2,424 research outputs found
A Layman's guide to SUSY GUTs
The determination of the most straightforward evidence for the existence of
the Superworld requires a guide for non-experts (especially experimental
physicists) for them to make their own judgement on the value of such
predictions. For this purpose we review the most basic results of Super-Grand
unification in a simple and clear way. We focus the attention on two specific
models and their predictions. These two models represent an example of a direct
comparison between a traditional unified-theory and a string-inspired approach
to the solution of the many open problems of the Standard Model. We emphasize
that viable models must satisfy {\em all} available experimental constraints
and be as simple as theoretically possible. The two well defined supergravity
models, and , can be described in terms of only a few
parameters (five and three respectively) instead of the more than twenty needed
in the MSSM model, \ie, the Minimal Supersymmetric extension of the Standard
Model. A case of special interest is the strict no-scale
supergravity where all predictions depend on only one parameter (plus the
top-quark mass). A general consequence of these analyses is that supersymmetric
particles can be at the verge of discovery, lurking around the corner at
present and near future facilities. This review should help anyone distinguish
between well motivated predictions and predictions based on arbitrary choices
of parameters in undefined models.Comment: 25 pages, Latex, 11 figures (not included), CERN-TH.7077/93,
CTP-TAMU-65/93. A complete ps file (1.31MB) with embedded figures is
available by request from [email protected]
Threshold Effects And Perturbative Unification
We discuss the effect of the renormalization procedure in the computation of
the unification point for running coupling constants. We explore the effects of
threshold--crossing on the --functions. We compute the running of the
coupling constants of the Standard Model, between and , using a mass
dependent subtraction procedure, and then compare the results with ,
and with the -- function approximation. We also do this for the Minimal
Supersymmetric extension of the Standard Model. In the latter, the bounds on
susy masses that one obtains by requiring perturbative unification are
dependent, to some extent, on the procedure.Comment: 22 pages, REVTEX-2.1, 6 Post-Script figures are include
Proton Decay and Cosmology Strongly Constrain the Minimal SU(5) Supergravity Model
We present the results of an extensive exploration of the five-dimensional
parameter space of the minimal supergravity model, including the
constraints of a long enough proton lifetime (\tau_p>1\times10^{32}\y) and a
small enough neutralino cosmological relic density ().
We find that the combined effect of these two constraints is quite severe,
although still leaving a small region of parameter space with m_{\tilde
g,\tilde q}<1\TeV. The allowed values of the proton lifetime extend up to
\tau_p\approx1\times10^{33}\y and should be fully explored by the
SuperKamiokande experiment. The proton lifetime cut also entails the following
mass correlations and bounds: m_h\lsim100\GeV,
m_\chi\approx{1\over2}m_{\chi^0_2}\approx0.15\gluino, , and m_\chi<85\,(115)\GeV,
m_{\chi^0_2,\chi^+_1}<165\,(225)\GeV for . Finally,
the {\it combined} proton decay and cosmology constraints predict that if
m_h\gsim75\,(80)\GeV then m_{\chi^+_1}\lsim90\,(110)\GeV for
. Thus, if this model is correct, at least one of
these particles will likely be observed at LEPII.Comment: 11 pages plus 5 figures (not included). CERN-TH.6628/92,
CTP-TAMU-61/92. A condensed version of this paper will appear in the
Proceedings of the XXVI International Conference on High Energy Physics,
Dallas--Texas, August 5--12, 199
Influence of Light and Heavy Thresholds on SUSY Unification
In this paper we study and compare susy unification using two different
approaches in order to take into account the effect of light particle
thresholds on the evolution of gauge couplings: the step--function
approximation, on the one hand, and a mass dependent procedure, which gives a
more accurate description of the dependence of the results on the masses, on
the other. We also include the effect of heavy thresholds, when is
chosen as the unifying group. We find that the mass--dependent procedure
excludes scenarios where all susy masses are below , and favors a value
of near its upper experimental bound, contrary to the results
obtained with the step--function approximation. We underline the dependence of
the results on the procedure chosen to deal with light thresholds.Comment: 18 pages,LAEFF-93/014,REVTEX-2.1, 5 figures not included, available
upon request (include FAX number)
The SuperWorlds of SU(5) and SU(5)xU(1): A Critical Assessment and Overview
We present an overview of the simplest supergravity models which enforce
radiative breaking of the electroweak symmetry, namely the minimal
supergravity model and the class of string-inspired/derived supergravity models
based on the flipped structure supplemented by a minimal set
of additional matter representations such that unification occurs at the string
scale (\sim10^{18}\GeV). These models can be fully parametrized in terms of
the top-quark mass, the ratio , and three supersymmetry
breaking parameters (). The latter are chosen in the minimal
model such that the stringent constraints from proton decay and
cosmology are satisfied. In the flipped case we consider two
string-inspired supersymmetry breaking scenaria: no-scale
supergravity and a dilaton-induced supersymmetry breaking scenario. Both imply
universal soft supersymmetry breaking parameters: and
m_0=\coeff{1}{\sqrt{3}}m_{1/2}, A=-m_{1/2} respectively. We present a
comparative study of the sparticle and Higgs spectra of both flipped
models and the minimal model and conclude that all can be partially
probed at the Tevatron and LEPII (and the flipped models at HERA too). In both
flipped cases there is a more constrained version which allows to
determine in terms of and which leads to much
sharper and readily accessible experimental predictions. We also discuss the
prospects for indirect experimental detection: a non-trivial fraction of the
parameter space of the flipped models is in conflict with the present
experimental allowed range for the rare decay mode, and the
one-loop electroweak radiative corrections imply the 90\% CL upper boundComment: CERN-TH.6934/93, CTP-TAMU-34/93, LaTeX, 58 pages, 20 embedded
figures. Complete ps file (~12000 blocks, 5.24MB) available via anonymous ftp
from site tamsun.tamu.edu in directory incoming, filename: CTP-TAMU-34-93.p
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