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Lisa Randall

Publications and source records attributed to Lisa Randall.

At least 127 records · Page 7Linked to original sources
arXiv1997

Comment on nonperturbative effects in $B\to X_sγ$

Uncertainties in the theoretical prediction for the inclusive $B\to X_sγ$ decay rate are discussed. We emphasize that there is no operator product expansion for this process. Nonetheless, some nonperturbative effects involving a virtual $c\bar c$ loop are calculable using the operator product expansion. They give a contribution to the decay rate that involves the B meson matrix element of an infinite tower of operators. The higher dimension operators give effects that are only suppressed by powers of $m_bΛ_{QCD}/m_c^2 \sim 0.6$, but come with small coefficients.

hep-ph↗
arXiv1997

Unity of Supersymmetry Breaking Models

We examine the models with gauge group U(1)^{k-1}\times\prod_{i=1}^k SU(n_i), which are obtained from decomposing the supersymmetry breaking model of Affleck, Dine and Seiberg containing an antisymmetric tensor field. We note that all of these models are distinct vacua of a single SU(N) gauge theory with an adjoint superfield. The dynamics of this model may be analyzed using the duality of Kutasov and Schwimmer and the deconfinement trick of Berkooz. This analysis leads to a simple picture for supersymmetry breaking for k=2, complementing that of previous work. We examine the flat directions of these models, and give straightforward criteria for lifting them, explaining the requisite peculiar form of the superpotential. For all cases with k>2, the duality argument fails to give supersymmetry breaking dynamics, and we identify a class of problematic flat directions, which we term 2m-baryons. We study in some detail the requirements for lifting these directions, and uncover some surprising facts regarding the relationship between R-symmetry and supersymmetry breaking in models with several gauge groups.

hep-ph↗
arXiv1996

New Mechanisms of Gauge-Mediated Supersymmetry Breaking

We introduce new mechanisms for the communication of supersymmetry breaking via gauge interactions. These models do not require complicated dynamics to induce a nonvanishing F term for a singlet. The first class of models communicates supersymmetry breaking to the visible sector through a ``mediator" field that transforms under both a messenger gauge group of the dynamical supersymmetry breaking sector and the standard model gauge group. This model has distinctive phenomenology; in particular, the scalar superpartners should be heavier by at least an order of magnitude than the gaugino superpartners. The second class of models has phenomenology more similar to the ``standard" messenger sectors. A singlet is incorporated, but the model does not require complicated mechanisms to generate a singlet F term. The role of the singlet is to couple fields from the dynamical symmetry breaking sector to fields transforming under the standard model gauge group. We also mention a potential solution to the $μ$ problem.

hep-ph↗
arXiv1996

Vacuum Instability in Low-Energy Supersymmetry Breaking Models

We show that for the simplest models of gauge mediated supersymmetry breaking, including all existing models, the true vacuum will not preserve QCD if it communicates supersymmetry breaking to the visible sector. We show that the desired supersymmetry breaking color preserving vacuum can nonetheless be stable cosmologically, but only if certain couplings are sufficiently small. We also present simple modifications to the sector which communicates supersymmetry breaking in which the true vacuum is acceptable, suggesting desirable properties to be sought in dynamical models of supersymmetry breaking.

hep-ph↗
arXiv1996

Holomorphic Anomalies and the Nonrenormalization Theorem

It has been argued that the superpotential can be renormalized in the presence of massless particles. Possible implications which have been considered include the restoration of supersymmetry at higher loops or a shift to a supersymmetric vacuum state. We argue that even in the presence of massless particles, there are no new contributions to the superpotential at any order in perturbation theory. This confirms the utility of the Wilsonian superpotential for analyzing the moduli space of the low energy theory.

hep-th↗
arXiv1996

Supersymmetry Breaking Through Confining and Dual Theory Gauge Dynamics

We show that theories in the confining, free magnetic, and conformal phases can break supersymmetry through dynamical effects. To illustrate this, we present theories based on the gauge groups $SU(n)\times SU(4)\times U(1)$ and $SU(n) \times SU(5) \times U(1)$ with the field content obtained by decomposing an $SU(m)$ theory with an antisymmetric tensor and $m-4$ antifundamentals.

hep-th↗
arXiv1996

More Dynamical Supersymmetry Breaking

In this paper we introduce a new class of theories which dynamically break supersymmetry based on the gauge group SU(n)xSU(3)xU(1) for even n. These theories are interesting in that no dynamical superpotential is generated in the absence of perturbations. For the example SU(4)xSU(3)xU(1) we explicitly demonstrate that all flat directions can be lifted through a renormalizable superpotential and that supersymmetry is dynamically broken. We derive the exact superpotential for this theory, which exhibits new and interesting dynamical phenomena. For example, modifications to classical constraints can be field dependent. We also consider the generalization to SU(n)xSU(3)xU(1) models (with even n>4). We present a renormalizable superpotential which lifts all flat directions. Because SU(3) is not confining in the absence of perturbations, the analysis of supersymmetry breaking is very different in these theories from the n=4 example. When the SU(n) gauge group confines, the Yukawa couplings drive the SU(3) theory into a regime with a dynamically generated superpotential. By considering a simplified version of these theories we argue that supersymmetry is probably broken.

hep-th↗
arXiv1996

Supernatural Inflation

Most models of inflation have small parameters, either to guarantee sufficient inflation or the correct magnitude of the density perturbations. In this paper we show that, in supersymmetric theories with weak scale supersymmetry breaking, one can construct viable inflationary models in which the requisite parameters appear naturally in the form of the ratio of mass scales that are already present in the theory. Successful inflationary models can be constructed from the flat-direction fields of a renormalizable supersymmetric potential, and such models can be realized even in the context of a simple GUT extension of the MSSM. We evade naive ``naturalness" arguments by allowing for more than one field to be relevant to inflation, as in ``hybrid inflation" models, and we argue that this is the most natural possibility if inflaton fields are to be associated with flat direction fields of a supersymmetric theory. Such models predict a very low Hubble constant during inflation, of order $10^3$-$10^4$ GeV, a scalar density perturbation index $n$ which is very close to or greater than unity, and negligible tensor perturbations. In addition, these models lead to a large spike in the density perturbation spectrum at short wavelengths.

hep-ph↗
arXiv1996

Supernatural Inflation: Inflation from Supersymmetry with No (Very) Small Parameters

Most models of inflation have small parameters, either to guarantee sufficient inflation or the correct magnitude of the density perturbations. In this paper we show that, in supersymmetric theories with weak scale supersymmetry breaking, one can construct viable inflationary models in which the requisite parameters appear naturally in the form of the ratio of mass scales that are already present in the theory. Successful inflationary models can be constructed from the flat-direction fields of a renormalizable supersymmetric potential, and such models can be realized even in the context of a simple GUT extension of the MSSM. We evade naive ``naturalness" arguments by allowing for more than one field to be relevant to inflation, as in ``hybrid inflation" models, and we argue that this is the most natural possibility if inflaton fields are to be associated with flat direction fields of a supersymmetric theory. Such models predict a very low Hubble constant during inflation, of order $10^3$-$10^4$ GeV, a scalar density perturbation index $n$ which is very close to or greater than unity, and negligible tensor perturbations. In addition, these models lead to a large spike in the density perturbation spectrum at short wavelengths.

hep-ph↗
arXiv1995

Phenomenological Constraints on the Higgs as Pseudo-Goldstone Boson Mechanism in Supersymmetric GUT Theories

There are few robust solutions to the doublet-triplet splitting problem in supersymmetric GUT theories. One of the more promising solutions is the Higgs as pseudo-Goldstone boson mechanism. In its minimal implementation, such a solution places an additional restriction on the parameter space of the minimal supersymmetric standard model. A testable consequence of this constraint is an equation for $\tan β$. We present this restriction and study its solutions in order to constrain the allowed parameter space. Thus the assumptions on the GUT scale Higgs sector should yield testable predictions for weak scale physics. If the SUSY parameters are measured then it should be possible to check the predictions, yielding insight into GUT scale physics.

hep-ph↗
arXiv1995

Destabilizing Divergences in Supergravity Theories at Two Loops

We examine the stability of the mass hierarchy in hidden-sector supergravity theories. We show that a quadratically divergent tadpole can appear at two loops, even in minimal supergravity theories, provided the theory has a gauge- and global-symmetry singlet with renormalizable couplings to the visible fields. This tadpole can destabilize the hierarchy. We also find a quadratically divergent two-loop contribution to the field-dependent vacuum energy. This result casts doubt on the efficacy of the "LHC mechanism" for controlling quadratic divergences. We carry out the two-loop calculation in a manifestly supersymmetric formalism, and explain how to apply the formalism in the presence of supersymmetry breaking to derive radiative corrections to the supersymmetric and soft supersymmetry-breaking operators. Our approach greatly simplifies the calculation and guarantees consistency of our results with the underlying supergravity framework.

hep-ph↗
arXiv1995

The Doublet-Triplet Splitting Problem and Higgses as Pseudogoldstone Bosons

The doublet-triplet splitting problem is probably the most significant challenge to supersymmetric GUT theories. In this talk, we review potential solutions and their problematic aspects. We also present a complete consistent realization of our preferred solution, higgses as pseudogoldstone bosons, and discuss some distinctive aspects of its phenomenology.

hep-ph↗
arXiv1995

Baryogenesis from Flat Directions of the Supersymmetric Standard Model

Baryogenesis from the coherent production of a scalar condensate along a flat direction of the supersymmetric extension of the standard model (Affleck-Dine mechanism) is investigated. Two important effects are emphasized. First, nonrenormalizable terms in the superpotential can lift standard model flat directions at large field values. Second, the finite energy density in the early universe induces soft potentials with curvature of order the Hubble constant. Both these have important implications for baryogenesis, which requires large squark or slepton expectation values to develop along flat directions. In particular, the induced mass squared must be negative. The resulting baryon to entropy ratio is very insensitive to the details of the couplings and initial conditions, but depends on the dimension of the nonrenormalizable operator in the superpotential which stabilizes the flat direction and the reheat temperature after inflation. Unlike the original scenario, an acceptable baryon asymmetry can result without subsequent entropy releases. In the simplest scenario the baryon asymmetry is generated along the $LH_u$ flat direction, and is related to the mass of the lightest neutrino.

hep-ph↗
arXiv1995

Flat Directions and Baryogenesis in Supersymmetric Theories

Flat directions are a generic feature of supersymmetric theories. They are of cosmological interest because they can lead to coherent production of scalars. In the early universe such flat directions could be dangerous due to the potentially large energy density and the late decay of the associated scalars when they have only $1/M_p$ couplings (Polonyi problem). On the other hand, flat directions among the standard model fields can carry baryon number and lead to a possible mechanism for baryogenesis (Affleck Dine baryogenesis). When considering the cosmological consequences of the flat directions, it is important to take into account the soft potential with curvature of order the Hubble constant due to supersymmetry breaking in the early universe. In this talk, we discuss flat directions, their potential cosmological implications focusing on Affleck-Dine baryogenesis, and how the standard picture of their evolution must be modified in the presence of the large supersymmetry breaking in the early universe.

hep-ph↗
arXiv1995

Supersymmetry Breaking in the Early Universe

Supersymmetry breaking in the early universe induces scalar soft potentials with curvature of order the Hubble constant. This has a dramatic effect on the coherent production of scalar fields along flat directions. For the moduli problem it generically gives a concrete realization of the problem by determining the field value subsequent to inflation. However it might suggest a solution if the minimum of the induced potential coincides with the true minimum. The induced Hubble scale mass also has important implications for the Affleck-Dine mechanism of baryogenesis. This mechanism requires large squark or slepton expectation values to develop along flat directions in the early universe. This is generally not the case if the induced mass squared is positive, but does occur if it is negative. The resulting baryon to entropy ratio depends mainly on the dimension of the nonrenormalizable operator in the superpotential which stabilizes the flat direction, and the reheat temperature after inflation. Unlike the original scenario, it is possible to obtain an acceptable baryon asymmetry without subsequent entropy releases.

hep-ph↗