131 research outputs found
Flux Compactifications of M-Theory on Twisted Tori
We find the bosonic sector of the gauged supergravities that are obtained
from 11-dimensional supergravity by Scherk-Schwarz dimensional reduction with
flux to any dimension D. We show that, if certain obstructions are absent, the
Scherk-Schwarz ansatz for a finite set of D-dimensional fields can be extended
to a full compactification of M-theory, including an infinite tower of
Kaluza-Klein fields. The internal space is obtained from a group manifold
(which may be non-compact) by a discrete identification. We discuss the
symmetry algebra and the symmetry breaking patterns and illustrate these with
particular examples. We discuss the action of U-duality on these theories in
terms of symmetries of the D-dimensional supergravity, and argue that in
general it will take geometric flux compactifications to M-theory on
non-geometric backgrounds, such as U-folds with U-duality transition functions.Comment: Latex, 47 page
Non-geometric backgrounds, doubled geometry and generalised T-duality
String backgrounds with a local torus fibration such as T-folds are naturally
formulated in a doubled formalism in which the torus fibres are doubled to
include dual coordinates conjugate to winding number. Here we formulate and
explore a generalisation of this construction in which all coordinates are
doubled, so that the doubled space is a twisted torus, i.e. a compact space
constructed from identifying a group manifold under a discrete subgroup. This
incorporates reductions with duality twists, T-folds and a class of flux
compactifications, together with the non-geometric backgrounds expected to
arise from these through T-duality. It also incorporates backgrounds that are
not even locally geometric, and suggests a generalisation of T-duality to a
more general context. We discuss the effective field theory arising from such
an internal sector, give a world-sheet sigma model formulation of string theory
on such backgrounds and illustrate our discussion with detailed examples.Comment: 81 page
Superstring partition functions in the doubled formalism
Computation of superstring partition function for the non-linear sigma model
on the product of a two-torus and its dual within the scope of the doubled
formalism is presented. We verify that it reproduces the partition functions of
the toroidally compactified type--IIA and type--IIB theories for appropriate
choices of the GSO projection.Comment: 15 page
Heterotic compactifications on SU(2)-structure backgrounds
In this paper we study the reduction of heterotic string theory on
SU(2)-structure backgrounds. We compute the bosonic low-energy gauged N=2
supergravity specified by the Killing vectors corresponding to the gauged
isometries. We check that the obtained Lagrangian is consistent with the one of
N=2 local supersymmetry. We also determine the Killing prepotentials.Comment: reference added, corrected typos and some factor
Area metric gravity and accelerating cosmology
Area metric manifolds emerge as effective classical backgrounds in quantum
string theory and quantum gauge theory, and present a true generalization of
metric geometry. Here, we consider area metric manifolds in their own right,
and develop in detail the foundations of area metric differential geometry.
Based on the construction of an area metric curvature scalar, which reduces in
the metric-induced case to the Ricci scalar, we re-interpret the
Einstein-Hilbert action as dynamics for an area metric spacetime. In contrast
to modifications of general relativity based on metric geometry, no continuous
deformation scale needs to be introduced; the extension to area geometry is
purely structural and thus rigid. We present an intriguing prediction of area
metric gravity: without dark energy or fine-tuning, the late universe exhibits
a small acceleration.Comment: 52 pages, 1 figure, companion paper to hep-th/061213
Type II compactifications on manifolds with SU(2) x SU(2) structure
We study compactifications of type II theories on SU(2) x SU(2) structure
manifolds to six, five and four spacetime dimensions. We use the framework of
generalized geometry to describe the NS-NS sector of such compactifications and
derive the structure of their moduli spaces. We show that in contrast to SU(3)
x SU(3) structure compactifications, there is no dynamical SU(2) x SU(2)
structure interpolating between an SU(2) structure and an identity structure.
Furthermore, we formulate type II compactifications on SU(2) x SU(2) structures
in the context of exceptional generalized geometry which makes the U-duality
group manifest and naturally incorporates the scalar degrees of freedom arising
in the Ramond-Ramond sector. Via this formalism we derive the structure of the
moduli spaces as it is expected from N=4 supergravity.Comment: 69 pages, v2 published versio
Conformal Field Theories in Six-Dimensional Twistor Space
This article gives a study of the higher-dimensional Penrose transform
between conformally invariant massless fields on space-time and cohomology
classes on twistor space, where twistor space is defined to be the space of
projective pure spinors of the conformal group. We focus on the 6-dimensional
case in which twistor space is the six-quadric Q in CP^7 with a view to
applications to the self-dual (0,2)-theory. We show how spinor-helicity
momentum eigenstates have canonically defined distributional representatives on
twistor space (a story that we extend to arbitrary dimension). These give an
elementary proof of the surjectivity of the Penrose transform. We give a direct
construction of the twistor transform between the two different representations
of massless fields on twistor space (H^2 and H^3) in which the H^3s arise as
obstructions to extending the H^2s off Q into CP^7.
We also develop the theory of Sparling's `\Xi-transform', the analogous
totally real split signature story based now on real integral geometry where
cohomology no longer plays a role. We extend Sparling's \Xi-transform to all
helicities and homogeneities on twistor space and show that it maps kernels and
cokernels of conformally invariant powers of the ultrahyperbolic wave operator
on twistor space to conformally invariant massless fields on space-time. This
is proved by developing the 6-dimensional analogue of the half-Fourier
transform between functions on twistor space and momentum space. We give a
treatment of the elementary conformally invariant \Phi^3 amplitude on twistor
space and finish with a discussion of conformal field theories in twistor
space.Comment: 37 pages, 3 figures, in v2 a number of inaccuracies are removed and
the discussion is improved; in v3, one reference added;v4 extra
acknowledgemen
T-duality, Generalized Geometry and Non-Geometric Backgrounds
We discuss the action of O(d,d), and in particular T-duality, in the context
of generalized geometry, focusing on the description of so-called non-geometric
backgrounds. We derive local expressions for the pure spinors descibing the
generalized geometry dual to an SU(3) structure background, and show that the
equations for N=1 vacua are invariant under T-duality. We also propose a local
generalized geometrical definition of the charges f, H, Q and R appearing in
effective four-dimensional theories, using the Courant bracket. We then address
certain global aspects, in particular whether the local non-geometric charges
can be gauged away in, for instance, backgrounds admitting a torus action, as
well as the structure of generalized parallelizable backgrounds.Comment: 33 page
On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection
A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)
Search for Tensor, Vector, and Scalar Polarizations in the Stochastic Gravitational-Wave Background
The detection of gravitational waves with Advanced LIGO and Advanced Virgo has enabled novel tests of general relativity, including direct study of the polarization of gravitational waves. While general relativity allows for only two tensor gravitational-wave polarizations, general metric theories can additionally predict two vector and two scalar polarizations. The polarization of gravitational waves is encoded in the spectral shape of the stochastic gravitational-wave background, formed by the superposition of cosmological and individually unresolved astrophysical sources. Using data recorded by Advanced LIGO during its first observing run, we search for a stochastic background of generically polarized gravitational waves. We find no evidence for a background of any polarization, and place the first direct bounds on the contributions of vector and scalar polarizations to the stochastic background. Under log-uniform priors for the energy in each polarization, we limit the energy densities of tensor, vector, and scalar modes at 95% credibility to Ω0T<5.58×10-8, Ω0V<6.35×10-8, and Ω0S<1.08×10-7 at a reference frequency f0=25 Hz. © 2018 American Physical Society
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