23 research outputs found
Non-perturbative Vacuum Destabilization and D-brane Dynamics
We analyze the process of string vacuum destabilization due to instanton
induced superpotential couplings which depend linearly on charged fields. These
non-perturbative instabilities result in potentials for the D-brane moduli and
lead to processes of D-brane recombination, motion and partial moduli
stabilization at the non-perturbative vacuum. By using techniques of D-brane
instanton calculus, we explicitly compute this scalar potential in toroidal
orbifold compactifications with magnetized D-branes by summing over the
possible discrete instanton configurations. We illustrate explicitly the
resulting dynamics in globally consistent models. These instabilities can have
phenomenological applications to breaking hidden sector gauge groups, open
string moduli stabilization and supersymmetry breaking. Our results suggest
that breaking supersymmetry by Polonyi-like models in string theory is more
difficult than expected.Comment: 61 pages, 6 figures, 5 tables; Minor corrections, version published
in JHE
Generalized Holographic Quantum Criticality at Finite Density
We show that the near-extremal solutions of Einstein-Maxwell-Dilaton
theories, studied in ArXiv:1005.4690, provide IR quantum critical geometries,
by embedding classes of them in higher-dimensional AdS and Lifshitz solutions.
This explains the scaling of their thermodynamic functions and their IR
transport coefficients, the nature of their spectra, the Gubser bound, and
regulates their singularities. We propose that these are the most general
quantum critical IR asymptotics at finite density of EMD theories.Comment: v4: Corrected the scaling equation for the conductivity in section
9.
Stringy instanton corrections to N=2 gauge couplings
We discuss a string model where a conformal four-dimensional N=2 gauge theory
receives corrections to its gauge kinetic functions from "stringy" instantons.
These contributions are explicitly evaluated by exploiting the localization
properties of the integral over the stringy instanton moduli space. The model
we consider corresponds to a setup with D7/D3-branes in type I' theory
compactified on T4/Z2 x T2, and possesses a perturbatively computable heterotic
dual. In the heteoric side the corrections to the quadratic gauge couplings are
provided by a 1-loop threshold computation and, under the duality map, match
precisely the first few stringy instanton effects in the type I' setup. This
agreement represents a very non-trivial test of our approach to the exotic
instanton calculus.Comment: 63 pages, 5 figures. V2: final version with minor corrections
published on JHEP05(2010)10
Non-perturbative transitions among intersecting-brane vacua
We investigate the transmutation of D-branes into Abelian magnetic
backgrounds on the world-volume of higher-dimensional branes, within the
framework of global models with compact internal dimensions. The phenomenon,
T-dual to brane recombination in the intersecting-brane picture, shares some
similarities to inverse small-instanton transitions in non-compact spaces,
though in this case the Abelian magnetic background is a consequence of the
compactness of the internal manifold, and is not ascribed to a zero-size
non-Abelian instanton growing to maximal size. We provide details of the
transition in various supersymmetric orientifolds and non-supersymmetric
tachyon-free vacua with Brane Supersymmetry Breaking, both from brane
recombination and from a field theory Higgs mechanism viewpoints.Comment: 52 pages, 2 figures. Typos correcte
Heterotic T-folds with a small number of neutral moduli
We discuss non-geometric supersymmetric heterotic string models in D=4, in
the framework of the free fermionic construction. We perform a systematic scan
of models with four a priori left-right asymmetric Z_2 projections and shifts.
We analyze some 2^{20} models, identifying 18 inequivalent classes and
addressing variants generated by discrete torsions. They do not contain
geometrical or trivial neutral moduli, apart from the dilaton. However, we show
the existence of flat directions in the form of exactly marginal deformations
and identify patterns of symmetry breaking where product gauge groups, realized
at level one, are broken to their diagonal at higher level. We also describe an
"inverse Gepner map" from Heterotic to Type II models that could be used, in
certain non geometric settings, to define "effective" topological invariants.Comment: 37 page
QCD and strongly coupled gauge theories : challenges and perspectives
We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe
Varying constants, Gravitation and Cosmology
Fundamental constants are a cornerstone of our physical laws. Any constant
varying in space and/or time would reflect the existence of an almost massless
field that couples to matter. This will induce a violation of the universality
of free fall. It is thus of utmost importance for our understanding of gravity
and of the domain of validity of general relativity to test for their
constancy. We thus detail the relations between the constants, the tests of the
local position invariance and of the universality of free fall. We then review
the main experimental and observational constraints that have been obtained
from atomic clocks, the Oklo phenomenon, Solar system observations, meteorites
dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic
microwave background and big bang nucleosynthesis. At each step we describe the
basics of each system, its dependence with respect to the constants, the known
systematic effects and the most recent constraints that have been obtained. We
then describe the main theoretical frameworks in which the low-energy constants
may actually be varying and we focus on the unification mechanisms and the
relations between the variation of different constants. To finish, we discuss
the more speculative possibility of understanding their numerical values and
the apparent fine-tuning that they confront us with.Comment: 145 pages, 10 figures, Review for Living Reviews in Relativit