30 research outputs found
ALEPH Four-Jet Excess, and Parity Violation
We review briefly the indications for some relatively light superpartners
based on the ~ ~anomaly and discuss the dependence of the potential
increase in on the assumption about ~parity (non)conservation. We
point out that the exotic 4-jet events reported by ALEPH may constitute a
signal for supersymmetry with such a light spectrum and with explicitly broken
parity. A parton level simulation shows that production of a pair of light
charginos with their subsequent baryon-number violating decays (either through
a stop or through a neutralino) could possibly give rise to this excess. The
decay \chi^- \ra \tilde{t}_R^\ast b \ra d s b with m_{\chi^-_1} \sim 60
\gev and m_{\tilde t} \sim 52 \gev leads to signatures very close to the
experimental observations.Comment: 17 pages, submited as uuencoded gz-compressed .tar file containing
LATEX file and figures. Some typos fixed and the important error in the
caption of Fig. 7 correcte
Flavour Changing Neutral Currents and Inverted Sfermion Mass Hierarchy
We study the contraints on non-flavour-blind soft supersymmetry breaking
terms coming from flavour and CP violating processes in the presence of
hierarchical Yukawa couplings, and quantify how much these constraints are
weakened in the regions of the MSSM parameter space characterized by heavy
gauginos and multi-TeV sfermion masses, respectively. We also study the
inverted sfermion mass hierarchy scenario in the context of D-term
supersymmetry breaking, and show that generic hierarchical Yukawa couplings
with arbitrary phases require first generation squarks in the few 10 TeV range.Comment: 9 pages, 6 figures. Talk given at the XLth Rencontres de Moriond on
Electroweak Interactions and Unified Theories, La Thuile, Aosta Valley,
Italy, 5-12 March 2005. V3: one reference correcte
The fine-tuning price of LEP
We quantify the amount of fine tuning of input parameters of the Minimal
Supersymmetric Extension of the Standard Model (MSSM) that is needed to respect
the lower limits on sparticle and Higgs masses imposed by precision electroweak
measurements at LEP, measurements of , and searches at LEP 2.
If universal input scalar masses are assumed in a gravity-mediated scenario, a
factor of \gappeq180 is required at , decreasing to
at . The amount of fine tuning is not greatly reduced
if non-universal input scalar Higgs masses are allowed, but may be
significantly reduced if some theoretical relations between MSSM parameters are
assumed.Comment: 13 pages, 5 figures. Two references added, one corrected. A typo
correcte
Haggling over the fine-tuning price
We amplify previous discussions of the fine-tuning price to be paid by
supersymmetric models in the light of LEP data, especially the lower bound on
the Higgs boson mass, studying in particular its power of discrimination
between different parameter regions and different theoretical assumptions. The
analysis is performed using the full one-loop effective potential. The whole
range of is discussed, including large values. In the minimal
supergravity model with universal gaugino and scalar masses, a small
fine-tuning price is possible only for intermediate values of .
However, the fine-tuning price in this region is significantly higher if we
require Yukawa-coupling unification. On the other hand, price
reductions are obtained if some theoretical relation between MSSM parameters is
assumed, in particular between , and . Significant price
reductions are obtained for large if non-universal soft Higgs mass
parameters are allowed. Nevertheless, in all these cases, the requirement of
small fine tuning remains an important constraint on the superpartner spectrum.
We also study input relations between MSSM parameters suggested in some
interpretations of string theory: the price may depend significantly on these
inputs, potentially providing guidance for building string models. However, in
the available models the fine-tuning price may not be reduced significantly.Comment: 35 pages, 16 figure
Bottom-up approach and supersymmetry breaking
We present a bottom-up approach to the question of supersymmetry breaking in
the MSSM. Starting with the experimentally measurable low-energy supersymmetry
breaking parameters, which can take any values consistent with present
experimental constraints, we evolve them up to an arbitrary high energy scale.
Approximate analytical expressions for such an evolution, valid for low and
moderate values of , are presented. We then discuss qualitative
properties of the high-energy parameter space and, in particular, identify the
conditions on the low energy spectrum that are necessary for the parameters at
the high energy scale to satisfy simple regular pattern such as universality or
partial universality. As an illustrative example, we take low energy parameters
for which light sparticles, within the reach of the LEP2 collider, appear in
the spectrum, and which do not affect the Standard Model agreement with the
precision measurement data. Comparison between supersymmetry breaking at the
GUT scale and at a low energy scale is made.Comment: 33 pages (14 figures
Neutrino Unification
Present neutrino data are consistent with neutrino masses arising from a
common seed at some ``neutrino unification'' scale . Such a simple
theoretical ansatz naturally leads to quasi-degenerate neutrinos that could lie
in the electron-volt range with neutrino mass splittings induced by
renormalization effects associated with supersymmetric thresholds. In such a
scheme the leptonic analogue of the Cabibbo angle describing
solar neutrino oscillations is nearly maximal. Its exact value is correlated
with the smallness of . These features agree both with latest
data on the solar neutrino spectra and with the reactor neutrino data. The two
leading mass-eigenstate neutrinos present in \ne form a pseudo-Dirac neutrino,
avoiding conflict with neutrinoless double beta decay.Comment: RevTex format, 2 figures, 4 pages, a few new references, no other
important change, figures unchanged, version to be published in PR