2,812 research outputs found

### Dirichlet Boundary State in Linear Dilaton Background

Dirichlet-branes have emerged as important objects in studying
nonperturbative string theory. It is important to generalize these objects to
more general backgrounds other than the usual flat background. The simplest
case is the linear dilaton condensate. The usual Dirichlet boundary condition
violates conformal invariance in such a background. We show that by switching
on a certain boundary interaction, conformal invariance is restored. An
immediate application of this result is to two dimensional string theory.Comment: 6 pages, harvmac, some remarks are modified and one reference is
added, formulas remain the sam

### Exact C=1 Boundary Conformal Field Theories

We present a solution of the problem of a free massless scalar field on the
half line interacting through a periodic potential on the boundary. For a
critical value of the period, this system is a conformal field theory with a
non-trivial and explicitly calculable S-matrix for scattering from the
boundary. Unlike all other exactly solvable conformal field theories, it is
non-rational ({\it i.e.} has infinitely many primary fields). It describes the
critical behavior of a number of condensed matter systems, including
dissipative quantum mechanics and of barriers in ``quantum wires''.Comment: harvmac, 10 pages, PUPT-1432/IASSNS-HEP-93/7

### CT-duality as a local property of the world-sheet

In the present article, we study the local features of the world-sheet in the
case when probe bosonic string moves in antisymmetric background field. We
generalize the geometry of surfaces embedded in space-time to the case when the
torsion is present. We define the mean extrinsic curvature for spaces with
Minkowski signature and introduce the concept of mean torsion. Its orthogonal
projection defines the dual mean extrinsic curvature. In this language, the
field equation is just the equality of mean extrinsic curvature and extrinsic
mean torsion, which we call CT-duality. To the world-sheet described by this
relation we will refer as CT-dual surface.Comment: Latex, 15 pages, 2 Figure

### A class of non-singular gravi-dilaton backgrounds

We present a class of static, spherically symmetric, non-singular solutions
of the tree-level string effective action, truncated to first order in
$\alpha'$. In the string frame the solutions approach asymptotically (at $r\to
0$ and $r\to \infty$) two different anti-de Sitter configurations, thus
interpolating between two maximally symmetric states of different constant
curvature. The radial-dependent dilaton defines a string coupling which is
everywhere finite, with a peak value that can be chosen arbitrarily small so as
to neglect quantum-loop corrections. This example stresses the possible
importance of finite-size $\alpha'$ corrections, typical of string theory, in
avoiding space-time singularities.Comment: 9 pages, LATEX, four figure included using EPSFIG. Essay written for
the 1997 Awards of the Gravity Research Foundation, and selected for
"Honorable Mention". To appear in Mod. Phys. Lett.

### S-Wave Scattering of Charged Fermions by a Magnetic Black Hole

We argue that, classically, $s$-wave electrons incident on a magnetically
charged black hole are swallowed with probability one: the reflection
coefficient vanishes. However, quantum effects can lead to both electromagnetic
and gravitational backscattering. We show that, for the case of extremal,
magnetically charged, dilatonic black holes and a single flavor of low-energy
charged particles, this backscattering is described by a perturbatively
computable and unitary $S$-matrix, and that the Hawking radiation in these
modes is suppressed near extremality. The interesting and much more difficult
case of several flavors is also discussed.Comment: 9p

### Information Loss and Anomalous Scattering

The approach of 't Hooft to the puzzles of black hole evaporation can be
applied to a simpler system with analogous features. The system is $1+1$
dimensional electrodynamics in a linear dilaton background. Analogues of black
holes, Hawking radiation and evaporation exist in this system. In perturbation
theory there appears to be an information paradox but this gets resolved in the
full quantum theory and there exists an exact $S$-matrix, which is fully
unitary and information conserving. 't Hooft's method gives the leading terms
in a systematic approximation to the exact result.Comment: 18 pages, 3 figures (postscript files available soon on request),
(earlier version got corrupted by mail system

### Junctions of three quantum wires and the dissipative Hofstadter model

We study a junction of three quantum wires enclosing a magnetic flux. This is
the simplest problem of a quantum junction between Tomonaga-Luttinger liquids
in which Fermi statistics enter in a non-trivial way. We present a direct
connection between this problem and the dissipative Hofstadter problem, or
quantum Brownian motion in two dimensions in a periodic potential and an
external magnetic field, which in turn is connected to open string theory in a
background electromagnetic field. We find non-trivial fixed points
corresponding to a chiral conductance tensor leading to an asymmetric flow of
the current.Comment: 4 pages, 1 figur

### Born-Regulated Gravity in Four Dimensions

Previous work involving Born-regulated gravity theories in two dimensions is
extended to four dimensions. The action we consider has two dimensionful
parameters. Black hole solutions are studied for typical values of these
parameters. For masses above a critical value determined in terms of these
parameters, the event horizon persists. For masses below this critical value,
the event horizon disappears, leaving a ``bare mass'', though of course no
singularity.Comment: LaTeX, 15 pages, 2 figure

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