2,517 research outputs found
A Stochastic Immersed Boundary Method for Fluid-Structure Dynamics at Microscopic Length Scales
In this work it is shown how the immersed boundary method of (Peskin2002) for
modeling flexible structures immersed in a fluid can be extended to include
thermal fluctuations. A stochastic numerical method is proposed which deals
with stiffness in the system of equations by handling systematically the
statistical contributions of the fastest dynamics of the fluid and immersed
structures over long time steps. An important feature of the numerical method
is that time steps can be taken in which the degrees of freedom of the fluid
are completely underresolved, partially resolved, or fully resolved while
retaining a good level of accuracy. Error estimates in each of these regimes
are given for the method. A number of theoretical and numerical checks are
furthermore performed to assess its physical fidelity. For a conservative
force, the method is found to simulate particles with the correct Boltzmann
equilibrium statistics. It is shown in three dimensions that the diffusion of
immersed particles simulated with the method has the correct scaling in the
physical parameters. The method is also shown to reproduce a well-known
hydrodynamic effect of a Brownian particle in which the velocity
autocorrelation function exhibits an algebraic tau^(-3/2) decay for long times.
A few preliminary results are presented for more complex systems which
demonstrate some potential application areas of the method.Comment: 52 pages, 11 figures, published in journal of computational physic
Explicit Actions for Electromagnetism with Two Gauge Fields with Only one Electric and one Magnetic Physical Fields
We extend the work of Mello et al. based in Cabbibo and Ferrari concerning
the description of electromagnetism with two gauge fields from a variational
principle, i.e. an action. We provide a systematic independent derivation of
the allowed actions which have only one magnetic and one electric physical
fields and are invariant under the discrete symmetries and . We conclude
that neither the Lagrangian, nor the Hamiltonian, are invariant under the
electromagnetic duality rotations. This agrees with the weak-strong coupling
mixing characteristic of the duality due to the Dirac quantization condition
providing a natural way to differentiate dual theories related by the duality
rotations (the energy is not invariant). Also the standard electromagnetic
duality rotations considered in this work violate both and by inducing
Hopf terms (theta terms) for each sector and a mixed Maxwell term. The
canonical structure of the theory is briefly addressed and the 'magnetic' gauge
sector is interpreted as a ghost sector.Comment: v2: 12 pages; References added, discussion concerning degrees of
freedom corrected; v3: is now used the standard normalization of 1/4 in the
actions; the possibility of theta being a pseudo-scalar implied a title
changing; eq (23) added; signs corrected in equations (39,45-47); references
adde
Impact parameter dependence of heavy ion e+ e- pair production to all orders in Z alpha
The heavy ion probability for continuum e+ e- pair production has been
calculated to all orders in Z alpha as a function of impact parameter. The
formula resulting from an exact solution of the semiclassical Dirac equation in
the ultrarelativistic limit is evaluated numerically. In a calculation of gamma
= 100 colliding Au ions the probability of e+ e- pair production is reduced
from the perturbation theory result throughout the impact parameter range.Comment: 20 pages, latex, revtex, 6 eps figures. Revised Phys. Rev. C version
with minor additions, one figure added, and added reference
Spontaneous symmetry breaking in the linear sigma model at finite chemical potential: One-loop corrections
We investigate spontaneous symmetry breaking within the linear sigma model
with the SU(2)xU(1) internal symmetry at finite chemical potential, which was
suggested as a model for kaon condensation in the CFL phase of dense quark
matter. One-loop corrections to the scalar field effective potential as well as
its propagator are calculated. Particular attention is paid to the type-II
Goldstone boson that appears in the Bose--Einstein condensed phase.
Furthermore, we show that the type-I Goldstone boson -- the superfluid phonon
-- is allowed to decay due to the nonlinearity of its dispersion relation at
high momentum, and determine its decay width.Comment: 13 pages, REVTeX4, 37 eps figures; v2: substantial error in Sec. IV
corrected, references added, other minor corrections; version to appear in
Phys. Rev.
Effects of Velocity-Dependent Dark Matter Annihilation on the Energy Spectrum of the Extragalactic Gamma-ray Background
We calculate the effects of velocity-dependent dark matter annihilation cross
sections on the intensity of the extragalactic gamma-ray background. Our
formalism does not assume a locally thermal distribution of dark matter
particles in phase space, and is valid for arbitrary velocity-dependent
annihilation. As concrete examples, we calculate the effects of p-wave
annihilation (with the -weighted cross section of ) on the
mean intensity of extragalactic gamma rays produced in cosmological dark matter
halos. This velocity variation makes the shape of the energy spectrum harder,
but this change in the shape is too small to see unless b/a\agt 10^6. While
we find no such models in the parameter space of the Minimal Supersymmetric
Standard Model (MSSM), we show that it is possible to find b/a\agt 10^6 in
the extension MSSM. However, we find that the most dominant
effect of the p-wave annihilation is the suppression of the amplitude of the
gamma-ray background. A non-zero at the dark matter freeze-out epoch
requires a smaller value of in order for the relic density constraint to be
satisfied, suppressing the amplitude by a factor as low as for a
thermal relic. Non-thermal relics will have weaker amplitude suppression. As
another velocity-dependent effect, we calculate the spectrum for s-wave
annihilation into fermions enhanced by the attractive Sommerfeld effect.
Resonances associated with this effect result in significantly enhanced
intensities, with a slightly softer energy spectrum.Comment: 18 pages, 10 figure
Properties of a Discrete Quantum Field Theory
A scalar quantum field theory defined on a discrete spatial coordinate is
examined. The renormalization of the lattice propagator is discussed with an
emphasis on the periodic nature of the associated momentum coordinate. The
analytic properties of the scattering amplitudes indicate the development of a
second branch point on which the branch cut from the optical theorem
terminates.Comment: 7 pages, 1 figur
Renormalization group approach to 2D Coulomb interacting Dirac fermions with random gauge potential
We argue that massless Dirac particles in two spatial dimensions with
Coulomb repulsion and quenched random gauge field are described by a manifold
of fixed points which can be accessed perturbatively in disorder and
interaction strength, thereby confirming and extending the results of
arXiv:0707.4171. At small interaction and small randomness, there is an
infra-red stable fixed curve which merges with the strongly interacting
infra-red unstable line at a critical endpoint, along which the dynamical
critical exponent .Comment: 4 pages, 4 figure
The effect of early dark matter halos on reionization
The annihilation of dark matter particles releases energy, ionizing some of
the gas in the Universe. We investigate the effect of dark matter halos on
reionization. We show that the effect depends on the assumed density profile,
the particle mass, and the assumed minimum halo mass. For NFW halos and typical
WIMPs, we find the effect to be quite small. However, light dark matter
candidates in the MeV range can contribute significantly to reionization and
can make an important contribution to the measured optical depth. This effect
may be used to constrain light dark matter models. We also study the effect of
varying the halo density profile on reionization.Comment: Minor changes from v2. Accepted for publication in Phys. Rev.
Trident pair production in strong laser pulses
We calculate the trident pair production amplitude in a strong laser
background. We allow for finite pulse duration, while still treating the laser
fields nonperturbatively in strong-field QED. Our approach reveals explicitly
the individual contributions of the one-step and two-step processes. We also
expose the role gauge invariance plays in the amplitudes and discuss the
relation between our results and the optical theorem.Comment: 4 pages, 1 .eps figure. Version 2: reference added, published versio
Spin-Dependent Antenna Splitting Functions
We consider parton showers based on radiation from QCD dipoles or `antennae'.
These showers are built from 2->3 parton splitting processes. The question then
arises of what functions replace the Altarelli-Parisi splitting functions in
this approach. We give a detailed answer to this question, applicable to
antenna showers in which partons carry definite helicity, and to both initial-
and final-state emissions.Comment: 31 pages, 12 figure
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