748 research outputs found
Two Loop Renormalization of Massive (p,q) Supersymmetric Sigma Models
We calculate the beta-functions of the general massive (p,q) supersymmetric
sigma model to two loop order using (1,0) superfields. The conditions for
finiteness are discussed in relation to (p,q) supersymmetry. We also calculate
the effective potential using component fields to one loop order and consider
the possibility of perturbative breaking of supersymmetry. The effect of one
loop finite local counter terms and the ultra-violet behaviour of the off-shell
(p,q) models to all orders in perturbation theory are also addressed.Comment: 43 pages phyzzx with 5 figure
Axisymmetric versus Non-axisymmetric Vortices in Spinor Bose-Einstein Condensates
The structure and stability of various vortices in F=1 spinor Bose-Einstein
condensates are investigated by solving the extended Gross-Pitaevskii equation
under rotation. We perform an extensive search for stable vortices, considering
both axisymmetric and non-axisymmetric vortices and covering a wide range of
ferromagnetic and antiferromagnetic interactions. The topological defect called
Mermin-Ho (Anderson-Toulouse) vortex is shown to be stable for ferromagnetic
case. The phase diagram is established in a plane of external rotation Omega vs
total magnetization M by comparing the free energies of possible vortices. It
is shown that there are qualitative differences between axisymmetric and
non-axisymmetric vortices which are manifested in the Omega- and M-dependences.Comment: 9 pages, 9 figure
Winding effects on brane/anti-brane pairs
We study a brane/anti-brane configuration which is separated along a compact
direction by constructing a tachyon effective action which takes into account
transverse scalars. Such an action is relevant in the study of HQCD model of
Sakai and Sugimoto of chiral symmetry breaking, where the size of the compact
circle sets the confinement scale. Our approach is motivated by string theory
orbifold constructions and gives a route to model inhomogeneous tachyon decay.
We illustrate the techniques involved with a relatively simple example of a
harmonic oscillator on a circle. We will then repeat the analysis for the
Sakai-Sugimoto model and show that by integrating out the winding modes will
provide us with a renormalized action with a lower energy than that of
truncating to zero winding sector.Comment: 21 pages, 3 figures. v3: discussion and references added, published
versio
Scattered Results in 2D String Theory
The nonperturbative tachyon scattering amplitude in 2D type 0A
string theory is computed. The probability that particles are produced is a
monotonically decreasing function of whenever is large enough that
statistical methods apply. The results are compared with expectations from
black hole thermodynamics.Comment: 22 pages, 5 figures, harvmac. v2: minor comments added, typos
correcte
Why social networks are different from other types of networks
We argue that social networks differ from most other types of networks,
including technological and biological networks, in two important ways. First,
they have non-trivial clustering or network transitivity, and second, they show
positive correlations, also called assortative mixing, between the degrees of
adjacent vertices. Social networks are often divided into groups or
communities, and it has recently been suggested that this division could
account for the observed clustering. We demonstrate that group structure in
networks can also account for degree correlations. We show using a simple model
that we should expect assortative mixing in such networks whenever there is
variation in the sizes of the groups and that the predicted level of
assortative mixing compares well with that observed in real-world networks.Comment: 9 pages, 2 figure
Proposal for the Open String Tachyon Effective Action in the Linear Dilaton Background
In this paper we propose tachyon effective actions for unstable D-branes in
superstring and bosonic string theories in the presence of the linear dilaton
background.Comment: 18 page
Nonprofit governance: Improving performance in troubled economic times
Nonprofit management is currently pressured to perform effectively in a weak economy. Yet, nonprofit governance continues to suffer from unclear conceptions of the division of labor between board of directors and executive directors. This online survey of 114 executive directors aims to provide clarification and recommendations for social administration
A minimal set of invariants as a systematic approach to higher order gravity models: Physical and Cosmological Constraints
We compare higher order gravity models to observational constraints from
magnitude-redshift supernova data, distance to the last scattering surface of
the CMB, and Baryon Acoustic Oscillations. We follow a recently proposed
systematic approach to higher order gravity models based on minimal sets of
curvature invariants, and select models that pass some physical acceptability
conditions (free of ghost instabilities, real and positive propagation speeds,
and free of separatrices). Models that satisfy these physical and observational
constraints are found in this analysis and do provide fits to the data that are
very close to those of the LCDM concordance model. However, we find that the
limitation of the models considered here comes from the presence of
superluminal mode propagations for the constrained parameter space of the
models.Comment: 12 pages, 6 figure
Concept of temperature in multi-horizon spacetimes: Analysis of Schwarzschild-De Sitter metric
In case of spacetimes with single horizon, there exist several
well-established procedures for relating the surface gravity of the horizon to
a thermodynamic temperature. Such procedures, however, cannot be extended in a
straightforward manner when a spacetime has multiple horizons. In particular,
it is not clear whether there exists a notion of global temperature
characterizing the multi-horizon spacetimes. We examine the conditions under
which a global temperature can exist for a spacetime with two horizons using
the example of Schwarzschild-De Sitter (SDS) spacetime. We systematically
extend different procedures (like the expectation value of stress tensor,
response of particle detectors, periodicity in the Euclidean time etc.) for
identifying a temperature in the case of spacetimes with single horizon to the
SDS spacetime. This analysis is facilitated by using a global coordinate chart
which covers the entire SDS manifold. We find that all the procedures lead to a
consistent picture characterized by the following features: (a) In general, SDS
spacetime behaves like a non-equilibrium system characterized by two
temperatures. (b) It is not possible to associate a global temperature with SDS
spacetime except when the ratio of the two surface gravities is rational (c)
Even when the ratio of the two surface gravities is rational, the thermal
nature depends on the coordinate chart used. There exists a global coordinate
chart in which there is global equilibrium temperature while there exist other
charts in which SDS behaves as though it has two different temperatures. The
coordinate dependence of the thermal nature is reminiscent of the flat
spacetime in Minkowski and Rindler coordinate charts. The implications are
discussed.Comment: 12 page
Magnetogenesis and the dynamics of internal dimensions
The dynamical evolution of internal space-like dimensions breaks the
invariance of the Maxwell's equations under Weyl rescaling of the (conformally
flat) four-dimensional metric. Depending upon the number and upon the dynamics
of internal dimensions large scale magnetic fields can be created. The
requirements coming from magnetogenesis together with the other cosmological
constraints are examined under the assumption that the internal dimensions
either grow or shrink (in conformal time) prior to a radiation dominated epoch.
If the internal dimensions are growing the magnitude of the generated magnetic
fields can seed the galactic dynamo mechanism.Comment: 27 in RevTex style, four figure
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