arXiv subjects

Shaaban Khalil

Publications and source records attributed to Shaaban Khalil.

At least 109 records · Page 6Linked to original sources

arXiv2002

CP violation in supersymmetric theories

We review the present status of CP violating problem in supersymmetric extensions of the standard model. We analyze the constraints imposed by the experimental limits of the electron, neutron, and mercury electric dipole moments on the supersymmetric CP phases and show that only the scenarios with flavour-off- diagonal CP violation remain attractive. These scenarios require hermitian Yukawa matrices which naturally arise in models with left-right symmetry or a SU(3) flavour symmetry. In this case, epsilon_K and epsilon'/epsilon can be saturated by a small non-universality of the soft scalar masses through the gluino and chargino contributions respectively. The model also predicts a strong correlation between A_{CP}(b-> s gamma) and the neutron electric dipole moment. In this framework, the standard model gives a the leading contribution to the CP asymmetry in B ->psi K_S decay, while the dominant chargino contribution to this asymmetry is < 0.2. Thus, no constraint is set on the non-universality of this model by the recent BaBar and Belle measureets.

hep-ph↗

arXiv2002

CP violation in supersymmetric models with Hermitian Yukawa couplings and A-terms

We analyse the CP violation in supersymmetric models with Hermitian Yukawa and trilinear couplings. We show that Hermitian Yukawa matrices can be implemented in supersymmetric models with SU(3) flavor symmetry. An important feature of this class of models is that the supersymmetric contributions to the EDM of the neutron and mercury atom are suppressed. In this framework, epsilon_K can be saturated by a small non--universality of the soft scalar masses through the gluino contribution. We perform a detailed analysis for the supersymmetric contributions to epsilon'/epsilon. Although, the gluino contribution is found to be negligible due to a severe cancellation between LR and RL mass insertions, the chargino contribution can be significant and accommodate the experimental results. Additionally, we find that the SUSY contributions to epsilon'/epsilon from the effective \bar{s}dZ vertex and the ΔI =3/2 operators are insignificant. We point out that the standard model gives the leading contribution to the CP asymmetry in B -> ψK_S decay, while the dominant chargino contribution to this asymmetry is < 0.2 . Thus, no constraint is set on the non--universality of this model by the recent BaBar and Belle measurements.

hep-ph↗

arXiv2001

The Enigma of the Dark Matter

One of the great scientific enigmas still unsolved, the existence of dark matter, is reviewed. Simple gravitational arguments imply that most of the mass in the Universe, at least 90%, is some (unknown) non-luminous matter. Some particle candidates for dark matter are discussed with particular emphasis on the neutralino, a particle predicted by the supersymmetric extension of the Standard Model of particle physics. Experiments searching for these relic particles, carried out by many groups around the world, are also discussed. These experiments are becoming more sensitive every year and in fact one of the collaborations claims that the first direct evidence for dark matter has already been observed.

hep-ph↗

arXiv2001

Lepton-Flavor Violation with Non-universal Soft Terms

We study the lepton-flavor violation processes tau ->mu gamma and mu->e gamma in two different examples of models with non-universal soft breaking terms derived from strings. We show that the predictions are quite different from those of universal scenarios. Non-universal A-terms provide an interesting framework to enhance the supersymmetric contributions to CP violation effects. We observe that in the case of the lepton-flavor violation we study, the non-universality of the scalar masses enhances the branching ratios more significantly than the non-universality of the A-terms. We find that the current experimental bounds on these processes restrict both the parameter space of the models and the texture of the Yukawa couplings which predicts the lepton masses, providing at the same time an interesting experimental test for physics beyond the Standard Model.

hep-ph↗

arXiv2001

Present and Future CP Measurements

We review theoretical and experimental results on CP violation summarizing the discussions in the working group on CP violation at the UK phenomenology workshop 2000 in Durham.

hep-ph↗

arXiv2000

Non universal gaugino phases and the LSP relic density

The cosmological relic density of the lightest supersymmetric particle (LSP) of type I string derived model is calculated. This model can accommodate large values of CP violating phases, and the electron and neutron electric dipole moments satisfy the experimental constraint. We show that the constraint from the electric dipole moment on the ratio between the gaugino masses implies that the mass of the LSP, which is bino like, is close to the lightest chargino. The co-annihilation between them is very important to reduce the LSP relic density to an interesting region. We show that the SUSY phases, although they are large, have no significant effect on the relic density and on the the upper bound imposed on the LSP mass. However, they are very significant for the detection rates. We emphasize that the phase of the trilinear coupling increase the direct and indirect detection rates.

hep-ph↗

arXiv2000

EDM-free supersymmetric CP violation with non-universal soft terms

Non-universality in the soft breaking terms is a common feature in most superstring inspired SUSY models. This property is required to obtain sizeable CP violation effects from SUSY and, on the other hand, can be used to avoid the Electric Dipole Moment constraints. We take advantage of these qualities and explore a class of SUSY models based on type I string theory where scalar masses, gaugino masses and trilinear couplings are non-universal. In this framework, we show that, in the presence of large SUSY phases, the bounds on the Electric Dipole Moments can be controled without fine-tuning. At the same time, we find that these phases, free from EDM constraints, lead to large contributions to the observed CP phenomena in Kaon system and, in particular, to direct CP violation in epsilon'/epsilon.

hep-ph↗

arXiv1999

Cosmological implications of supersymmetric CP violating phases

We show that large SUSY phases have no significant effect on the relic density of the lightest supersymmetric particle (LSP). However, they are very significant for the detection rates. We emphasise that the phase of the trilinear coupling increase the direct and indirect detection rates.

hep-ph↗

arXiv1999

Examining the cancellation mechanism of neutron EDM in a model with dilaton-dominated susy breaking

We examine the cancellation mechanism between the different contributions to the electric dipole moment of the neutron in a model with dilaton-dominated SUSY breaking. We find these accidental cancellations occur at few points in parameter space. For a wide region of this space we must constrain the phase of $μ$ to be of order $10^{-1}$ and have the phases of $A$ and $μ$ strongly correlated in order to have small neutron EDM. Moreover, we consider the indirect CP violation parameter $ε$ in this region where the electric dipole moment is less than the experimental limit and find that we can generate $ε$ of order $10^{-6}$.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

arXiv1999

Supersymmetric CP violation $ε'/ε$ due to asymmetric A-matrix

We study contributions of supersymmetric CP phases to the CP violation $ε'/ε$ in models with asymmetric $A$-matrices. We consider asymmetric $A$-matrices, which are obtained from string-inspired supergravity. We show that a certain type of asymmetry of $A$-matrices enhances supersymmetric contributions to the CP violation $ε'/ε$ and the supersymmetric contribution to $ε'/ε$ can be of order of the KTeV result, $ε'/ε\sim 10^{-3}$.

hep-ph↗

arXiv1999

CP violation in supersymmetric model with non-degenerate A-terms

We study the CP phases of the soft supersymmetry breaking terms in string-inspired models with non-universal trilinear couplings. We show that such non-universality plays an important role on all CP violating processes. In particular these new supersymmetric sources of CP violation may significantly contribute to the observed CP phenomena in kaon physics while respecting the severe bound on the electric dipole moment of the neutron.

hep-ph↗

arXiv1997

Phenomenological implications of moduli-dominant SUSY breaking

We study moduli-dominated SUSY breaking within the framework of string models. This type of SUSY breaking in general leads to non-universal soft masses, i.e. soft scalar masses and gaugino masses. Further gauginos are lighter than sfermions. This non-universality has phenomenologically important implications. We investigate radiative electroweak symmetry breaking in the mass spectrum derived from moduli-dominated SUSY breaking, where the lightest chargino and neutralino are almost gauginos. Moreover, constraints from the branching ratio of $b \to s γ$ and the relic abundance of the LSP are also considered. The mass spectrum of moduli-dominated SUSY breaking is favorable to the experimental bound of the $b \to s γ$ decay decreasing its branching ratio. We obtain an upper bound for the gravitino mass from the cosmological constraint.

hep-ph↗

arXiv1997

Yukawa Unification in Moduli-dominant SUSY Breaking

We study Yukawa unification in string models with moduli-dominant SUSY breaking. This type of SUSY breaking in general leads to non-universal soft masses, i.e., soft scalar masses and gaugino masses. Such non-universality is important for phenomenological aspects of Yukawa unification, i.e., successful electroweak breaking, SUSY corrections to the bottom mass and the branching ratio of $b \to s γ$. We show three regions in the whole parameter space which lead to successful electroweak breaking and allow small SUSY corrections to the bottom mass. For these three regions we investigate the $b \to s γ$ decay and mass spectra.

hep-ph↗

arXiv1997

From $b \to s γ$ to the LSP Detection Rates in Minimal String Unification Models

We exploit the measured branching ratio for $b \rightarrow sγ$ to derive lower limits on the sparticle and Higgs masses in the minimal string unification models. For the LSP('bino'), chargino and the lightest Higgs, these turn out to be 50, 90 and 75 GeV respectively. Taking account of the upper bounds on the mass spectrum from the LSP relic abundance, we estimate the direct detection rate for the latter to vary from 10^{-1} to 10^{-4} events/kg/day. The muon flux, produced by neutrinos from the annihilating LSP's, varies in the range 10^{-2} - 10^{-9} muons/m^2/day.

hep-ph↗

arXiv1997

Sparticle and Higgs masses within minimal string unification

We consider the sparticle and higgs spectroscopy in a class of superstring inspired models in which the string threshold corrections ensure the consistency of the string unification scale with the low energy data. The lightest neutralino is almost a pure bino and it is predicted to be the lightest sparticle (LSP). Requiring that $Ω_{LSP}\leq 0.9$, we find an upper bound on its mass which, in the case of dilaton supersymmetry breaking, turns out to be 160 GeV. The LEP 1.5 experimental lower bound on the chargino mass, $m_{χ^{\pm}} > 65$ GeV, implies that the lower bound on the LSP mass is $m_{LSP} > 32 (45)$ GeV, corresponding to $μ< (>)0$. We also determine the lower and upper bounds on the higgs and other sparticle masses. For instance, the lightest higgs lies between 65 and 115 GeV, while the mass of the lightest charged sparticle satisfies 47 GeV $ < m_{\tilde{e}_R} < 325$ GeV. With only the top Yukawa coupling of order unity we find that $ 1.5 \leq \tan β\leq 3.5$.

hep-ph↗

arXiv1996

Supersymmetric Sum Rules In Minimal Superstring Unification

The Minimal superstring unification, assuming orbifold compactification, provides interesting and rather detailed implications on physics at low energy. The interesting feature of this model is that the masses of the spectrum are related since all of them are functions of only two parameters: the goldstino angle theta and the gravitino mass m_{3/2}. This fact will help us in studying the modification of the supersymmetric magnetic moments sum rules which are very sensitive to the supersymmetry breaking. We write these rules in case of exact supersymmetry in a form close to the supersymmetric mass relations, namely $ \sum_J (-1)^{(2J)} (2 J) A_J=0$, where A_J is the anomalous magnetic moment of the spin J particle. We show that the anomalous magnetic moments of the W-boson and the gauginos can be written as functions of theta and m_{3/2}. Then we obtain a modified version of the supersymmetric magnetic moment sum rule in the context of the minimal superstring unification.

hep-ph↗