346 research outputs found
'History as a Political Act': 100 Years of US Empire 1898-1998 and Radical Hopes for the Future
On November 24, 1998, Raymond Lotta talked with Howard Zinn about 100 years of U.S. empire and radical prospects for the future
NMR Spectroscopy as a Tool to Provide Mechanistic Clues About Protein Function and Disease Pathogenesis
Generating Function for Particle-Number Probability Distribution in Directed Percolation
We derive a generic expression for the generating function (GF) of the
particle-number probability distribution (PNPD) for a simple reaction diffusion
model that belongs to the directed percolation universality class. Starting
with a single particle on a lattice, we show that the GF of the PNPD can be
written as an infinite series of cumulants taken at zero momentum. This series
can be summed up into a complete form at the level of a mean-field
approximation. Using the renormalization group techniques, we determine
logarithmic corrections for the GF at the upper critical dimension. We also
find the critical scaling form for the PNPD and check its universality
numerically in one dimension. The critical scaling function is found to be
universal up to two non-universal metric factors.Comment: (v1,2) 8 pages, 5 figures; one-loop calculation corrected in response
to criticism received from Hans-Karl Janssen, (v3) content as publishe
A prescription for the asteroseismic surface correction
In asteroseismology, the surface effect is a disparity between the observed
and the modelled oscillation frequencies. It originates from improper modelling
of the surface layers in stars with solar-like oscillations. Correcting the
surface effect usually requires using functions with free parameters, which are
conventionally fitted to the observed frequencies. On the basis that the
correction should vary smoothly across the H--R diagram, we parameterize it as
a simple function of three stellar surface properties: surface gravity,
effective temperature, and metallicity. We determine this function by fitting
stars ranging from main-sequence dwarfs to red-giant-branch stars. The absolute
amount of the surface correction increases with surface gravity, but the ratio
between it and decreases. Applying the prescription has an
advantage of eliminating unrealistic surface correction, which improves
parameter estimations with stellar modelling. Using two open clusters, we found
that adopting the prescription can help reduce the scatter of the model-derived
ages for each star in the same cluster. For an application, we provide a new
revision for the scaling relation, using our prescription to
account for the surface effect in models. The values of the correction factor,
, are up to 2\% smaller than those determined without the
surface effect considered, suggesting decreases of up to 4\% in asteroseismic
scaling radii and up to 8\% in asteroseismic scaling masses. This revision
brings the asteroseismic properties into agreement with those determined from
eclipsing binaries. Finally, the new correction factor and the stellar models
with the corrected frequencies are made publicly available.Comment: 11 pages, 9 figures. Submitted to MNRAS. All comments (including on
refs) are welcom
Path integral evaluation of the one-loop effective potential in field theory of diffusion-limited reactions
The well-established effective action and effective potential framework from
the quantum field theory domain is adapted and successfully applied to
classical field theories of the Doi and Peliti type for diffusion controlled
reactions. Through a number of benchmark examples, we show that the direct
calculation of the effective potential in fixed space dimension to
one-loop order reduces to a small set of simple elementary functions,
irrespective of the microscopic details of the specific model. Thus the
technique, which allows one to obtain with little additional effort, the
potentials for a wide variety of different models, represents an important
alternative to the standard model dependent diagram-based calculations. The
renormalized effective potential, effective equations of motion and the
associated renormalization group equations are computed in spatial
dimensions for a number of single species field theories of increasing
complexity.Comment: Plain LaTEX2e, 32 pages and three figures. Submitted to Journal of
Statistical Physic
State-dependent diffusion: thermodynamic consistency and its path integral formulation
The friction coefficient of a particle can depend on its position as it does
when the particle is near a wall. We formulate the dynamics of particles with
such state-dependent friction coefficients in terms of a general Langevin
equation with multiplicative noise, whose evaluation requires the introduction
of specific rules. Two common conventions, the Ito and the Stratonovich,
provide alternative rules for evaluation of the noise, but other conventions
are possible. We show the requirement that a particle's distribution function
approach the Boltzmann distribution at long times dictates that a drift term
must be added to the Langevin equation. This drift term is proportional to the
derivative of the diffusion coefficient times a factor that depends on the
convention used to define the multiplicative noise. We explore the consequences
of this result in a number examples with spatially varying diffusion
coefficients. We also derive path integral representations for arbitrary
interpretation of the noise, and use it in a perturbative study of correlations
in a simple system.Comment: 18 pages, 8 figures, Accepted to PR
- …