Searcharxiv⌕ Search

arXiv subjects

S. Khalil

Publications and source records attributed to S. Khalil.

At least 91 records · Page 5Linked to original sources
arXiv2002

The String CP Problem

We show that string models with low energy supersymmetry which accommodate the fermion mass hierarchy generally give non-universal soft trilinear couplings (A-terms). In conjunction with the apparently large Cabibbo-Kobayashi-Maskawa (CKM) phase, this results in large fermion EDMs even in the absence of CP violating phases in the supersymmetry breaking terms and the mu-term. Nonobservation of the EDMs therefore implies that strings select special flavor and/or supersymmetry breaking patterns.

hep-ph↗
arXiv2002

New shifted hybrid inflation

A new shifted hybrid inflationary scenario is introduced which, in contrast to the older one, relies only on renormalizable superpotential terms. This scenario is automatically realized in a concrete extension of the "minimal" supersymmetric Pati-Salam model which naturally leads to a moderate violation of Yukawa unification so that, for mu>0, the predicted b-quark mass is acceptable even with universal boundary conditions. It is shown that this extended model possesses a classically flat "shifted" trajectory which acquires a slope via one-loop radiative corrections and can be used as inflationary path. The constraints from the cosmic background explorer can be met with natural values of the relevant parameters. Also, there is no disastrous production of magnetic monopoles after inflation since the Pati-Salam gauge group is already broken on the "shifted" path. The relevant part of inflation takes place at values of the inflaton field which are not much smaller than the "reduced" Planck scale and, thus, supergravity corrections could easily invalidate inflation. It is, however, shown that inflation can be kept intact provided that an extra gauge singlet with a superheavy vacuum expectation value, which originates from D-terms, is introduced and a specific form of the Kaehler potential is used. Moreover, it is found that, although the supergravity corrections are sizable, the constraints from the cosmic background explorer can again be met by readjusting the values of the parameters which were obtained with global supersymmetry.

hep-ph↗
arXiv2002

CP Violation and Dilaton Stabilization in Heterotic String Models

We study the possibility of spontaneous CP violation in string models with the dilaton field stabilized at a phenomenologically acceptable value. We consider three mechanisms to stabilize the dilaton: multiple gaugino condensates, a nonperturbative Kahler potential, and a superpotential based on S-duality, and analyze consequent CP phases in the soft SUSY breaking terms. Due to non-universality forced upon the theory by requiring a non-trivial CKM phase, the EDM problem becomes more severe. Even if there are no complex phases in the VEVs of the SUSY breaking fields, the electric dipole moments are overproduced by orders of magnitude. We also address the question of modular invariance of the physical CP phases.

hep-th↗
arXiv2002

Relic Neutralino Density in Scenarios with Intermediate Unification Scale

We analyse the relic neutralino density in supersymmetric models with an intermediate unification scale. In particular, we present concrete cosmological scenarios where the reheating temperature is as small as $\cal{O}$ $1 - 1000$ MeV). When this temperature is associated to the decay of moduli fields producing neutralinos, we show that the relic abundance increases considerably with respect to the standard thermal production. Thus the neutralino becomes a good dark matter candidate with $0.1\lsim Ωh^2 \lsim 0.3$, even for regions of the parameter space where large neutralino-nucleon cross sections, compatible with current dark matter experiments, are present. This is obtained for intermediate scales $M_I\sim 10^{11}-10^{14}$ GeV, and moduli masses $m_ϕ\sim 100-1000$ GeV. On the other hand, when the above temperature is associated to the decay of an inflaton field, the relic abundance is too small.

hep-ph↗
arXiv2002

On the B -> X_s l^+ l^- decays in general supersymmetric models

We analyze the inclusive semileptonic decays B -> X_s l^+ l^- in the framework of the supersymmetric standard model with non-universal soft-breaking terms at GUT scale. We show that the general trend of universal and non-universal models is a decreasing of branching ratio (BR) and increasing of energy asymmetry (AS). However, only non--universal models can have chances to get very large enhancements in BR and AS, corresponding to large (negative) SUSY contributions to the b -> s γamplitude.

hep-ph↗
arXiv2001

Large dark matter cross sections from supergravity and superstrings

We study the direct detection of supersymmetric dark matter in the light of recent experimental results. In particular, we show that regions in the parameter space of several scenarios with a neutralino-nucleon cross section of the order of $10^{-6}$ pb, i.e., where current dark matter detectors are sensitive, can be obtained. These are supergravity scenarios with intermediate unification scale, and superstring scenarios with D-branes.

hep-ph↗
arXiv2001

Determination of the String Scale in D-Brane Scenarios and Dark Matter Implications

We analyze different phenomenological aspects of D-brane constructions. First, we obtain that scenarios with the gauge group and particle content of the supersymmetric standard model lead naturally to intermediate values for the string scale, in order to reproduce the value of gauge couplings deduced from experiments. Second, the soft terms, which turn out to be generically non universal, and Yukawa couplings of these scenarios are studied in detail. Finally, using these soft terms and the string scale as the initial scale for their running, we compute the neutralino-nucleon cross section. In particular we find regions in the parameter space of D-brane scenarios with cross sections in the range of $10^{-6}$--$10^{-5}$ pb, i.e. where current dark matter experiments are sensitive. For instance, this can be obtained for $\tanβ> 5$.

hep-ph↗
arXiv2001

Supersymmetric dark matter and neutralino-nucleon cross section

We review the direct detection of supersymmetric dark matter in the light of recent experimental results. In particular, we show that regions in the parameter space of several supergravity scenarios with a neutralino-nucleon cross section of order of $10^{-6}$ pb, i.e., where current dark matter detectors are sensitive, can be obtained. These are scenarios with large $\tan β$, with non-universal soft supersymmetry-breaking terms, with multi-TeV masses for scalar superpartners known as `focus point' supersymmetry, and finally scenarios with intermediate unification scale which appear naturally in some superstring constructions.

hep-ph↗
arXiv2001

Muon anomalous magnetic moment in supersymmetric scenarios with an intermediate scale and nonuniversality

We analyze the anomalous magnetic moment of the muon (a_μ) in supersymmetric scenarios. First we concentrate on scenarios with universal soft terms. We find that a moderate increase of a_μ can be obtained by lowering the unification scale M_{GUT} to intermediate values 10^{10-12} GeV. However, large values of \tan βare still favored. Then we study the case of non-universal soft terms. For the usual value M_{GUT}~10^{16} GeV, we obtain a_μ in the favored experimental range even for moderate \tan βregions \tanβ~ 5$. Finally, we give an explicit example of these scenarios. In particular, we show that in a D-brane model, where the string scale is naturally of order 10^{10-12} GeV and the soft terms are non universal, a_μ is enhanced with low \tanβ.

hep-ph↗
arXiv2001

Chargino Contributions to Epsilon and Epsilon-Prime

We analyze the chargino contributions to the K-\bar K mixing and epsilon-prime in the mass insertion approximation and derive the corresponding bounds on the mass insertion parameters. We find that the chargino contributions can significantly enlarge the regions of the parameter space where CP violation can be fully supersymmetric. In principle, the observed values of epsilon and epsilon-prime may be entirely due to the chargino -- up-squark loops.

hep-ph↗
arXiv2001

Monopole Problem and Extensions of Supersymmetric Hybrid Inflation

We discuss, in the context of a concrete supersymmetric grand unified model based on the Pati-Salam gauge group SU(4)_c x SU(2)_L x SU(2)_R, two "natural" extensions of supersymmetric hybrid inflation, which avoid the cosmological disaster encountered in the standard hybrid inflationary scenario from the overproduction of monopoles at the end of inflation. Successful "reheating" which satisfies the gravitino constraint takes place after the end of inflation. Also, adequate baryogenesis via a primordial leptogenesis occurs consistently with the solar and atmospheric neutrino oscillation data as well as the SU(4)_c symmetry. Moreover, the mu-term is generated via a Peccei-Quinn symmetry and proton is practically stable.

hep-ph↗
arXiv2001

EDM Constraints in Supersymmetric Theories

We systematically analyze constraints on supersymmetric theories imposed by the experimental bounds on the electron, neutron, and mercury electric dipole moments. We critically reappraise the known mechanisms to suppress the EDMs and conclude that only the scenarios with approximate CP-symmetry or flavour-off-diagonal CP violation remain attractive after the addition of the mercury EDM constraint.

hep-ph↗
arXiv2001

On the EDM Cancellations in D-brane models

We analyze the possibility of simultaneous electron, neutron, and mercury electric dipole moment (EDM) cancellations in the mSUGRA and D--brane models. We find that the mercury EDM constraint practically rules out the cancellation scenario in D-brane models whereas in the context of mSUGRA it is still allowed with some fine-tuning.

hep-ph↗
arXiv2001

Leptogenesis in Smooth Hybrid Inflation

We present a concrete supersymmetric grand unified model based on the Pati-Salam gauge group SU(4)_c x SU(2)_L x SU(2)_R and leading naturally to smooth hybrid inflation, which avoids the cosmological disaster encountered in the standard hybrid inflationary scenario from the overproduction of monopoles at the end of inflation. Successful `reheating' which satisfies the gravitino constraint takes place after the termination of inflation. Also, adequate baryogenesis via a primordial leptogenesis occurs consistently with the solar and atmospheric neutrino oscillation data as well as the SU(4)_c symmetry.

hep-ph↗
arXiv2001

Inflation and Monopoles in Supersymmetric SU(4)_c x SU(2)_L x SU(2)_R

We show how hybrid inflation can be successfully realized in a supersymmetric model with gauge group G_{PS}= SU(4)_c x SU(2)_L x SU(2)_R. By including a non-renormalizable superpotential term, we generate an inflationary valley along which G_{PS} is broken to the standard model gauge group. Thus, catastrophic production of the doubly charged magnetic monopoles, which are predicted by the model, cannot occur at the end of inflation. The results of the cosmic background explorer can be reproduced with natural values (of order 10^{-3}) of the relevant coupling constant, and symmetry breaking scale of G_{PS} close to 10^{16} GeV. The spectral index of density perturbations lies between unity and 0.9. Moreover, the mu-term is generated via a Peccei-Quinn symmetry and proton is practically stable. Baryogenesis in the universe takes place via leptogenesis. The low deuterium abundance constraint on the baryon asymmetry, the gravitino limit on the reheat temperature and the requirement of almost maximal mu neutrino - tau neutrino mixing from SuperKamiokande can be simultaneously met with mu-neutrino, tau-neutrino and heaviest Dirac neutrino masses determined from the large angle MSW resolution of the solar neutrino problem, the SuperKamiokande results and SU(4)_c symmetry respectively.

hep-ph↗
arXiv2001

Cold Dark Matter and b --> s gamma in the Horava-Witten Theory

The minimal supersymmetric standard model with complete, partial or no Yukawa unification and radiative electroweak breaking with boundary conditions from the Horava-Witten theory is considered. The parameters are restricted by constraining the lightest sparticle relic abundance by cold dark matter considerations and requiring the b-quark mass after supersymmetric corrections and the branching ratio of b --> s gamma to be compatible with data. Complete Yukawa unification can be excluded. Also, t-b Yukawa unification is strongly disfavored since it requires almost degenerate lightest and next-to-lightest sparticle masses. However, the b-tau or no Yukawa unification cases avoid this degeneracy. The latter with mu<0 is the most natural case. The lightest sparticle mass, in this case, can be as low as about 77 GeV.

hep-ph↗
arXiv2001

Flavour-Dependent CP Violation and Natural Suppression of the Electric Dipole Moments

We revisit the supersymmetric CP problem and find that it can be naturally resolved if the origin of CP violation is closely related to the origin of flavour structures. In this case, the supersymmetry breaking dynamics do not bring in any new CP-violating phases. This mechanism requires hermitian Yukawa matrices which naturally arise in models with a U(3) flavour symmetry. The neutron electric dipole moment (NEDM) is predicted to be within one-two orders of magnitude below the current experimental limit. The model also predicts a strong correlation between A_{CP}(b --> s gamma)and the NEDM. The strong CP problem is mitigated although not completely solved.

hep-ph↗