93,024 research outputs found
Comparison of Nucleon Form Factors from Lattice QCD Against the Light Front Cloudy Bag Model and Extrapolation to the Physical Mass Regime
We explore the possibility of extrapolating state of the art lattice QCD
calculations of nucleon form factors to the physical regime. We find that the
lattice results can be reproduced using the Light Front Cloudy Bag Model by
letting its parameters be analytic functions of the quark mass. We then use the
model to extend the lattice calculations to large values of Q^{2} of interest
to current and planned experiments. These functions are also used to define
extrapolations to the physical value of the pion mass, thereby allowing us to
study how the predicted zero in G_{E}(Q^{2})/G_{M}(Q^{2}) varies as a function
of quark mass.Comment: 31 pages, 22 figure
Electromagnetic Gauge Invariance of the Cloudy Bag Model
We examine the question of the gauge invariance of electromagnetic form
factors calculated within the cloudy bag model. One of the assumptions of the
model is that electromagnetic form factors are most accurately evaluated in the
Breit frame. This feature is used to show that gauge invariance is respected in
this frame.Comment: 8 pages, RevTex, 1 figure, to be published in Phys. Rev.
Microscopic optical potential from chiral nuclear forces
The energy- and density-dependent single-particle potential for nucleons is
constructed in a medium of infinite isospin-symmetric nuclear matter starting
from realistic nuclear interactions derived within the framework of chiral
effective field theory. The leading-order terms from both two- and
three-nucleon forces give rise to real, energy-independent contributions to the
nucleon self-energy. The Hartree-Fock contribution from the two-nucleon force
is attractive and strongly momentum dependent, in contrast to the contribution
from the three-nucleon force which provides a nearly constant repulsive mean
field that grows approximately linearly with the nuclear density. Together, the
leading-order perturbative contributions yield an attractive single-particle
potential that is however too weak compared to phenomenology. Second-order
contributions from two- and three-body forces then provide the additional
attraction required to reach the phenomenological depth. The imaginary part of
the optical potential, which is positive (negative) for momenta below (above)
the Fermi momentum, arises at second-order and is nearly inversion-symmetric
about the Fermi surface when two-nucleon interactions alone are present. The
imaginary part is strongly absorptive and requires the inclusion of an
effective mass correction as well as self-consistent single-particle energies
to attain qualitative agreement with phenomenology.Comment: 12 pages, 7 figures, added references, corrected typo
An analytic model for the epoch of halo creation
In this paper we describe the Bayesian link between the cosmological mass
function and the distribution of times at which isolated halos of a given mass
exist. By assuming that clumps of dark matter undergo monotonic growth on the
time-scales of interest, this distribution of times is also the distribution of
`creation' times of the halos. This monotonic growth is an inevitable aspect of
gravitational instability. The spherical top-hat collapse model is used to
estimate the rate at which clumps of dark matter collapse. This gives the prior
for the creation time given no information about halo mass. Applying Bayes'
theorem then allows any mass function to be converted into a distribution of
times at which halos of a given mass are created. This general result covers
both Gaussian and non-Gaussian models. We also demonstrate how the mass
function and the creation time distribution can be combined to give a joint
density function, and discuss the relation between the time distribution of
major merger events and the formula calculated. Finally, we determine the
creation time of halos within three N-body simulations, and compare the link
between the mass function and creation rate with the analytic theory.Comment: 7 pages, 2 figures, submitted to MNRA
Long-term energy capture and the effects of optimizing wind turbine operating strategies
Methods of increasing energy capture without affecting the turbine design were investigated. The emphasis was on optimizing the wind turbine operating strategy. The operating strategy embodies the startup and shutdown algorithm as well as the algorithm for determining when to yaw (rotate) the axis of the turbine more directly into the wind. Using data collected at a number of sites, the time-dependent simulation of a MOD-2 wind turbine using various, site-dependent operating strategies provided evidence that site-specific fine tuning can produce significant increases in long-term energy capture as well as reduce the number of start-stop cycles and yawing maneuvers, which may result in reduced fatigue and subsequent maintenance
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