arXiv subjects

S. P. de Alwis

Publications and source records attributed to S. P. de Alwis.

At least 37 records · Page 2Linked to original sources

arXiv2011

A Unified Approach to Supersymmetry Breaking

General formulae for the soft SUSY breaking terms, valid in any SUGRA context, were derived in the mid-nineties. Since SUSY is not expected to have quantum anomalies, they should be valid in the quantum theory and be RG invariant down to the soft SUSY breaking scale. This observation enables us to give a uniform treatment of all phenomenological models for SUSY breaking and transmission, such as AMSB, GMSB, etc. In particular we find that the much discussed RG invariant formulae for soft SUSY breaking parameters in AMSB, effectively depend on a strong assumption of factorizability of the matter Kaehler metric. We then argue that there is no necessity for having ad hoc constructions such as mAMSB to counteract the negative squared slepton mass problem, since the natural framework that emerges in a sequestered model is one in which gaugino masses are as in AMSB, and the other soft terms are generated by RG running as in gaugino mediation.

hep-th↗

arXiv2011

Gauge Mediated Supersymmetry Breaking and String Theory

We discuss the possibility of finding scenarios, within type IIB string theory compactified on Calabi-Yau orientifolds with fluxes, for realizing gauge mediated supersymmetry breaking. We find that while in principle such scenarios are not ruled out, in practice it is hard to get acceptable constructions, since typically, supersymmetry breaking cannot be separated from the stabilization of the light modulus.

hep-th↗

arXiv2010

A Comparison of Supersymmetry Breaking and Mediation Mechanisms

We give a unified treatment of different models of supersymmetry breaking and mediation from a four dimensional effective field theory standpoint. In particular a comparison between GMSB and various gravity mediated versions of SUSY breaking shows that, once the former is embedded within a SUGRA framework, there is no particular advantage to that mechanism from the point of view of FCNC suppression. We point out the difficulties of all these scenarios - in particular the cosmological modulus problem. We end with a discussion of possible string theory realizations.

hep-th↗

arXiv2010

Classical and Quantum SUSY Breaking Effects in IIB Local Models

We discuss the calculation of soft supersymmetry breaking terms in type IIB string theoretic models in the Large Volume Scenario (LVS). The suppression of FCNC gives a lower bound on the size of the compactification volume. This leads to soft terms which are strongly suppressed relative to the gravitino mass so that the dominant contribution to the gaugino masses comes from the Weyl anomaly. The other soft terms are essentially generated by the renormalization group running from the string scale to the TeV scale.

hep-th↗

arXiv2008

Heterotic Strings on Generalized Calabi-Yau Manifolds and Kaehler Moduli Stabilization

Compactifications of heterotic string theory on Generalized Calabi-Yau manifolds have been expected to give the same type of flexibility that type IIB compactifications on Calabi-Yau orientifolds have. In this note we generalize the work done on half-flat manifolds by other authors, to show how flux quantization occurs in the general case, by starting with a basis of harmonic forms and then extending it. However it turns out that only the axions associated with the non-harmonic directions in the space of Kaehler moduli, can be stabilized by the geometric (torsion) terms. Also we argue that there are no supersymmetric extrema of the potential when the second (and fourth) cohomology groups on the manifold are non-trivial. We suggest that threshold corrections to the classical gauge coupling function could solve these problems.

hep-th↗

arXiv2008

Electronic structure and exotic exchange splitting in spin-density-wave states of BaFe$_2$As$_2$

The magnetic properties in the parent compounds are often intimately related to the microscopic mechanism of superconductivity. Here we report the first direct measurements on the electronic structure of a parent compound of the newly discovered iron-based superconductor, BaFe$_2$As$_2$, which provides a foundation for further studies. We show that the energy of the spin density wave (SDW) in BaFe$_2$As$_2$ is lowered through exotic exchange splitting of the band structure, rather than Fermi surface nesting of itinerant electrons. This clearly demonstrates that a metallic SDW state could be solely induced by interactions of local magnetic moments, resembling the nature of antiferromagnetic order in cuprate parent compounds.

cond-mat.supr-con↗

arXiv2008

On Anomaly Mediated SUSY Breaking

A discrepancy between the Anomaly Mediated Supersymmetry Breaking (AMSB) gaugino mass calculated from the work of Kaplunovsky and Louis (hep-th/9402005) (KL) and other calculations in the literature is explained, and it is argued that the KL expression is the correct one relevant to the Wilsonian action. Furthermore it is argued that the AMSB contribution to the squark and slepton masses should be replaced by the contribution pointed out by Dine and Seiberg (DS) which has nothing to do with Weyl anomalies. This is not in general equivalent to the AMSB expression, and it is shown that there are models in which the usual AMSB expression would vanish but the DS one is non-zero. In fact the latter has aspects of both AMSB and gauge mediated SUSY breaking. In particular like the latter, it gives positive squared masses for sleptons.

hep-th↗

arXiv2008

Warped Supersymmetry Breaking

We address the size of supersymmetry-breaking effects within higher-dimensional settings where the observable sector resides deep within a strongly warped region, with supersymmetry breaking not necessarily localized in that region. Our particular interest is in how the supersymmetry-breaking scale seen by the observable sector depends on this warping. We obtain this dependence in two ways: by computing within the microscopic (string) theory supersymmetry-breaking masses in supermultiplets; and by investigating how warping gets encoded into masses within the low-energy 4D effective theory. We find that the lightest gravitino mode can have mass much less than the straightforward estimate from the mass shift of the unwarped zero mode. This lightest Kaluza-Klein excitation plays the role of the supersymmetric partner of the graviton and has a warped mass m_{3/2} proportional to e^A, with e^A the warp factor, and controls the size of the soft SUSY breaking terms. We formulate the conditions required for the existence of a description in terms of a 4D SUGRA formulation, or in terms of 4D SUGRA together with soft-breaking terms, and describe in particular situations where neither exist for some non-supersymmetric compactifications. We suggest that some effects of warping are captured by a linear $A$ dependence in the Kahler potential. We outline some implications of our results for the KKLT scenario of moduli stabilization with broken SUSY.

hep-th↗

arXiv2007

On Gauge Mediated SUSY Breaking and Moduli Stabilization

A generic lesson of string theory is that the coupling constants of an effective low energy theory are determined by the vacuum values of a set of fields - the so-called moduli - some of which are stabilized at relatively low masses by non-perturbative effects. We argue that the physics of these moduli cannot be separated from the issues of dynamical and gauge mediated supersymmetry breaking. To illustrate this point we present a modified version of the type IIB KKLT model where the criteria for gauge mediated SUSY breaking may be realized.

hep-th↗

arXiv2007

String Phenomenology and the Cosmological Constant

It is argued that classical string solutions should not be fine tuned to have a positive cosmological constant (CC) at the observed size, since even the quantum corrections from standard model effects will completely negate any classical string theory solution with such a CC. In fact it is even possible that there is no need at all for any ad hoc uplifting term in the potential since these quantum effects may well take care of this. Correspondingly any calculation of the parameters of the MSSM has to be rethought to take into account the evolution of the CC. This considerably complicates the issue since the initial conditions for RG evolution of these parameters are determined by the final condition on the CC! The Anthropic Principle is of no help in addressing these issues.

hep-th↗

arXiv2006

Transitions Between Flux Vacua

A dynamical description of the transitions between different backgrounds requires the existence of a background independent action which propagates the correct number of degrees of freedom and couples bulk supergravity to certain higher dimensional branes. We present classical equations for configurations that separate the world into regions with different flux parameters etc. and discuss the difficulties of trying to construct an action that describes the transitions between them within the framework of supergravity.

hep-th↗

arXiv2006

The Scales of Brane Nucleation Processes

The scales associated with Brown-Teitelboim-Bousso-Polchinski processes of brane nucleation, which result in changes of the flux parameters and the number of D-branes, are discussed in the context of type IIB models with all moduli stabilized. It is argued that such processes are unlikely to be described by effective field theory.

hep-th↗

arXiv2006

Potentials for Light Moduli in Supergravity and String Theory

This is an account of lectures that were given at TASI 2005, the Shanghai Summer School in M-theory 2005 and the Perimeter Institute. I review 1) the derivation of the potential for chiral scalar fields in ${\cal N}$=1 supergravity 2) the relation between F and D terms for chiral scalars, Weyl anomalies and the generation of non-perturbative terms in the superpotential and 3) the derivation of effective potentials for light moduli in type IIB string theory.

hep-th↗

arXiv2005

The Landscape of String Theory and The Wave Function of the Universe

We explore the possibility that quantum cosmology considerations could provide a selection principle in the landscape of string vacua. We propose that the universe emerged from the string era in a thermally excited state and determine, within a mini-superspace model, the probability of tunneling to different points on the landscape. We find that the potential energy of the tunneling end point from which the universe emerges and begins its classical evolution is determined by the primordial temperature. By taking into account some generic properties of the moduli potential we then argue that the tunneling to the tail of the moduli potentials is disfavored, that the most likely emergence point is near an extremum, and that this extremum is not likely to be in the outer region of moduli space where the compact volume is very large and the string coupling very weak. As a concrete example we discuss the application of our arguments to the KKLT model of moduli stabilization.

hep-th↗

arXiv2005

Effective Potentials for Light Moduli

We examine recent work on compactifications of string theory with fluxes, where effective potentials for light moduli have been derived after integrating out moduli that are assumed to be heavy at the classical level, and then adding non-perturbative (NP) corrections to the superpotential. We find that this two stage procedure is not valid and that the correct potential has additional terms. Althought this does not affect the conclusion of Kachru et al (KKLT) that the Kaehler moduli may be stabilized by NP effects, it can affect the detailed physics. In particular it is possible to get metastable dS minima without adding uplifting terms.

hep-th↗

arXiv2005

On Integrating Out Heavy Fields in SUSY Theories

We examine the procedure for integrating out heavy fields in supersymmetric (both global and local) theories. We find that the usual conditions need to be modified in general and we discuss the restrictions under which they are valid. These issues are relevant for recent work in string compactification with fluxes.

hep-th↗

arXiv2004

Brane Worlds in 5D and Warped Compactifications in IIB

In computing potentials for moduli in for instance type IIB string theory in the presence of fluxes and branes a factorisable ansatz for the ten dimensional metric is usually made. We investigate the validity of this ansatz by examining the cosmology of a brane world in a five dimensional bulk and find that it contradicts the results obtained by using a factorizable ansatz. We explicitly identify the problem with the latter in the IIB case. These arguments support our previous work on this question.

hep-th↗

arXiv2004

Cosmological stabilization of moduli with steep potentials

A scenario which overcomes the well-known cosmological overshoot problem associated with stabilizing moduli with steep potentials in string theory is proposed. Our proposal relies on the fact that moduli potentials are very steep and that generically their kinetic energy quickly becomes dominant. However, moduli kinetic energy red-shifts faster than other sources when the universe expands. So, if any additional sources are present, even in very small amounts, they will inevitably become dominant. We show that in this case cosmic friction allows the dissipation of the large amount of moduli kinetic energy that is required for the field to be able to find an extremely shallow minimum. We present the idea using analytic methods and verify with some numerical examples.

hep-th↗