20 research outputs found

### Unification with Low String Scale

I argue that in open-string theory with hierarchically small (or large) extra
dimensions, gauge groups can unify naturally with logarithmically-running
coupling constants at the high Kaluza-Klein (or string-winding) scale.
This opens up the possibility of rescuing the standard logarithmic
unification at $M_U\sim 10^{15-18}$ GeV even if the fundamental-string scale is
much lower, at intermediate or possibly even electroweak scales. I also
explain, however, why a low type-I string scale may not suffice to obliterate
the ultraviolet problems usually associated with the gauge hierarchy.Comment: 6 pages, uses harvmac. Two minor changes. Final version to appear in
JHE

### (8,0) Quantum mechanics and symmetry enhancement in type I' superstrings

The low-energy supersymmetric quantum mechanics describing D-particles in the
background of D8-branes and orientifold planes is analyzed in detail, including
a careful discussion of Gauss' law and normal ordering of operators. This
elucidates the mechanism that binds D-particles to an orientifold plane, in
accordance with the predictions of heterotic/type I duality. The ocurrence of
enhanced symmetries associated with massless bound states of a D-particle with
one orientifold plane is illustrated by the enhancement of $SO(14) \times U(1)$
to $E_8$ and $SO(12)\times U(1)$ to $E_7$ at strong type I' coupling.
Enhancement to higher-rank groups involves both orientifold planes. For
example, the enhanced $E_8 \times E_8 \times SU(2)$ symmetry at the self-dual
radius of the heterotic string is seen as the result of two D8-branes
coinciding midway between the orientifold planes, while the enhanced $SU(18)$
symmetry results from the coincidence of all sixteen D8-branes and $SO(34)$
when they also coincide with an orientifold plane. As a separate by-product,
the s-rule of brane-engineered gauge theories is derived by relating it through
a chain of dualities to the Pauli exclusion principle.Comment: 30 pages LaTeX, Five figures. Two references added as well as some
Comments in section4. v4: Missing backslashes added to four reference
citations

### Wetting and Minimal Surfaces

We study minimal surfaces which arise in wetting and capillarity phenomena.
Using conformal coordinates, we reduce the problem to a set of coupled boundary
equations for the contact line of the fluid surface, and then derive simple
diagrammatic rules to calculate the non-linear corrections to the Joanny-de
Gennes energy. We argue that perturbation theory is quasi-local, i.e. that all
geometric length scales of the fluid container decouple from the
short-wavelength deformations of the contact line. This is illustrated by a
calculation of the linearized interaction between contact lines on two opposite
parallel walls. We present a simple algorithm to compute the minimal surface
and its energy based on these ideas. We also point out the intriguing
singularities that arise in the Legendre transformation from the pure Dirichlet
to the mixed Dirichlet-Neumann problem.Comment: 22 page

### Loop Operators and the Kondo Problem

We analyse the renormalisation group flow for D-branes in WZW models from the
point of view of the boundary states. To this end we consider loop operators
that perturb the boundary states away from their ultraviolet fixed points, and
show how to regularise and renormalise them consistently with the global
symmetries of the problem. We pay particular attention to the chiral operators
that only depend on left-moving currents, and which are attractors of the
renormalisation group flow. We check (to lowest non-trivial order in the
coupling constant) that at their stable infrared fixed points these operators
measure quantum monodromies, in agreement with previous semiclassical studies.
Our results help clarify the general relationship between boundary transfer
matrices and defect lines, which parallels the relation between
(non-commutative) fields on (a stack of) D-branes and their push-forwards to
the target-space bulk.Comment: 22 pages, 2 figure

### Permeable conformal walls and holography

We study conformal field theories in two dimensions separated by domain
walls, which preserve at least one Virasoro algebra. We develop tools to study
such domain walls, extending and clarifying the concept of `folding' discussed
in the condensed-matter literature. We analyze the conditions for unbroken
supersymmetry, and discuss the holographic duals in AdS3 when they exist. One
of the interesting observables is the Casimir energy between a wall and an
anti-wall. When these separate free scalar field theories with different
target-space radii, the Casimir energy is given by the dilogarithm function of
the reflection probability. The walls with holographic duals in AdS3 separate
two sigma models, whose target spaces are moduli spaces of Yang-Mills
instantons on T4 or K3. In the supergravity limit, the Casimir energy is
computable as classical energy of a brane that connects the walls through AdS3.
We compare this result with expectations from the sigma-model point of view.Comment: Latex file, 34 pages, 8 figures, uses JHEP3.cls. Typos corrected and
references adde

### World-sheet duality for D-branes with travelling waves

We study D-branes with plane waves of arbitrary profiles as examples of
time-dependent backgrounds in string theory. We show how to reproduce the
quantum mechanical (one-to-one) open-string S-matrix starting from the
closed-string boundary state for the D-branes, thereby establishing the channel
duality of this calculation. The required Wick rotation to a Lorentzian
worldsheet singles out as 'prefered' time coordinate the open-string light-cone
time.Comment: 17 pages, Latex file, uses JHEP3.cls, two figures. Added references
and corrected two typo

### Spin-2 spectrum of defect theories

We study spin-2 excitations in the background of the recently-discovered
type-IIB solutions of D'Hoker et al. These are holographically-dual to defect
conformal field theories, and they are also of interest in the context of the
Karch-Randall proposal for a string-theory embedding of localized gravity. We
first generalize an argument by Csaki et al to show that for any solution with
four-dimensional anti-de Sitter, Poincare or de Sitter invariance the spin-2
excitations obey the massless scalar wave equation in ten dimensions. For the
interface solutions at hand this reduces to a Laplace-Beltrami equation on a
Riemann surface with disk topology, and in the simplest case of the
supersymmetric Janus solution it further reduces to an ordinary differential
equation known as Heun's equation. We solve this equation numerically, and
exhibit the spectrum as a function of the dilaton-jump parameter $\Delta\phi$.
In the limit of large $\Delta\phi$ a nearly-flat linear-dilaton dimension grows
large, and the Janus geometry becomes effectively five-dimensional. We also
discuss the difficulties of localizing four-dimensional gravity in the more
general backgrounds with NS5-brane or D5-brane charge, which will be analyzed
in detail in a companion paper.Comment: 41 pages, 6 figure