Minimal Supergravity Model Research Articles

Gaugino-higgsino mixing and slepton-squark production in ep collisions

We consider gaugino-higgsino mixing in the minimal supersymmetric extension of the standard model and determine the mass eigenstates and eigenvalues taking the physical masses of the lightest chargino and neutralino as input in the diagonalization procedure. Using the results we calculate cross sections and charge asymmetries for the associated production of sleptons and squarks in ep collisions and investigate the dependence on the chargino and neutralino properties and on the slepton and squark spectrum. The numerical analysis is performed within the minimal supergravity model as well as in a more general framework. Although predictions are made also for higher energies, the main focus is on the HERA energy range. We indicate the values of sparticle masses accessible to SUSY searches at HERA and illustrate the expected reach beyond the present bounds.

Supersymmetry phenomenology and the nature of the lightest supersymmetric particle.

We show that in addition to the photinolike and Higgsino-like states usually considered as candidates for the lightest supersymmetric particle (LSP), there is yet one more state in the neutralino sector which could be the LSP in minimal supergravity models. In this paper, we study how the phenomenology of supersymmetry is altered depending on the nature of the LSP. Within the framework of minimal supergravity models, we demonstrate that if the mass of the LSP is small compared to that of the W boson, the lightest chargino and the next to lightest neutralino states are always lighter than ${M}_{W}$ and ${M}_{Z}$ independent of the soft-supersymmetry-breaking gaugino masses. We show that the W- and Z-boson decay widths into these particles depends quite significantly on the nature of the LSP. We further show that the bound on the scalar-electron mass of the ASP detector at the SLAC storage ring PEP is considerably weakened if the LSP is the new state discussed above, even if this state is dominantly a gaugino. Finally, we briefly study the phenomenology of the LSP in the context of superstring-inspired E(6) models.

Low energy supergravity: R-parity breaking and the top quark mass

We study the process of spontaneous R-parity breaking in minimal low energy supergravity models. We show that it is very hard to obtain models with heavy top quarks if one wants to preserve the radiative breaking of SU(2) L⊗U(1) Y without breaking R-parity.

No future for the fourth generation?

We discuss bounds on and implications of a possible fourth generation of quarks and leptons. We derive lower bounds on the masses of fourth generation quarks from the recent experimental bound mτ' ⩾ 41 GeV, for both nonsupersymmetric and supersymmetric models. We also derive upper bounds on these masses from the requirement of a perturbative unification, and briefly discuss the non-perturbative case. We then show that a fourth generation renders the conventional mechanism of cosmological baryogenesis via Higgs-decay problematic, and tends to result in a large supersymmetry breaking scale in minimal supergravity models. We finally show that the incorporation of a fourth generation in superstring models can only work if the superpotential fulfills certain specific conditions, which appear to be in conflict with cosmological requirements.

Low energy properties of superstring-inspired SU(5) supergravity models

The spontaneous breaking of the electroweak SU(2) L × U(1) Y gauge symmetry in supergravity theories is investigated with a rank-four gauge group inspired by the superstring. It is shown that the resulting low energy particle spectrum is essentially identical to that of the common minimal low energy supergravity models except for the fact that the upper bound on the mass of the lightest neutral Higgs boson is near m z 0 independent of the (unconstrained!) top quark mass.

Effects of Left-Right Scalar Quark Flavor Mixings on CP Violation in Supersymmetry

Studies of flavor mixings among left squarks (superpartners of left-handed components of quarks) and right squarks are made in the minimal low energy supergravity model, where all one-loop corrections are taken into account from renormalization group equations. So far, their effects on physics have been presumed to be negligible. Ii is found, contrary to that presumption, that they play major roles in two phenomena, the electric dipole moment of neutron and the asymmetry of muon polarization in K L .... fJ.+ fJ.decay.

Is spontaneous breaking of R-parity feasible in minimal low-energy supergravity?

Spontaneous violation of lepton number without breaking Lorentz invariance can, in principle, be incorporated in models with softly broken supersymmetry. We study the situation for minimal low-energy supergravity models coming from a GUT (hence not having hierarchy destabilizing light singlets) and where the SU(2) × U(1) breaking is radiative. It is found that for this type of model, R-parity breaking requires either too heavy a top quark for a realistic superpartner spectrum or too light a superpartner spectrum for a realistic top quark, making the spontaneous violation of lepton number in the third generation incompatible with present experimental data. We do not discard the possibility of having it in a fourth, heavier, generation.

Cosmological constraints on supergravity models

We delineate the domain of supersymmetry breaking parameters in minimal supergravity models for which the cosmological relic photino density is no larger than the closure density. Demanding that the relic density equal the closure density as suggested by inflationary cosmology suggests m q ̃ ∼ 42 GeV + 0.89 m g ̃ . We point out that several supergravity scenarios for the monojet events at the CERN p p Collider would yield a relic density considerably greater than the closure density.

Are Higgs couplings enhanced in minimal supergravity models?

We consider the couplings of Higgs bosons to quarks and vector bosons in minimal N=1 supergravity models with two Higgs doublets. We show that in the limit of one of the Higgs bosons being naturally light, the Higgs quark couplings cannot be enhanced to explain the recently observed ζ (8.3 GeV) in upsilon decay. Phenomenologically, if one enhances the couplings of the light Higgs scalar to the bottom quarks, the model predicts the existence of a light charged fermion, in contradiction with the experimental bounds on such fermions.