59,396 research outputs found
Closing the Universe by relaxing the cosmological constant
We propose a string-inspired model which correlates several aspects of
particle physics and cosmology. Inspired by the flat directions and the absence
of adjoint Higgs representations found in typical string models, we consider a
no-scale supergravity model. This model entails well
determined low-energy phenomenology, such as the value of the neutralino dark
matter relic abundance and a negative contribution to the vacuum energy. A
positive contribution to the vacuum energy is also typically present in string
theory as a consequence of the running of the fundamental constants towards
their fixed point values. If these two contributions cancel appropriately, one
may end up with a vacuum energy which brings many cosmological observations
into better agreement with theoretical expectations. The present abundance of
neutralinos would then be fixed. We delineate the regions of parameter space
allowed in this scenario, and study the ensuing predictions for the sparticle
and Higgs-boson masses in this model.Comment: 5 pages, 2 figures (included), Latex, CTP-TAMU-06/9
A supergravity explanation of the CDF event
We present a unified no-scale supergravity model with a light gravitino that
can naturally explain the observed event at CDF via
right-handed selectron pair-production. The full spectrum of our model can be
described in terms of a single parameter and can be distinguished from
alternative proposals in the literature. Ongoing and future runs at LEP 2
should be able to probe the full allowed parameter space via acoplanar diphoton
events from production.Comment: 10 pages, LaTeX, 4 figures (included
in supergravity models
We compute the supersymmetric contribution to
in a
variety of supergravity models. We find R^{\rm susy}_b\lsim0.0004, which does
not shift significantly the Standard Model prediction
( for m_t=160\GeV). An improvement in experimental
precision by a factor of four would be required to be sensitive to such an
effect.Comment: 5 pages, LaTeX, 1 figure (included)
No-scale supergravity confronts LEP diphoton events
We examine the possibility that some anomalous acoplanar diphoton events
observed at LEP may be consistent with the predictions of our previously
proposed one-parameter no-scale supergravity model with a light gravitino, via
the process e^+e^- -> chi chi -> gamma gamma+E_miss. We find that one such
event may indeed be consistent with the model predictions for m_chi \approx
(60-70) GeV. This region of parameter space is also consistent with the
selectron and chargino interpretations of the CDF ee gamma gamma + E_T,miss
event.Comment: 9 pages, LaTeX, 4 figures (included). Updated to include full LEP161
dat
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
Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition
Transiting planet surveys like Kepler have provided a wealth of information
on the distribution of planetary radii, particularly for the new populations of
super-Earth and sub-Neptune sized planets. In order to aid in the physical
interpretation of these radii, we compute model radii for low-mass rocky
planets with hydrogen-helium envelopes. We provide model radii for planets 1-20
Earth masses, with envelope fractions from 0.01-20%, levels of irradiation
0.1-1000x Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide
simple analytic fits that summarize how radius depends on each of these
parameters. Most importantly, we show that at fixed composition, radii show
little dependence on mass for planets with more than ~1% of their mass in their
envelope. Consequently, planetary radius is to first order a proxy for
planetary composition for Neptune and sub-Neptune sized planets. We recast the
observed mass-radius relationship as a mass-composition relationship and
discuss it in light of traditional core accretion theory. We discuss the
transition from rocky super-Earths to sub-Neptune planets with large volatile
envelopes. We suggest 1.75 Earth radii as a physically motivated dividing line
between these two populations of planets. Finally, we discuss these results in
light of the observed radius occurrence distribution found by Kepler.Comment: 17 pages, 9 figures, 7 tables, submitted to Ap
SU(5)xU(1): a string paradigm of a TOE and its experimental consequences
We present a string-inspired/derived supergravity model 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). This model is complemented by two string supersymmetry
breaking scenaria: the no-scale supergravity model 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. In either case the
models depend on only three parameters: , , and .
We present a comparative study of the sparticle and Higgs spectra of both
models and conclude that even though both can be partially probed at the
Tevatron, LEPII, and HERA, a larger fraction of the parameter space of the
no-scale model is actually accessible. In both cases there is a more
constrained version which allows to determine in terms of
. In the strict no-scale case we find that the value of
determines the sign of (\mu>0:\,m_t\lsim135\GeV,
\mu<0:\,m_t\gsim140\GeV) and whether the lightest Higgs boson mass is above
or below 100\GeV. In the more constrained version of the dilaton scenario,
and m_t\lsim155\GeV, 61\GeV\lsim m_h\lsim91\GeV
follow. Thus, continuing Tevatron top-quark searches and LEPI,II Higgs searches
could probe this restricted scenario completely.Comment: CERN-TH.6926/93, CTP-TAMU-33/93, LaTeX, 33 pages, 11 embedded
figures. Significant numerical correction to the dilaton scenario. Complete
ps file (7943 blocks,3.33MB) available via anonymous ftp from site
tamsun.tamu.edu in directory incoming, filename: CTP-TAMU-33-93.p
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