27,263 research outputs found
The stability of the spectator, Dirac, and Salpeter equations for mesons
Mesons are made of quark-antiquark pairs held together by the strong force.
The one channel spectator, Dirac, and Salpeter equations can each be used to
model this pairing. We look at cases where the relativistic kernel of these
equations corresponds to a time-like vector exchange, a scalar exchange, or a
linear combination of the two. Since the model used in this paper describes
mesons which cannot decay physically, the equations must describe stable
states. We find that this requirement is not always satisfied, and give a
complete discussion of the conditions under which the various equations give
unphysical, unstable solutions
Break-up fragment topology in statistical multifragmentation models
Break-up fragmentation patterns together with kinetic and configurational
energy fluctuations are investigated in the framework of a microcanonical model
with fragment degrees of freedom over a broad excitation energy range. As far
as fragment partitioning is approximately preserved, energy fluctuations are
found to be rather insensitive to both the way in which the freeze-out volume
is constrained and the trajectory followed by the system in the excitation
energy - freeze-out volume space. Due to hard-core repulsion, the freeze-out
volume is found to be populated un-uniformly, its highly depleted core giving
the source a bubble-like structure. The most probable localization of the
largest fragments in the freeze-out volume may be inferred experimentally from
their kinematic properties, largely dictated by Coulomb repulsion
Two-pion exchange potential and the amplitude
We discuss the two-pion exchange potential which emerges from a box diagram
with one nucleon (the spectator) restricted to its mass shell, and the other
nucleon line replaced by a subtracted, covariant scattering amplitude
which includes , Roper, and isobars, as well as contact terms
and off-shell (non-pole) dressed nucleon terms. The amplitude satisfies
chiral symmetry constraints and fits data below 700 MeV pion
energy. We find that this TPE potential can be well approximated by the
exchange of an effective sigma and delta meson, with parameters close to the
ones used in one-boson-exchange models that fit data below the pion
production threshold.Comment: 9 pages (RevTex) and 7 postscript figures, in one uuencoded gzipped
tar fil
Continuous phase transitions with a convex dip in the microcanonical entropy
The appearance of a convex dip in the microcanonical entropy of finite
systems usually signals a first order transition. However, a convex dip also
shows up in some systems with a continuous transition as for example in the
Baxter-Wu model and in the four-state Potts model in two dimensions. We
demonstrate that the appearance of a convex dip in those cases can be traced
back to a finite-size effect. The properties of the dip are markedly different
from those associated with a first order transition and can be understood
within a microcanonical finite-size scaling theory for continuous phase
transitions. Results obtained from numerical simulations corroborate the
predictions of the scaling theory.Comment: 8 pages, 7 figures, to appear in Phys. Rev.
Thermal fluctuations in the lattice Boltzmann method for non-ideal fluids
We introduce thermal fluctuations in the lattice Boltzmann method for
non-ideal fluids. A fluctuation-dissipation theorem is derived within the
Langevin framework and applied to a specific lattice Boltzmann model that
approximates the linearized fluctuating Navier-Stokes equations for fluids
based on square-gradient free energy functionals. The obtained thermal noise is
shown to ensure equilibration of all degrees of freedom in a simulation to high
accuracy. Furthermore, we demonstrate that satisfactory results for most
practical applications of fluctuating hydrodynamics can already be achieved
using thermal noise derived in the long wavelength-limit.Comment: 15 pages, 5 figure
Mode 2 fatigue crack growth specimen development
A Mode II test specimen was developed which has potential application in understanding phemonena associated with mixed mode fatigue failures in high performance aircraft engine bearing races. The attributes of the specimen are: it contains one single ended notch, which simplifiers data gathering and reduction; the fatigue crack grous in-line with the direction of load application; a single axis test machine is sufficient to perform testing; and the Mode I component is vanishingly small
Witness Recantation Study: Preliminary Findings
In September 2012, the National Registry of Exonerations began a research study of all the cases in our database that involve post-conviction recantations by witnesses or victims. This is the first systematic study of recantations ever conducted. Its purpose is to identify patterns and trends among these cases, with a particular focus on the circumstances that first elicit the false testimony, and on the official reactions to the recantations by judges and other authorities. Our data set includes all the cases in the Registry as of February 28, 2013 – a total of 1,068 cases, 250 of which involve recantations. We developed a coding system to track various features of these cases, from the crime (or alleged crime) and initial investigation through the conviction, post-conviction proceedings, re-investigation, and exoneration of the defendant. In early April, we finished coding the cases, and we presented initial findings at the Innocence Network Conference on April 20th. This memo provides background on the project and a brief summary of our findings thus far
Witness Recantation Study: Preliminary Findings
In September 2012, the National Registry of Exonerations began a research study of all the cases in our database that involve post-conviction recantations by witnesses or victims. This is the first systematic study of recantations ever conducted. Its purpose is to identify patterns and trends among these cases, with a particular focus on the circumstances that first elicit the false testimony, and on the official reactions to the recantations by judges and other authorities. Our data set includes all the cases in the Registry as of February 28, 2013 – a total of 1,068 cases, 250 of which involve recantations. We developed a coding system to track various features of these cases, from the crime (or alleged crime) and initial investigation through the conviction, post-conviction proceedings, re-investigation, and exoneration of the defendant. In early April, we finished coding the cases, and we presented initial findings at the Innocence Network Conference on April 20th. This memo provides background on the project and a brief summary of our findings thus far
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