Superstring Extension of the Standard Model and Gravitation

Some of the four-dimensional Superstring solutions provide a consistent framework for a Supersymmetric Unification of all interactions including gravity. A class of them extends successfully the validity of the standard model up to the string scale ${\cal O}(10^{17})~GeV$. We stress the importance of string corrections which are relevant for low energy ${\cal O}(1)~TeV$ predictions of gauge and Yukawa couplings as well as the spectrum of the supersymmetric particles after supersymmetry breaking.Comment: plain LATEX, no figures, 22 page

Universal Thermal Instabilities and the High-Temperature Phase of the N=4 Superstrings

Using the properties of gauged N=4 supergravity, we show that it is possible to derive a universal thermal effective potential that describes all possible high-temperature instabilities of the known N=4 superstrings. These instabilities are due to non-perturbative dyonic modes, which become tachyonic in a region of the thermal moduli space M={s,t,u}; M is common to all non-perturbative dual-equivalent N=4 superstrings in five dimensions. We analyse the non-perturbative thermal potential and show the existence of a phase transition at high temperatures corresponding to a condensation of 5-branes. This phase is described in detail, using an effective non-critical string theory.Comment: 15 pages, LATEX file, no figure

Construction of String Solutions around Non-trivial Backgrounds

We present a way of constructing string solutions around non-trivial gravitational backgrounds. The proposed solutions are constructed using $N = 4$ superconformal building blocks with $\hat c = 4$. We give two different and inequivalent realizations of non-trivial four-dimensional subspaces, and we show the emergence of the $N = 4$ globally defined superconformal symmetry. The existence of $N = 4$ world-sheet symmetry stabilizes our solutions and implies in target space a number of covariantized supersymmetries around space-time dependent gravitational and dilaton backgrounds.Comment: Latex file, 15pp, CERN-TH.6790/9

String and Particle with Two Times

An action for a string and a particle with two timelike dimensions is proposed and analyzed. Due to new gauge symmetries and associated constraints, the motion of each system in the background of the other is equivalent to effective motion with a single timelike dimension. The quantum constraints are consistent only in critical dimensions. For the bosonic system in flat spacetime the critical dimension is 27 or 28, with signature (25,2) or (26,2), depending on whether the particle is massive or massless respectively. For the supersymmetric case the critical dimensions are 11 or 12, with signature (9,2) or (10,2), under the same circumstances. Generalizations to multiparticles, strings and p-branes are outlined.Comment: Latex, 19 pages, misprints and references correcte

Instabilities in Strong Magnetic Fields in String Theory

We construct groundstates of the string with non-zero mass gap and non-trivial chromo-magnetic fields as well as curvature. The exact spectrum as function of the chromo-magnetic fields and curvature is derived. We examine the behavior of the spectrum, and find that there is a maximal value for the magnetic field $H_{\rm max}\sim M_{\rm Plank}^2$. At this value all states that couple to the magnetic field become infinitely massive and decouple. We also find tachyonic instabilities for strong background fields of the order ${\cal O}(\mu M_{\rm Plank})$ where $\mu$ is the mass gap of the theory. Unlike the field theory case, we find that such ground states become stable again for magnetic fields of the order ${\cal O}(M^2_{\rm Plank})$. The implications of these results are discussed.Comment: To appear in the proceedings of the Conference on Gauge Theories, Applied Supersymmetry and Quantum Gravity, Leuven, July 199

Infrared-Regulated String Theory and Loop Corrections to Coupling Constants

Exact superstring solutions are constructed in 4-D space-time, with positive curvature and non-trivial dilaton and antisymmetric tensor fields. The full spectrum of string excitations is derived as a function of moduli fields $T^{i}$ and the scale $\mu^2=1/(k+2)$ which is induced by the non-zero background fields. The spectrum of string excitations has a non-zero mass gap $\mu^2$ and in the weak curvature limit ($\mu$ small) $\mu^2$ plays the role of a well defined infrared regulator, consistent with modular invariance, gauge invariance, supersymmetry and chirality. The effects of a covariantly constant (chomo)magnetic field $H$ as well as additional curvature can be derived exactly up to one string-loop level. Thus, the one-loop corrections to all couplings (gravitational, gauge and Yukawas) are unambiguously computed and are finite both in the UltraViolet and the InfraRed regime. These corrections are necessary for quantitative string superunification predictions at low energies. The one-loop corrections to the couplings are also found to satisfy Infrared Flow Equations. Having in our disposal an exact description which goes beyond the leading order in the $\alpha'$-expansion or the linearized approximation in the magnetic field, we find interesting clues about the physics of string theory in strong gravitational and magnetic fields. In particular, the nature of gravitational or magnetic instabilities is studied.Comment: Based on talks given by the authors at the Strings-95 conference in USC, Los Angeles. Contains some new result

A new supersymmetry

We propose a new supersymmetry in field theory that generalizes standard supersymmetry and we construct field theoretic models that provide some of its representations. This symmetry combines a finite number of standard 4D supersymmetry multiplets into a single multiplet with a new type of Kaluza-Klein embedding in higher dimensions. We suggest that this mechanism may have phenomenological applications in understanding family unification. The algebraic structure, which has a flavor of W-algebras, is directly motivated by S-theory and its application in black holes. We show connections to previous proposals in the literature for 12 dimensional supergravity, Yang-Mills, (2,1) heterotic superstrings and Matrix models that attempt to capture part of the secret theory behind string theory.Comment: Latex, 27 page