arXiv subjects
Publications and source records attributed to M. M. Nojiri.
We study the decays of a scalar \stst\ bound state \sigst, where \st\ is the lighter stop eigenstate. If \st\ has no tree--level 2--body decays, the dominant decay modes of \sigst\ are $gg$ or, if $m_h < \mst \ll \mstt$, a pair of light scalar Higgs bosons $h$. The best signal for \sigst\ production at hadron colliders is probably its decay into two photons.
We present a detailed discussion of the elastic scattering of a supersymmetric neutralino off a nucleon or nucleus, with emphasis on the spin--independent interaction. We carefully treat QCD effects on the squark exchange contribution. In particular, we identify a class of terms that survive even in the absence of mixing in both the neutralino and squark sectors; the corresponding quark and gluon operators also appear in analyses of deep--inelastic lepton--nucleon scattering (``twist--2 operators''), so their hadronic matrix elements are well known. We also re--emphasize the importance of mixing between the superpartners of left-- and right--handed quarks, and of the contribution from the heavier scalar Higgs boson. We use our refined calculation of the scattering amplitude to update predictions of signal rates for cosmic relic neutralino searches with Germanium detectors. In general the counting rate varies strongly with the values (even the signs) of model parameters; typical results lie between a few times $10^{-4}$ and a few times $10^{-1}$ events/(kg$\cdot$day).
We compute the cosmic relic (dark matter) density of the lightest supersymmetric particle (LSP) in the framework of minimal $N=1$ Supergravity models with radiative breaking of the electroweak gauge symmetry. To this end, we re--calculate the cross sections for all possible annihilation processes for a general, mixed neutralino state with arbitrary mass. Our analysis includes effects of all Yukawa couplings of third generation fermions, and allows for a fairly general set of soft SUSY breaking parameters at the Planck scale. We find that a cosmologically interesting relic density emerges naturally over wide regions of parameter space. However, the requirement that relic neutralinos do not overclose the universe does not lead to upper bounds on SUSY breaking parameters that are strictly valid for all combinations of parameters and of interest for existing or planned collider experiments; in particular, gluino and squark masses in excess of 5 TeV cannot strictly be excluded. On the other hand, in the ``generic'' case of a gaugino--like neutralino whose annihilation cross sections are not ``accidentally'' enhanced by a nearby Higgs or $Z$ pole, all sparticles should lie within the reach of the proposed $pp$ and $e^+e^-$ supercolliders. We also find that requiring the LSP to provide all dark matter predicted by inflationary models imposes a strict lower bound of 40 GeV on the common scalar mass $m$ at the Planck scale, while the lightest sleptons would have to be heavier