F-enomenology

The advantages of Flipped SU(5) over conventional Supersymmetric GUTs, like SU(5), are discussed. Recent values of the strong coupling at M_Z, sin-squared theta-Weinberg, g-2 of the muon, and the lower limit on the proton lifetime for the (K+, anti-neutrino) mode point directly to Flipped SU(5) as the simplest way to avoid potential pitfalls. It is shown that "F(lipped)-enomenology" accomodates easily all presently available low-energy data, favoring a rather "light" supersymmetric spectrum while yielding the right amount of Cold Dark Matter and a proton lifetime in the ((e+/muon+), pi-zero) mode which is beyond the present experimental limit yet still possibly accessible to a further round of experiments.Comment: 22 pages; 3 figures and 2 diagrams prepared with feynmf.mf & feynmf.sty; Invited talk given at: 1st Intl. Conf. on String Phenomenology, Oxford, England, July 6-11, 2002, -and- NeSS 2002, Washington D.C., USA, September 19-21, 200

M-Phenomenology

Recent developments involving strongly coupled superstrings are discussed from a phenomenological point of view. In particular, strongly coupled $E_8\times E'_8$ is described as an appropriate long-wavelength limit of M-theory, and some generic phenomenological implications are analyzed, including a long sought downward shift of the string unification scale and a novel way to break supersymmetry. A specific scenario is presented that leads to a rather light, and thus presently experimentally testable, sparticle spectrum.Comment: 22 pages, 2 figure

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

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

Supercriticality of a Class of Critical String Cosmological Solutions

For a class of Friedmann-Robertson-Walker type string solutions with compact hyperbolic spatial slices formulated in critical dimension, we find the world sheet conformal field theory which involves the linear dilaton and Wess-Zumino-Witten type model with the compact hyperbolic target space. By analyzing the infrared spectrum, we conclude that the theory is actually supercritical due to the modular invariance of string theory. Thus, taking into account previous results, we conclude that all the simple nontrivial string cosmological solutions are supercritical. A possible explanation of why we are living in D=4 is provided. The interesting relation of this background with the Supercritical String Cosmology (SSC) is pointed out

The Electroweak Phase Transition in Minimal Supergravity Models

We have explored the electroweak phase transition in minimal supergravity models by extending previous analysis of the one-loop Higgs potential to include finite temperature effects. Minimal supergravity is characterized by two higgs doublets at the electroweak scale, gauge coupling unification, and universal soft-SUSY breaking at the unification scale. We have searched for the allowed parameter space that avoids washout of baryon number via unsuppressed anomalous Electroweak sphaleron processes after the phase transition. This requirement imposes strong constraints on the Higgs sector. With respect to weak scale baryogenesis, we find that the generic MSSM is {\it not} phenomenologically acceptable, and show that the additional experimental and consistency constraints of minimal supergravity restricts the mass of the lightest CP-even Higgs even further to m_h\lsim 32\GeV (at one loop), also in conflict with experiment. Thus, if supergravity is to allow for baryogenesis via any other mechanism above the weak scale, it {\it must} also provide for B-L production (or some other `accidentally' conserved quantity) above the electroweak scale. Finally, we suggest that the no-scale flipped $SU(5)$ supergravity model can naturally and economically provide a source of B-L violation and realistically account for the observed ratio $n_B/n_\gamma\sim 10^{-10}$.Comment: 14 pages (not including two postscript figures available upon request