59,827 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]
New Precision Electroweak Tests of SU(5) x U(1) Supergravity
We explore the one-loop electroweak radiative corrections in supergravity via explicit calculation of vacuum-polarization and
vertex-correction contributions to the and
parameters. Experimentally, these parameters are obtained from a global fit to
the set of observables , and . We
include -dependent effects, which induce a large systematic negative shift
on for light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). The
(non-oblique) supersymmetric vertex corrections to \Zbb, which define the
parameter, show a significant positive shift for light chargino
masses, which for can be nearly compensated by a negative
shift from the charged Higgs contribution. We conclude that at the 90\%CL, for
m_t\lsim160\GeV the present experimental values of and
do not constrain in any way supergravity in
both no-scale and dilaton scenarios. On the other hand, for m_t\gsim160\GeV
the constraints on the parameter space become increasingly stricter. We
demonstrate this trend with a study of the m_t=170\GeV case, where only a
small region of parameter space, with \tan\beta\gsim4, remains allowed and
corresponds to light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). Thus
supergravity combined with high-precision LEP data would
suggest the presence of light charginos if the top quark is not detected at the
Tevatron.Comment: LaTeX, 11 Pages+4 Figures(not included), the figures available upon
request as an uuencoded file(0.4MB) or 4 PS files from [email protected],
CERN-TH.7078/93, CTP-TAMU-68/93, ACT-24/9
M-theory Inspired No-scale Supergravity
We propose a supergravity model that contains elements recently shown to
arise in the strongly-coupled limit of the heterotic string
(M-theory), including a no-scale--like K\"ahler potential, the identification
of the string scale with the gauge coupling unification scale, and the onset of
supersymmetry breaking at an intermediate scale determined by the size of the
eleventh dimension of M-theory. We also study the phenomenological consequences
of such scenario, which include a rather constrained sparticle spectrum within
the reach of present-generation particle accelerators.Comment: 8 pages, LaTeX, 3 figures (included
SUSY signals at HERA in the no-scale flipped SU(5) supergravity model
Sparticle production and detection at HERA are studied within the recently
proposed no-scale flipped supergravity model. Among the various
reaction channels that could lead to sparticle production at HERA, only the
following are within its limit of sensitivity in this model: , where are the
two lightest neutralinos and is the lightest chargino. We study the
elastic and deep-inelastic contributions to the cross sections using the
Weizs\"acker-Williams approximation. We find that the most promising
supersymmetric production channel is right-handed selectron ()
plus first neutralino (), with one hard electron and missing energy
signature. The channel leads to comparable rates but also
allows jet final states. A right-handedly polarized electron beam at HERA would
shut off the latter channel and allow preferentially the former one. With an
integrated luminosity of {\cal L}=100\ipb, HERA can extend the present LEPI
lower bounds on by
\approx25\GeV, while {\cal L}=1000\ipb will make HERA competitive with
LEPII. We also show that the Leading Proton Spectrometer (LPS) at HERA is an
excellent supersymmetry detector which can provide indirect information about
the sparticle masses by measuring the leading proton longitudinal momentum
distribution.Comment: 11 pages, 8 figures (available upon request as uuencoded file or
separate ps files), tex (harvmac) CTP-TAMU-15/93, CERN/LAA/93-1
Supersymmetry dileptons and trileptons at the Tevatron
We consider the production of supersymmetry neutralinos and charginos in
collisions at the Tevatron, and their subsequent decay via
hadronically quiet dileptons and trileptons. We perform our computations in the
context of a variety of supergravity models, including generic four-parameter
supergravity models, the minimal supergravity model, and supergravity with string inspired two- and one-parameter moduli and
dilaton scenarios. Our results are contrasted with estimated experimental
sensitivities for dileptons and trileptons for integrated luminosities of
100\ipb and 1\ifb, which should be available in the short and long terms.
We find that the dilepton mode is a needed complement to the trilepton signal
when the latter is suppressed by small neutralino leptonic branching ratios.
The estimated reaches in chargino masses can be as large as 100\,(150)\GeV
for 100\ipb\,(1\ifb). We also discuss the task left for LEPII once the
Tevatron has completed its short-term search for dilepton and trilepton
production.Comment: 12 pages (LaTeX), 6 figures (included
New Precision Electroweak Tests in Supergravity Models
We update the analysis of the precision electroweak tests in terms of 4
epsilon parameters, , to obtain more accurate experimental
values of them by taking into account the new LEP data released at the 28th
ICHEP (1996, Poland). We also compute and in the
context of the no-scale supergravity model to obtain the
updated constraints by imposing the correlated constraints in terms of the
experimental ellipses in the plane and also by imposing
the new bound on the lightest chargino mass, .
Upon imposing these new experimental results, we find that the situations in
the no-scale model are much more favorable than those in the standard model,
and if , then the allowed regions at the 95% C.~L. in
the no-scale model are and for , which are in fact much more stringent than in
our previous analysis. Therefore, assuming that , if the
lightest chargino mass bound were to be pushed up only by a few GeV, the sign
on the Higgs mixing term in the no-scale model could well be determined
from the constraint to be positive at the 95% C.~L. At
any rate, better accuracy in the measured from the Tevatron in the near
future combined with the LEP data is most likely to provide a decisive test of
the no-scale supergravity model.Comment: 15 pages, REVTEX, 1 figure (not included but available as a ps file
from [email protected]
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