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 $SU(5)\times U(1)$ 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 eeγγee\gamma\gamma event

We present a unified no-scale supergravity model with a light gravitino that can naturally explain the observed $ee\gamma\gamma$ 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 $e^+e^-\to\chi^0_1\chi^0_1$ production.Comment: 10 pages, LaTeX, 4 figures (included

RbR_b in supergravity models

We compute the supersymmetric contribution to $R_{b}\equiv \Gamma (Z\to b{\bar b})/\Gamma (Z\to {\rm hadrons})$ in a variety of supergravity models. We find R^{\rm susy}_b\lsim0.0004, which does not shift significantly the Standard Model prediction ($R^{\rm SM}_b=0.2162$ 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 $SU(5)$ supergravity model and the class of string-inspired/derived supergravity models based on the flipped $SU(5)\times U(1)$ 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 $\tan\beta=v_2/v_1$, and three supersymmetry breaking parameters ($m_{1/2},m_0,A$). The latter are chosen in the minimal $SU(5)$ model such that the stringent constraints from proton decay and cosmology are satisfied. In the flipped $SU(5)$ case we consider two string-inspired supersymmetry breaking scenaria: $SU(N,1)$ no-scale supergravity and a dilaton-induced supersymmetry breaking scenario. Both imply universal soft supersymmetry breaking parameters: $m_0=A=0$ 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 $SU(5)$ models and the minimal $SU(5)$ model and conclude that all can be partially probed at the Tevatron and LEPII (and the flipped models at HERA too). In both flipped $SU(5)$ cases there is a more constrained version which allows to determine $\tan\beta$ in terms of $m_t,m_{\tilde g}$ 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 $SU(5)$ models is in conflict with the present experimental allowed range for the $b\to s\gamma$ 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 $SU(5)\times U(1)$ 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 $SU(N,1)$ no-scale supergravity model and a dilaton-induced supersymmetry breaking scenario. Both imply universal soft supersymmetry breaking parameters: $m_0=0, A=0$ 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: $m_t$, $\tan\beta$, and $m_{\tilde g}$. 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 $\tan\beta$ in terms of $m_t,m_{\tilde g}$. In the strict no-scale case we find that the value of $m_t$ determines the sign of $\mu$ (\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, $\tan\beta\approx1.4-1.6$ 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