Susha L. Parameswaran

Lifshitz solutions in supergravity and string theory

We derive Lifshitz configurations in string theory for general dynamical exponents z ≥ 1. We begin by obtaining simple Li × Ω solutions to supergravities in diverse dimensions, with Ω a compact constant curvature manifold. Then we uplift the solutions to ten dimensions, providing configurations that correspond to warped compactifications in Type II string theory.

Stability and negative tensions in 6D brane worlds

We investigate the dynamical stability of warped, axially symmetric compactifications in anomaly free 6D gauged supergravity. The solutions have conical defects, which we source by 3-branes placed on orbifold fixed points, and a smooth limit to the classic sphere-monopole compactification. Like for the sphere, the extra fields that are generically required by anomaly freedom are especially relevant for stability. With positive tension branes only, there is a strict stability criterion (identical to the sphere case) on the charges present under the monopole background. Thus brane world models with positive tensions can be embedded into anomaly free theories in only a few ways. Meanwhile, surprisingly, in the presence of a negative tension brane the stability criteria can be relaxed. We also describe in detail the geometries induced by negative tension codimension two branes.

On Moduli Stabilisation and de Sitter Vacua in MSSM Heterotic Orbifolds

We study the problem of moduli stabilisation in explicit heterotic orbifold compactifications, whose spectra contain the MSSM plus some vector-like exotics that can be decoupled. Considering all the bulk moduli, we obtain the 4D low energy effective action for the compactification, which has contributions from various, computable, perturbative and non-perturbative effects. Hidden sector gaugino condensation and string worldsheet instantons result in a combination of racetrack, KKLT-like and cusp-form contributions to the superpotential, which lift all the bulk moduli directions. We point out the properties observed in our concrete models, which tend to be missed when only “generic” features of a model are assumed. We search for interesting vacua and find several de Sitter solutions, but so far, they all turn out to be unstable.

Revisiting coupling selection rules in heterotic orbifold models

A bstractWe study L-point couplings between twisted sector fields in heterotic orbifold compactifications, using conformal field theory. Selection rules provide an easy way to identify which couplings are non-vanishing. Those used in the current literature are gauge invariance, R-charge conservation and the space group selection rule, but they are not the whole story. We revive and refine a fourth selection rule, due to symmetries in the underlying torus lattice, and introduce a fifth one, due to the existence or not of classical worldsheet instanton solutions to mediate the couplings. We consider briefly the phenomenological consequences of the additional rules, in particular for recent orbifold constructions whose field content correspond to that of the MSSM. The structure of the exotic mass matrices is unaltered and many dimension-5 proton-decay operators vanish.

R-charge conservation and more in factorizable and non-factorizable orbifolds

A bstractWe consider the string theory origin of R-charge conservation laws in heterotic orbifold compactifications, deriving the corresponding string coupling selection rule for factorizable and non-factorizable orbifolds, with prime ordered and non-prime ordered point groups. R-charge conservation arises due to symmetries among the worldsheet instantons that can mediate the couplings. Among our results is a previously missed non-trivial contribution to the conserved R-charges from the γ-phases in non-prime orbifolds, which weakens the R-charge selection rule. Symmetries among the worldsheet instantons can also lead to additional selection rules for some couplings. We make a similar analysis for Rule 4 or the “torus lattice selection rule”. Moreover, we identify a new string selection rule, that we call Rule 6 or the “coset vector selection rule”.

Accidental SUSY: Enhanced Bulk Supersymmetry from Brane Back-reaction

A bstractWe compute how bulk loops renormalize both bulk and brane effective interactions for codimension-two branes in 6D gauged chiral supergravity, as functions of the brane tension and brane-localized flux. We do so by explicitly integrating out hyper- and gauge-multiplets in 6D gauged chiral supergravity compactified to 4D on a flux-stabilized 2D rugby-ball geometry, specializing the results of a companion paper, arXiv:1210.3753, to the supersymmetric case. While the brane back-reaction generically breaks supersymmetry, we show that the bulk supersymmetry can be preserved if the amount of brane- localized flux is related in a specific BPS-like way to the brane tension, and verify that the loop corrections to the brane curvature vanish in this special case. In these systems it is the brane-bulk couplings that fix the size of the extra dimensions, and we show that in some circumstances the bulk geometry dynamically adjusts to ensure the supersymmetric BPS-like condition is automatically satisfied. We investigate the robustness of this residual supersymmetry to loops of non-supersymmetric matter on the branes, and show that supersymmetry-breaking effects can enter only through effective brane-bulk interactions involving at least two derivatives. We comment on the relevance of this calculation to proposed applications of codimension-two 6D models to solutions of the hierarchy and cosmological constant problems.

Lifshitz black holes in IIA supergravity

A bstractWe compute string theoretic black hole solutions having Lifshitz asymptotics with a general dynamical exponent z > 1. We start by constructing solutions in a flux compactification of six dimensional supergravity, then uplift them to massive type IIA supergravity. Alongside the Lifshitz black holes we study the simpler anti-de Sitter solutions, of which there are a 1-parameter family in this supergravity, and compare and contrast their properties. The black holes are characterized by a two-form and scalar charge, and we numerically explore their configuration space and thermodynamical aspects.

Discrete R-symmetries and anomaly universality in heterotic orbifolds

A bstractWe study discrete R-symmetries, which appear in the 4D low energy effective field theory derived from heterotic orbifold models. We derive the R-symmetries directly from the geometrical symmetries of the orbifolds. In particular, we obtain the corresponding R-charges by requiring that the couplings be invariant under these symmetries. This allows for a more general treatment than the explicit computations of correlation functions made previously by the authors, including models with discrete Wilson lines, and orbifold symmetries beyond plane-by-plane rotational invariance. The R-charges obtained in this manner differ from those derived in earlier explicit computations. We study the anomalies associated with these R-symmetries, and comment on the results.

Prospects for Primordial Gravitational Waves in String Inflation

Assuming that the early universe had (i) a description using perturbative string theory and its field theory limit (ii) an epoch of slow-roll inflation within a four-dimensional effective field theory and a hierarchy of scales

Wilson lines and Chern-Simons flux in explicit heterotic Calabi-Yau compactifications

M_{inf} < m_{kk} < m_s \lesssim M_{pl}