well as with the unification at $M_{GUT}$ of the three gauge couplings.

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arXiv1999

Atmospheric and Solar Neutrino Oscillations in \nuMSSM and Beyond

We show how a unified description of the various two-flavor neutrino oscillation solutions, allowed by the atmospheric and solar neutrino experiments, are naturally realized within the framework of $ν$MSSM (MSSM augmented with the seesaw mechanism) and beyond, especially grand unified theories. A general mechanism for achieving maximal mixing to resolve the atmospheric anomaly is discussed, and applied to the flipped SU(5) model. Except in the case of MSSM and SU(5), a light sterile neutrino is an inevitable consequence of our considerations. The bi-maximal neutrino mixing scenario is one of the options considered. Neutrino hot dark matter can arise in models with maximal $ν_μ-ν_s$ oscillations. A ${\cal U}(1)$ flavor symmetry, motivated by the charged fermion mass hierarchies and the magnitudes of the CKM matrix elements, plays a central role.

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arXiv1998

Neutrino Oscillations and Other Key Issues in Supersymmetric SU(4)_c \times SU(2)_L \times$ SU(2)_R

We try to gain an understanding of the recent Superkamiokande data on neutrino oscillations and several other important phenomenological issues within the framework of supersymmetric $SU(4)_c \times SU(2)_L \times SU(2)_R$ ($\equiv G_{422}$). By supplementing $G_{422}$ with a U(1)-${\cal R}$ symmetry, we can provide an explanation of the magnitude $M_G$ ($\sim 10^{16}$ GeV) of the $G_{422}$- symmetry breaking scale, resolve the MSSM $μ$ problem, and understand why proton decay has not been seen ($τ_p \gg 10^{34}$ yr). The family dependent ${\cal R}$ - symmetry also helps provide an explanation of the charged fermion mass hierarchies as well as the magnitudes of the CKM matrix elements. Several additional heavy states in the mass range $10^4-10^7$ GeV are predicted, and the MSSM parameter $\tan β$ turns out to be of order unity. The atmospheric neutrino problem is explained through $ν_μ-ν_τ$ mixing with $\sin^2 2θ_{μτ}\simeq 1$. The resolution of the solar neutrino puzzle is via the small angle MSW oscillations and necessarily requires a sterile neutrino $ν_s$ which, thanks to the ${\cal R}$ - symmetry, has a tiny mass.

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arXiv1998

TeV and Superheavy Mass-Scale Particles from Supersymmetric Topological Defects, the Extragalactic Gamma-ray Background, and the Highest Energy Cosmic Rays

Cosmic topological defects in a wide class of supersymmetric theories can simultaneously be sources of higgs particles of mass comparable to the supersymmetry breaking scale $\sim$ TeV, as well as superheavy gauge bosons of mass $\simη$, where $η$ (>> 1 TeV) denotes the associated gauge symmetry breaking scale. For cosmic strings with $η\sim 10^{14}$ GeV, the higgs decay can account for the extragalactic diffuse gamma ray background above $\sim$ 10 GeV, while the gauge boson decay may explain the highest energy cosmic ray flux above $\sim 10^{11}$ GeV, provided that particle production is the dominant energy loss mechanism for cosmic strings, as recent simulations suggest. By the same token, cosmic strings with $η$ much above $\sim 10^{14}$ GeV will be ruled out.

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