171 research outputs found
Open-closed string correspondence: D-brane decay in curved space
This paper analyzes the effect of curved closed string backgrounds on the
stability of D-branes within boundary string field theory. We identify the
non-local open string background that implements shifts in the closed string
background and analyze the tachyonic sector off-shell. The renormalization
group flow reveals some characteristic properties, which are expected for a
curved background, like the absence of a stable space-filling brane. In
3-dimensions we describe tachyon condensation processes to lower-dimensional
branes, including a curved 2-dimensional brane. We argue that this 2-brane is
perturbatively stable. This is in agreement with the known maximally symmetric
WZW-branes and provides further support to the bulk-boundary factorization
approach to open-closed string correspondence.Comment: 23 pages, harvma
Spectral flow and boundary string field theory for angled D-branes
D-branes intersecting at an arbitrary fixed angle generically constitute a
configuration unstable toward recombination. The reconnection of the branes
nucleates at the intersection point and involves a generalization of the
process of brane decay of interest to non-perturbative string dynamics as well
as cosmology. After reviewing the string spectrum of systems of angled branes,
we show that worldsheet twist superfields may be used in the context of
Boundary Superstring Field Theory to describe the dynamics. Changing the angle
between the branes is seen from the worldsheet as spectral flow with boundary
insertions flowing from bosonic to fermionic operators. We calculate the
complete tachyon potential and the low energy effective action as a function of
angle and find an expression that interpolates between the brane-antibrane and
the Dirac-Born-Infeld actions. The potential captures the mechanism of D-brane
recombination and provides for interesting new physics for tachyon decay.Comment: 32 pages, 9 figures; v2 references added; v3 discussion clarifie
The 6D SuperSwirl
We present a novel supersymmetric solution to a nonlinear sigma model coupled
to supergravity. The solution represents a static, supersymmetric,
codimension-two object, which is different to the familiar cosmic strings. In
particular, we consider 6D chiral gauged supergravity, whose spectrum contains
a number of hypermultiplets. The scalar components of the hypermultiplet are
charged under a gauge field, and supersymmetry implies that they experience a
simple paraboloid-like (or 2D infinite well) potential, which is minimised when
they vanish. Unlike conventional vortices, the energy density of our
configuration is not localized to a string-like core. The solutions have two
timelike singularities in the internal manifold, which provide the necessary
boundary conditions to ensure that the scalars do not lie at the minimum of
their potential. The 4D spacetime is flat, and the solution is a continuous
deformation of the so-called ``rugby ball'' solution, which has been studied in
the context of the cosmological constant problem. It represents an unexpected
class of supersymmetric solutions to the 6D theory, which have gravity, gauge
fluxes and hyperscalars all active in the background.Comment: 26 pages, 2 figures, JHEP3 class. Typos corrected, analysis expanded,
references adde
An Improved Brane Anti-Brane Action from Boundary Superstring Field Theory and Multi-Vortex Solutions
We present an improved effective action for the D-brane-anti-D-brane system
obtained from boundary superstring field theory. Although the action looks
highly non-trivial, it has simple explicit multi-vortex (i.e. codimension-2
multi-BPS D-brane) multi-anti-vortex solutions. The solutions have a curious
degeneracy corresponding to different ``magnetic'' fluxes at the core of each
vortex. We also generalize the brane anti-brane effective action that is
suitable for the study of the inflationary scenario and the production of
defects in the early universe. We show that when a brane and anti-brane are
distantly separated, although the system is classically stable it can decay via
quantum tunneling through the barrier.Comment: 24 pages, 1 figure, JHEP3.cls; v2: references added, tunneling rate
discussion expande
Towards a Naturally Small Cosmological Constant from Branes in 6D Supergravity
We investigate the possibility of self-tuning of the effective 4D
cosmological constant in 6D supergravity, to see whether it could naturally be
of order 1/r^4 when compactified on two dimensions having Kaluza-Klein masses
of order 1/r. In the models we examine supersymmetry is broken by the presence
of non-supersymmetric 3-branes (on one of which we live). If r were
sub-millimeter in size, such a cosmological constant could describe the
recently-discovered dark energy. A successful self-tuning mechanism would
therefore predict a connection between the observed size of the cosmological
constant, and potentially observable effects in sub-millimeter tests of gravity
and at the Large Hadron Collider. We do find self tuning inasmuch as 3-branes
can quite generically remain classically flat regardless of the size of their
tensions, due to an automatic cancellation with the curvature and dilaton of
the transverse two dimensions. We argue that in some circumstances
six-dimensional supersymmetry might help suppress quantum corrections to this
cancellation down to the bulk supersymmetry-breaking scale, which is of order
1/r. We finally examine an explicit realization of the mechanism, in which
3-branes are inserted into an anomaly-free version of Salam-Sezgin gauged 6D
supergravity compactified on a 2-sphere with nonzero magnetic flux. This
realization is only partially successful due to a topological constraint which
relates bulk couplings to the brane tension, although we give arguments why
these relations may be stable against quantum corrections.Comment: 31 pages, 1 figure. Uses JHEP class. Expanded discussions in
Introduction, Section 3.2 (Quantum Corrections) and Section 4.2 (Topological
Constraint). Note added on subsequent related articles. Results unchange
Digital Drugs: an anatomy of new medicines
Medicines are digitalized as aspects of their regulation and use are embodied in or draw from interlinked computerized systems and databases. This paper considers how this development changes the delivery of health care, the pharma industry, and regulatory and professional structures, as it reconfigures the material character of drugs themselves. It draws on the concept of assemblage in presenting a theory-based analysis that explores digital drugsâ ontological status including how they embody benefit and value. The paper addresses three interconnected domains â that of use of drugs (practice), of research (epistemology) and of regulation (structures)
Model for Particle Masses, Flavor Mixing, and CP Violation Based on Spontaneously Broken Discrete Chiral Symmetry as the Origin of Families
We construct extensions of the standard model based on the hypothesis that
the Higgs bosons also exhibit a family structure, and that the flavor weak
eigenstates in the three families are distinguished by a discrete chiral
symmetry that is spontaneously broken by the Higgs sector. We study in detail
at the tree level models with three Higgs doublets, and with six Higgs doublets
comprising two weakly coupled sets of three. In a leading approximation of
cyclic permutation symmetry the three Higgs model gives a ``democratic''
mass matrix of rank one, while the six Higgs model gives either a rank one mass
matrix, or in the case when it spontaneously violates CP, a rank two mass
matrix corresponding to nonzero second family masses. In both models, the CKM
matrix is exactly unity in leading approximation. Allowing small explicit
violations of cyclic permutation symmetry generates small first family masses
in the six Higgs model, and first and second family masses in the three Higgs
model, and gives a non-trivial CKM matrix in which the mixings of the first and
second family quarks are naturally larger than mixings involving the third
family. Complete numerical fits are given for both models, flavor changing
neutral current constraints are discussed in detail, and the issues of
unification of couplings and neutrino masses are addressed. On a technical
level, our analysis uses the theory of circulant and retrocirculant matrices,
the relevant parts of which are reviewed.Comment: Revtex, 59 pages including four tables at en
Spectral and transport properties of doped Mott-Hubbard systems with incommensurate magnetic order
We present spectral and optical properties of the Hubbard model on a
two-dimensional square lattice using a generalization of dynamical mean-field
theory to magnetic states in finite dimension. The self-energy includes the
effect of spin fluctuations and screening of the Coulomb interaction due to
particle-particle scattering. At half-filling the quasiparticles reduce the
width of the Mott-Hubbard `gap' and have dispersions and spectral weights that
agree remarkably well with quantum Monte Carlo and exact diagonalization
calculations. Away from half-filling we consider incommensurate magnetic order
with a varying local spin direction, and derive the photoemission and optical
spectra. The incommensurate magnetic order leads to a pseudogap which opens at
the Fermi energy and coexists with a large Mott-Hubbard gap. The quasiparticle
states survive in the doped systems, but their dispersion is modified with the
doping and a rigid band picture does not apply. Spectral weight in the optical
conductivity is transferred to lower energies and the Drude weight increases
linearly with increasing doping. We show that incommensurate magnetic order
leads also to mid-gap states in the optical spectra and to decreased scattering
rates in the transport processes, in qualitative agreement with the
experimental observations in doped systems. The gradual disappearence of the
spiral magnetic order and the vanishing pseudogap with increasing temperature
is found to be responsible for the linear resistivity. We discuss the possible
reasons why these results may only partially explain the features observed in
the optical spectra of high temperature superconductors.Comment: 22 pages, 18 figure
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