Exponentially Small Couplings and the Hierarchy Problem

We propose a stringy mechanism whereby a large hierarchy between symmetry breaking scales is generated. This mechanism is based upon the existence of a fifth dimension compactified on a segment. We focus on a simple supersymmetric model with one massless Higgs field in the 3 of SU(3) and another one in the $\bar 3$ on each four dimensional end-point of the fifth dimension. Along supersymmetric flat directions the gauge symmetry is broken down to SU(2) due to the vacuum expectation value of the Higgs fields on one of the end points. The remaining massless mode on the other end point acquires a potential due to a massive five dimensional state propagating between the end points. This potential breaks the SU(2) symmetry at an exponentially suppressed scale compared to the SU(3) breaking scale. The suppression factor depends exponentially on the mass M of the massive state and the length $\pi R$ of the fifth dimension. For reasonably large values of the length scale R one can achieve a factor of order M_{W}/M_GUT}.Comment: 8 pages, latex, 2 figure

Modular Symmetries of Threshold Corrections for Abelian Orbifolds with Discrete Wilson Lines

The modular symmetries of string loop threshold corrections for gauge coupling constants are studied in the presence of discrete Wilson lines for all examples of abelian orbifolds, where the point group is realised by the action of Coxeter elements or generalised Coxeter elements on the root lattices of the Lie groups.Comment: 36 pages, Late

Challenges for Superstring Cosmology

We consider whether current notions about superstring theory below the Planck scale are compatible with cosmology. We find that the anticipated form for the dilaton interaction creates a serious roadblock for inflation and makes it unlikely that the universe ever reaches a state with zero cosmological constant and time-independent gravitational constant.Comment: 14 pages, 2 figures available as eps files on reques

A Cosmological Mechanism for Stabilizing Moduli

In this paper, we show how the generic coupling of moduli to the kinetic energy of ordinary matter fields results in a cosmological mechanism that influences the evolution and stability of moduli. As an example, we reconsider the problem of stabilizing the dilaton in a non-perturbative potential induced by gaugino condensates. A well-known difficulty is that the potential is so steep that the dilaton field tends to overrun the correct minimum and to evolve to an observationally unacceptable vacuum. We show that the dilaton coupling to the thermal energy of matter fields produces a natural mechanism for gently relaxing the dilaton field into the correct minimum of the potential without fine-tuning of initial conditions. The same mechanism is potentially relevant for stabilizing other moduli fields.Comment: 15 pages, 3 figures, revised discussio

Erratum for "Fractionally Charged Particles and Supersymmetry Breaking in 4D Strings."

The original paper (hep-th/9209020) contains an early example of gauge mediated supersymmetry breaking motivated by string model building. In this erratum we point out an important misprint in the abstract of the published version (Phys. Lett. B295 (1992) 219) which may confuse the reader. We also correct the computation of scalar masses and the numerical examples. Qualitative results remain the same.Comment: 2 pages, Te

Supersymmetry Breaking by Hidden Matter Condensation in Superstrings

We show that supersymmetry can be broken mainly by hidden matter condensates in the observable matter direction in generic superstring models. This happens only when the fields whose VEVs give masses to hidden matter do not decouple at the condensation scale. We find how the parameters of the string model and the vacuum determine whether supersymmetry is broken mainly by hidden matter or gaugino condensates and in the matter or moduli directions.Comment: 14 pages, uses phyzzx.tex, WIS-94/29/JUL-P

Dilaton Supersymmetry Breaking

We argue that dilaton supersymmetry breaking in string derived supergravity requires an effective superpotential which is not separable as a function of the dilaton times a function of the moduli. We show that in a simple model with hidden sector matter condensation and a dilaton independent term one can easily obtain $|F_S| \not=0$. For a wide range of realistic model parameters $|F_S|>>|F_T|$ and supersymmetry is mainly broken in the dilaton direction.Comment: 11 pages, uses phyzzx.te

Supersymmetric Models with a Gauge Singlet and Non-Universal Soft Terms from Orbifold String Theory

The particle spectrum of the supersymmetric extension of the standard model with a gauge singlet is studied. Soft supersymmetry breaking terms are explicitly chosen to be non-universal according to orbifold string theory. they depend on modular weights of chiral fields and on an angle $\theta$ specifying the supersymmetry breaking sector. Imposing radiative weak symmetry breaking and requiring that standard model Yukawa couplings should be allowed yield constraints on modular weights and almost specifies the angle $\theta$. We then perform a numerical anlysis of the low energy spectrum. It turns out that the spectrum is very constrained, revealing salient features such as light Higgses and neutralinos. The latter turns out to be essentially gaugino-like.Comment: 13 pages, uuencoded ps-fil

String gauge unification and infrared fixed points in MSSM + X models

In order to achieve gauge unification at the string scale M_X sim 5.10^17 GeV in the minimal supersymmetric standard model (MSSM) it is necessary to add extra gauge non-singlet representations at an intermediate scale M_I<M_X, leading to a class of models which we refer to as MSSM+X models. We perform a detailed analysis of a large class of MSSM+X models and find that the number of (3,1) representations added must be greater than the total of the number of (3,2) and (1,2) representations. Predictions of M_I, M_X and alpha(M_X) are obtained for models with up to 5 extra vector representations than the MSSM\@. Upper bounds on the U(1) string gauge normalisation k_1 and the sum of the squares of the hypercharge assignments of the extra matter are also obtained for the models. We also study the infra-red fixed point behaviour of the top quark Yukawa coupling in a large class of MSSM+X models and find that the low energy MSSM quasi-fixed point prediction of the top quark mass is more likely to be realised in these theories than in the MSSM. In other words the top quark tends to be heavier in MSSM+X models than in the MSSM. The implementation of a U(1)_X family symmetry into MSSM+X models to account for the Standard Model fermion masses is discussed and a particular viable model is presented.Comment: 30 pages LaTeX, uses epsf.sty. Minor changes and extra field content in model X, leaving all results qualitatively unchange