4,160 research outputs found
Interacting branes, dual branes, and dyonic branes: a unifying lagrangian approach in D dimensions
This paper presents a general covariant lagrangian framework for the dynamics
of a system of closed n-branes and dual (D-n-4)-branes in D dimensions,
interacting with a dynamical (n+1)-form gauge potential. The framework proves
sufficiently general to include also a coupling of the branes to (the bosonic
sector of) a dynamical supergravity theory. We provide a manifestly
Lorentz-invariant and S-duality symmetric Lagrangian, involving the (n+1)-form
gauge potential and its dual (D-n-3)-form gauge potential in a symmetric way.
The corresponding action depends on generalized Dirac-strings. The requirement
of string-independence of the action leads to Dirac-Schwinger quantization
conditions for the charges of branes and dual branes, but produces also
additional constraints on the possible interactions. It turns out that a system
of interacting dyonic branes admits two quantum mechanically inequivalent
formulations, involving inequivalent quantization conditions. Asymmetric
formulations involving only a single vector potential are also given. For the
special cases of dyonic branes in even dimensions known results are easily
recovered. As a relevant application of the method we write an effective action
which implements the inflow anomaly cancellation mechanism for interacting
heterotic strings and five-branes in D=10. A consistent realization of this
mechanism requires, in fact, dynamical p-form potentials and a systematic
introduction of Dirac-strings.Comment: 36 pages, LaTeX, no figure
Spin-statistics transmutation in relativistic quantum field theories of dyons
We analyse spin and statistics of quantum dyon fields, i.e. fields carrying
both electric and magnetic charge, in 3+1 space-time dimensions. It has been
shown long time ago that, at the quantum mechanical level, a composite dyon
made out of a magnetic pole of charge g and a particle of electric charge e
possesses half-integral spin and fermionic statistics, if the constituents are
bosons and the Dirac quantization condition holds, with n odd. This
phenomenon is called spin-statistics transmutation. We show that the same
phenomenon occurs at the quantum field theory level for an elementary dyon.
This analysis requires the construction of gauge invariant charged dyon fields.
Dirac's proposal for such fields, relying on a Coulomb-like photon cloud, leads
to quantum correlators exhibiting an unphysical dependence on the Dirac-string.
Recently Froehlich and Marchetti proposed a recipe for charged dyon fields,
based on a sum over Mandelstam-strings, which overcomes this problem. Using
this recipe we derive explicit expressions for the quantum field theory
correlators and we provide a proof of the occurrence of spin-statistics
transmutation. The proof reduces to a computation of the self-linking numbers
of dyon worldlines and Mandelstam strings, projected on a fixed time
three-space. Dyon composites are also analysed. The transmutation discussed in
this paper bares some analogy with the appearance of anomalous spin and
statistics for particles or vortices in Chern-Simons theories in 2+1
dimensions. However, peculiar features appear in 3+1 dimensions e.g. in the
spin addition rule.Comment: 32 pages, LaTeX, no figure
A Stochastic Geometry Framework for LOS/NLOS Propagation in Dense Small Cell Networks
The need to carry out analytical studies of wireless systems often motivates
the usage of simplified models which, despite their tractability, can easily
lead to an overestimation of the achievable performance. In the case of dense
small cells networks, the standard single slope path-loss model has been shown
to provide interesting, but supposedly too optimistic, properties such as the
invariance of the outage/coverage probability and of the spectral efficiency to
the base station density. This paper seeks to explore the performance of dense
small cells networks when a more accurate path-loss model is taken into
account. We first propose a stochastic geometry based framework for small cell
networks where the signal propagation accounts for both the Line-of-Sight (LOS)
and Non-Line-Of-Sight (NLOS) components, such as the model provided by the 3GPP
for evaluation of pico-cells in Heterogeneous Networks. We then study the
performance of these networks and we show the dependency of some metrics such
as the outage/coverage probability, the spectral efficiency and Area Spectral
Efficiency (ASE) on the base station density and on the LOS likelihood of the
propagation environment. Specifically, we show that, with LOS/NLOS propagation,
dense networks still achieve large ASE gain but, at the same time, suffer from
high outage probability.Comment: Typo corrected in eq. (3); Typo corrected in legend of Fig. 1-2;
Typos corrected and definitions of some variables added in Section III.E;
Final result unchanged; Paper accepted to IEEE ICC 201
ILP-based approaches to partitioning recurrent workloads upon heterogeneous multiprocessors
The problem of partitioning systems of independent constrained-deadline sporadic tasks upon heterogeneous multiprocessor platforms is considered. Several different integer linear program (ILP) formulations of this problem, offering different tradeoffs between effectiveness (as quantified by speedup bound) and running time efficiency, are presented
Spontaneous rotating vortex rings in a parametrically driven polariton fluid
We present the theoretical prediction of spontaneous rotating vortex rings in
a parametrically driven quantum fluid of polaritons -- coherent superpositions
of coupled quantum well excitons and microcavity photons. These rings arise not
only in the absence of any rotating drive, but also in the absence of a
trapping potential, in a model known to map quantitatively to experiments. We
begin by proposing a novel parametric pumping scheme for polaritons, with
circular symmetry and radial currents, and characterize the resulting
nonequilibrium condensate. We show that the system is unstable to spontaneous
breaking of circular symmetry via a modulational instability, following which a
vortex ring with large net angular momentum emerges, rotating in one of two
topologically distinct states. Such rings are robust and carry distinctive
experimental signatures, and so they could find applications in the new
generation of polaritonic devices.Comment: 6 pages, 4 figure
Substrate rigidity deforms and polarizes active gels
We present a continuum model of the coupling between cells and substrate that
accounts for some of the observed substrate-stiffness dependence of cell
properties. The cell is modeled as an elastic active gel, adapting recently
developed continuum theories of active viscoelastic fluids. The coupling to the
substrate enters as a boundary condition that relates the cell's deformation
field to local stress gradients. In the presence of activity, the coupling to
the substrate yields spatially inhomogeneous contractile stresses and
deformations in the cell and can enhance polarization, breaking the cell's
front-rear symmetry.Comment: 6 pages, 4 figures, EPL forma
Unlocking the deployment of spectrum sharing with a policy enforcement framework
Spectrum sharing has been proposed as a promising way to increase the efficiency of spectrum usage by allowing incumbent operators (IOs) to share their allocated radio resources with licensee operators (LOs), under a set of agreed rules. The goal is to maximize a common utility, such as the sum rate throughput, while maintaining the level of service required by the IOs. However, this is only guaranteed under the assumption that all “players”respect the agreed sharing rules. In this paper, we propose a comprehensive framework for licensed shared access (LSA) networks that discourages LO misbehavior. Our framework is built around three core functions: misbehavior detection via the employment of a dedicated sensing network; a penalization function; and, a behavior-driven resource allocation. To the best of our knowledge, this is the first time that these components are combined for the monitoring/policing of the spectrum under the LSA framework. Moreover, a novel simulator for LSA is provided as an open access tool, serving the purpose of testing and validating our proposed techniques via a set of extensive system-level simulations in the context of mobile network operators, where IOs and several competing LOs are considered. The results demonstrate that violation of the agreed sharing rules can lead to a great loss of resources for the misbehaving LOs, the amount of which is controlled by the system. Finally, we promote that including a policy enforcement function as part of the spectrum sharing system can be beneficial for the LSA system, since it can guarantee compliance with the spectrum sharing rules and limit the short-term benefits arising from misbehavior
Critical Hysteresis in Random Field XY and Heisenberg Models
We study zero-temperature hysteresis in random-field XY and Heisenberg models
in the zero-frequency limit of a cyclic driving field. We consider three
distributions of the random field and present exact solutions in the mean field
limit. The results show a strong effect of the form of disorder on critical
hysteresis as well as the shape of hysteresis loops. A discrepancy with an
earlier study based on the renormalization group is resolved.Comment: 10 pages, 6 figures; this is published version (added some text and
references
Bridging the microscopic and the hydrodynamic in active filament solutions
Hydrodynamic equations for an isotropic solution of active polar filaments
are derived from a microscopic mean-field model of the forces exchanged between
motors and filaments. We find that a spatial dependence of the motor stepping
rate along the filament is essential to drive bundle formation. A number of
differences arise as compared to hydrodynamics derived (earlier) from a
mesoscopic model where relative filament velocities were obtained on the basis
of symmetry considerations. Due to the anisotropy of filament diffusion, motors
are capable of generating net filament motion relative to the solvent. The
effect of this new term on the stability of the homogeneous state is
investigated.Comment: 7 pages, 2 figures, submitted to Europhys. Let
- …