13,767 research outputs found
WMAP Constraints on the Generalized Chaplygin Gas Model
The generalized Chaplygin gas (GCG) model explains the recent accelerated
expansion of the Universe via an exotic background fluid whose equation of
state is given by p=-A/\rho^\alpha, where A is a positive constant and
0<\alpha\le 1. The model is an interesting alternative to scenarios involving
scalar field potentials, with the ensuing unnatural fine tuning conditions for
the underlying particle physics theories. We derive constraints on the
parameter space of the model from bounds on the location of the first few peaks
and troughs of the the Cosmic Microwave Background Radiation (CMBR) power
spectrum arising from recent WMAP and BOOMERanG data.Comment: 18 pages, 5 figures, version to appear in Phys. Lett.
Exactly stable non-BPS spinors in heterotic string theory on tori
Considering SO(32) heterotic string theory compactified on a torus of
dimension 4 and less, stability of non-supersymmetric states is studied. A
non-supersymmetric state with robust stability is constructed, and its exact
stability is proven in a large region of moduli space against all the possible
decay mechanisms allowed by charge conservation. Using various T-duality
transform matrices, we translate various selection rules about conserved
charges into simpler problems resembling partition and parity of integers. For
heterotic string on T^4, we give a complete list of BPS atoms with elementary
excitations, and we study BPS and non-BPS molecules with various binding
energies. Using string-string duality, the results are interpreted in terms of
Dirichlet-branes in type IIA string theory compactified on an orbifold limit of
a K3 surface.Comment: 47 pages, 14 figures, LaTe
Stable non-BPS states in string theory: a pedagogical review
We present a pedagogical review of the stable non-BPS states in string theory
which have recently attracted some attention in the literature. In particular,
following the analysis of A. Sen, we discuss in detail the case of the stable
non-BPS D-particle of Type I theory whose existence is predicted (and required)
by the heterotic/Type I duality. We show that this D-particle originates from
an unstable bound state formed by a D1/anti-D1 pair of Type IIB in which the
tachyon field acquires a solitonic kink configuration. The mechanism of tachyon
condensation is discussed first at a qualitative level and then with an exact
conformal field theory analysis.Comment: 58 pages, 1 figure; minor correction
Flows and Duality Symmetries in N=1 Supersymmetric Gauge Theories
We present more examples of dual N=1 SUSY gauge theories. This set of
theories is connected by flows to both Seiberg's and Kutasov's dual theories.
This provides a unifying picture of the various dual theories. We investigate
the dual theories, their flat directions and mass perturbations.Comment: 31 pages, uses harvmac.tex and tables.tex Minor changes - version
accepted to Nucl. Phys.
Evolution of Tachyon Kink with Electric Field
We investigate the decay of an inhomogeneous D1-brane wrapped on a with
an electric field. The model that we consider consists of an array of tachyon
kink and anti-kink with a constant electric flux. Beginning with an initially
static configuration, we numerically evolve the tachyon field with some
perturbations under a fixed boundary condition at diametrically opposite points
on the circle . When the electric flux is smaller than the critical value,
the tachyon kink becomes unstable; the tachyon field rolls down the potential,
and the lower dimensional D0- and -brane become thin, which
resembles the caustic formation known for this type of the system in the
literature. For the supercritical values of the electric flux, the tachyon kink
remains stable.Comment: 27 pages, 8 figures, some changes, one reference added, version to
appear in JHE
D-Brane Probe and Closed String Tachyons
We consider a D-brane probe in unstable string background associated with
flux branes. The twist in spacetime metric reponsible for the supersymmetry
breaking is shown to manifest itself in mixing of open Wilson lines with the
phases of some adjoint matter fields, resulting in a nonlocal and
nonsupersymmetric form of Yang-Mills theory as the probe dynamics. This
provides a setup where one can study fate of a large class of unstable closed
string theories that include as a limit type 0 theories and various orbifolds
of type II and type 0 theories. We discuss the limit of orbifold
in some detail and speculate on couplings with closed string tachyons.Comment: LaTeX, 17 pages, typos fixed, references update
Perturbation Theory for Singular Potentials in Quantum Mechanics
We study perturbation theory in certain quantum mechanics problems in which
the perturbing potential diverges at some points, even though the energy
eigenvalues are smooth functions of the coefficient of the potential. We
discuss some of the unusual techniques which are required to obtain
perturbative expansions of the energies in such cases. These include a
point-splitting prescription for expansions around the Dirichlet (fermionic)
limit of the -function potential, and performing a similarity
transformation to a non-Hermitian potential in the Calogero-Sutherland model.
As an application of the first technique, we study the ground state of the
-function Bose gas near the fermionic limit.Comment: LaTeX, 19 pages, no figure
Lectures on Non-BPS Dirichlet branes
A comprehensive introduction to the boundary state approach to Dirichlet
branes is given. Various examples of BPS and non-BPS Dirichlet branes are
discussed. In particular, the non-BPS states in the duality of Type IIA on K3
and the heterotic string on T4 are analysed in detail.Comment: 46 pages, 5 figures, LaTeX; lectures given at the TMR network school
on `Quantum aspects of gauge theories, supersymmetry and quantum gravity',
Torino, 26 January - 2 February 2000, and at the `Spring workshop on
Superstrings and related matters', Trieste, 27 March - 4 April 2000;
references adde
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