74 research outputs found

### Active Topology Inference using Network Coding

Our goal is to infer the topology of a network when (i) we can send probes
between sources and receivers at the edge of the network and (ii) intermediate
nodes can perform simple network coding operations, i.e., additions. Our key
intuition is that network coding introduces topology-dependent correlation in
the observations at the receivers, which can be exploited to infer the
topology. For undirected tree topologies, we design hierarchical clustering
algorithms, building on our prior work. For directed acyclic graphs (DAGs),
first we decompose the topology into a number of two-source, two-receiver
(2-by-2) subnetwork components and then we merge these components to
reconstruct the topology. Our approach for DAGs builds on prior work on
tomography, and improves upon it by employing network coding to accurately
distinguish among all different 2-by-2 components. We evaluate our algorithms
through simulation of a number of realistic topologies and compare them to
active tomographic techniques without network coding. We also make connections
between our approach and alternatives, including passive inference, traceroute,
and packet marking

### The Dynamics of Small Instanton Phase Transitions

The small instanton transition of a five-brane colliding with one end of the
S1/Z2 interval in heterotic M-theory is discussed, with emphasis on the
transition moduli, their potential function and the associated non-perturbative
superpotential. Using numerical methods, the equations of motion of these
moduli coupled to an expanding Friedmann-Robertson-Walker spacetime are solved
including non-perturbative interactions. It is shown that the five-brane
collides with the end of the interval at a small instanton. However, the moduli
then continue to evolve to an isolated minimum of the potential, where they are
trapped by gravitational damping. The torsion free sheaf at the small instanton
is ``smoothed out'' into a vector bundle at the isolated minimum, thus
dynamically completing the small instanton phase transition. Radiative damping
at the origin of moduli space is discussed and shown to be insufficient to trap
the moduli at the small instanton point.Comment: LaTeX, 23 pages, 7 figures; minor corrections, references adde

### Nonlinear Magnetohydrodynamics from Gravity

We apply the recently established connection between nonlinear fluid dynamics
and AdS gravity to the case of the dyonic black brane in AdS_4. This yields the
equations of fluid dynamics for a 2+1 dimensional charged fluid in a background
magnetic field. We construct the gravity solution to second order in the
derivative expansion. From this we find the fluid dynamical stress tensor and
charge current to second and third order in derivatives respectively, along
with values for the associated transport coefficients.Comment: 20 pages. v3: Added section 2.3 on comparison to other approaches and
definition of viscosit

### Perspectives on Pfaffians of Heterotic World-sheet Instantons

To fix the bundle moduli of a heterotic compactification one has to
understand the Pfaffian one-loop prefactor of the classical instanton
contribution. For compactifications on elliptically fibered Calabi-Yau spaces X
this can be made explicit for spectral bundles and world-sheet instantons
supported on rational base curves b: one can express the Pfaffian in a closed
algebraic form as a polynomial, or it may be understood as a theta-function
expression. We elucidate the connection between these two points of view via
the respective perception of the relevant spectral curve, related to its
extrinsic geometry in the ambient space (the elliptic surface in X over b) or
to its intrinsic geometry as abstract Riemann surface. We identify, within a
conceptual description, general vanishing loci of the Pfaffian, and derive
bounds on the vanishing order, relevant to solutions of W=dW=0.Comment: 40 pages; minor changes, discussion section 1.1 adde

### Quantum N=3, d=3 Chern-Simons Matter Theories in Harmonic Superspace

We develop the background field method for studying classical and quantum
aspects of N=3, d=3 Chern-Simons and matter theories in N=3 harmonic
superspace. As one of the immediate consequences, we prove a nonrenormalization
theorem implying the ultra-violet finiteness of the corresponding supergraph
perturbation theory. We also derive the general hypermultiplet and gauge
superfield propagators in a Chern-Simons background. The leading supergraphs
with two and four external lines are evaluated. In contrast to the
non-supersymmetric theory, the leading quantum correction to the massive
charged hypermultiplet proves to be the super Yang-Mills action rather than the
Chern-Simons one. The hypermultiplet mass is induced by a constant triplet of
central charges in the N=3, d=3 Poincare superalgebra.Comment: 1+37 pages, 3 figures; v2: a reference added, to appear in JHE

### Induced mass in N=2 super Yang-Mills theories

The masses of the matter fields of N=2 Super-Yang-Mills theories can be
defined as parameters of deformed supersymmetry transformations. The
formulation used involves central charges for the matter fields. The explicit
form of the deformed supersymmetry transformations and of the invariant
Lagrangian in presence of the gauge supermultiplet are constructed. This works
generalizes a former one, due to the same authors, which presented the free
matter case.Comment: 15 pages, Late

### The Particle Spectrum of Heterotic Compactifications

Techniques are presented for computing the cohomology of stable, holomorphic
vector bundles over elliptically fibered Calabi-Yau threefolds. These
cohomology groups explicitly determine the spectrum of the low energy,
four-dimensional theory. Generic points in vector bundle moduli space manifest
an identical spectrum. However, it is shown that on subsets of moduli space of
co-dimension one or higher, the spectrum can abruptly jump to many different
values. Both analytic and numerical data illustrating this phenomenon are
presented. This result opens the possibility of tunneling or phase transitions
between different particle spectra in the same heterotic compactification. In
the course of this discussion, a classification of SU(5) GUT theories within a
specific context is presented.Comment: 77 pages, 3 figure

### On Low-Energy Effective Actions in N = 2, 4 Superconformal Theories in Four Dimensions

We study some aspects of low-energy effective actions in 4-d superconformal
gauge theories on the Coulomb branch. We describe superconformal invariants
constructed in terms of N=2 abelian vector multiplet which play the role of
building blocks for the N=2,4 supersymmetric low-energy effective actions. We
compute the one-loop effective actions in constant N=2 field strength
background in N=4 SYM theory and in N=2 SU(2) SYM theory with four
hypermultiplets in fundamental representation. Using the classification of
superconformal invariants we then find the manifestly N=2 superconformal form
of these effective actions. While our explicit computations are done in the
one-loop approximation, our conclusions about the structure of the effective
actions in N=2 superconformal theories are general. We comment on some
applications to supergravity - gauge theory duality in the description of
D-brane interactions.Comment: 18 pages, latex, comments/reference adde

### World-sheet Instanton Superpotentials in Heterotic String theory and their Moduli Dependence

To understand in detail the contribution of a world-sheet instanton to the
superpotential in a heterotic string compactification, one has to understand
the moduli dependence (bundle and complex structure moduli) of the one-loop
determinants from the fluctuations, which accompany the classical exponential
contribution (involving K\"ahler moduli) when evaluating the world-volume
partition function. Here we use techniques to describe geometrically these
Pfaffians for spectral bundles over rational base curves in elliptically
fibered Calabi-Yau threefolds, and provide a (partially exhaustive) list of
cases involving {\em factorising} (or vanishing) superpotential. This gives a
conceptual explanation and generalisation of the few previously known cases
which were obtained just experimentally by a numerical computation.Comment: 57 pages; minor changes, discussion section 1.3 adde

### The Spectra of Heterotic Standard Model Vacua

A formalism for determining the massless spectrum of a class of realistic
heterotic string vacua is presented. These vacua, which consist of SU(5)
holomorphic bundles on torus-fibered Calabi-Yau threefolds with fundamental
group Z_2, lead to low energy theories with standard model gauge group (SU(3)_C
x SU(2)_L x U(1)_Y)/Z_6 and three families of quarks and leptons. A methodology
for determining the sheaf cohomology of these bundles and the representation of
Z_2 on each cohomology group is given. Combining these results with the action
of a Z_2 Wilson line, we compute, tabulate and discuss the massless spectrum.Comment: 41+1pp, 2 fig

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