1,133 research outputs found
The effect of wind tunnel turbulence upon the forces measured on models
1. Reasons for inquiry: The tests were undertaken to find the effect of turbulence in the air stream upon the lift and drag forces measured on models in the four-foot wind tunnel at the Massachusetts Institute of Technology. 2. Range of investigation: Maximum lifts and minimum drags were measured on Gottingen-387 and R.A.F.-15 airfoils, minimum drag on a streamlined strut, and the static pressure gradients for different conditions of turbulence. 3. Results and further developments: The results show that the scale of the turbulence (as defined in this report) has a marked effect upon the measured forces on models tested in the tunnel as well as on the pressure gradient, and it is recommended that further investigation of the phenomena be made with the aid of smoke and small wind vanes
Perturbation theory vs. simulation for tadpole improvement factors in pure gauge theories
We calculate the mean link in Landau gauge for Wilson and improved SU(3)
anisotropic gauge actions, using two loop perturbation theory and Monte Carlo
simulation employing an accelerated Langevin algorithm. Twisted boundary
conditions are employed, with a twist in all four lattice directions
considerably improving the (Fourier accelerated) convergence to an improved
lattice Landau gauge. Two loop perturbation theory is seen to predict the mean
link extremely well even into the region of commonly simulated gauge couplings
and so can be used remove the need for numerical tuning of self-consistent
tadpole improvement factors. A three loop perturbative coefficient is inferred
from the simulations and is found to be small. We show that finite size effects
are small and argue likewise for (lattice) Gribov copies and double Dirac
sheets.Comment: 13 pages of revtex
Few-body physics in effective field theory
Effective Field Theory (EFT) provides a powerful framework that exploits a
separation of scales in physical systems to perform systematically improvable,
model-independent calculations. Particularly interesting are few-body systems
with short-range interactions and large two-body scattering length. Such
systems display remarkable universal features. In systems with more than two
particles, a three-body force with limit cycle behavior is required for
consistent renormalization already at leading order. We will review this EFT
and some of its applications in the physics of cold atoms and nuclear physics.
In particular, we will discuss the possibility of an infrared limit cycle in
QCD. Recent extensions of the EFT approach to the four-body system and N-boson
droplets in two spatial dimensions will also be addressed.Comment: 10 pages, 5 figures, Proceedings of the INT Workshop on "Nuclear
Forces and the Quantum Many-Body Problem", Oct. 200
Diffusive transport in networks built of containers and tubes
We developed analytical and numerical methods to study a transport of
non-interacting particles in large networks consisting of M d-dimensional
containers C_1,...,C_M with radii R_i linked together by tubes of length l_{ij}
and radii a_{ij} where i,j=1,2,...,M. Tubes may join directly with each other
forming junctions. It is possible that some links are absent. Instead of
solving the diffusion equation for the full problem we formulated an approach
that is computationally more efficient. We derived a set of rate equations that
govern the time dependence of the number of particles in each container
N_1(t),N_2(t),...,N_M(t). In such a way the complicated transport problem is
reduced to a set of M first order integro-differential equations in time, which
can be solved efficiently by the algorithm presented here. The workings of the
method have been demonstrated on a couple of examples: networks involving
three, four and seven containers, and one network with a three-point junction.
Already simple networks with relatively few containers exhibit interesting
transport behavior. For example, we showed that it is possible to adjust the
geometry of the networks so that the particle concentration varies in time in a
wave-like manner. Such behavior deviates from simple exponential growth and
decay occurring in the two container system.Comment: 21 pages, 18 figures, REVTEX4; new figure added, reduced emphasis on
graph theory, additional discussion added (computational cost, one
dimensional tubes
Improved lattice operators for non-relativistic fermions
In this work I apply a recently proposed improvement procedure, originally
conceived to reduce finite lattice spacing effects in transfer matrices for
dilute Fermi systems, to tuning operators for the calculation of observables. I
construct, in particular, highly improved representations for the energy and
the contact, as a first step in an improvement program for finite-temperature
calculations. I illustrate the effects of improvement on those quantities with
a ground-state lattice calculation at unitarity.Comment: 11 pages, 7 figures; replaced with published versio
Four Fermion Field Theories and the Chern-Simons Field: A Renormalization Group Study
In (2+1) dimensions, we consider the model of a flavor, two-component
fermionic field interacting through a Chern-Simons field besides a four fermion
self-interaction which consists of a linear combination of the Gross-Neveu and
Thirring like terms. The four fermion interaction is not perturbatively
renormalizable and the model is taken as an effective field theory in the
region of low momenta. Using Zimmerman procedure for reducing coupling
constants, it is verified that, for small values of the Chern-Simons parameter,
the origin is an infrared stable fixed point but changes to ultraviolet stable
as becomes bigger than a critical . Composite operators are
also analyzed and it is shown that a specific four fermion interaction has an
improved ultraviolet behavior as increases.Comment: 9 pages, revte
Quasiparticle Properties in Effective Field Theory
The quasiparticle concept is an important tool for the description of
many-body systems. We study the quasiparticle properties for dilute Fermi
systems with short-ranged, repulsive interactions using effective field theory.
We calculate the proper self-energy contributions at order (K_f/Lambda)^3,
where Lambda is the short-distance scale that sets the size of the effective
range parameters and K_f the Fermi momentum. The quasiparticle energy, width,
and effective mass to order O(K_f/Lambda)^3 are derived from the calculated
self-energy.Comment: 15 pages, revtex4, 4 PS figure
The Scalar and Form Factors in QCD
QCD sum rules on the light-cone are derived for the sum of the
and form factors taking into account contributions up to
twist four. Combining the results with the corresponding form factors
calculated previously by the same method, we obtain the scalar form factors
. Our sum rule predictions are compared with lattice results,
current-algebra constraints, and quark-model calculations. Furthermore, we
calculate decay distributions and the integrated width for the semileptonic
decay which is sensitive to . Finally, the
dependence of the sum rules on the heavy quark mass and the asymptotic scaling
laws are discussed.Comment: 19 pages, 10 figures, Latex, epsfi
Re-examination of the Perturbative Pion Form Factor with Sudakov Suppression
The perturbative pion form factor with Sudakov suppression is re-examined.
Taking into account the multi-gluon exchange in the law regions, we
suggest that the running coupling constant should be frozen at
and is the
average transverse momentum which can be determined by the pionic wave
function. In addition, we correct the previous calculations about the Sudakov
suppression factor which plays an important role in the perturbative
predictions for the pion form factor.Comment: 11 pages, LaTex file, 2 figures as uu-encoded postscript file
On the correlation between the binding energies of the triton and the alpha-particle
We consider the correlation between the binding energies of the triton and
the alpha-particle which is empirically observed in calculations employing
different phenomenological nucleon-nucleon interactions. Using an effective
quantum mechanics approach for short-range interactions with large scattering
length |a| >> l, where l is the natural low-energy length scale, we construct
the effective interaction potential at leading order in l/|a|. In order to
renormalize the four-nucleon system, it is sufficient to include a
SU(4)-symmetric one-parameter three-nucleon interaction in addition to the
S-wave nucleon-nucleon interactions. The absence of a four-nucleon force at
this order explains the empirically observed correlation between the binding
energies of the triton and the alpha-particle. We calculate this correlation
and obtain a prediction for the alpha-particle binding energy. Corrections to
our results are suppressed by l/|a|.Comment: 4 pages, 1 ps figure, references update
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