1,010 research outputs found
Studies of Autumn calving suckler cows, bulls at pasture and winter grazing
End of project reportMost beef and dairy cows are spring calving leading to distinct seasonality of supply. Calving
a proportion of the beef herd in the autumn would lead to a more uniform annual supply of
cattle for slaughter and potentially increase the proportion of grazed grass in the diet of the
suckler progeny. Autumn calving sucklers also facilitate the use of AI, which should enhance
the product quality.
This project aimed to address the technical aspects of autumn calving sucklers, which differ
from those of spring calvers. The currently available international energy models were
evaluated for autumn calving lactating suckler cows using the type of cow typically found in
Irish suckler herds (Experiment 1). The winter accommodation of the suckler cow and calf
unit and its impact on cow reproductive performance was evaluated (Experiment 2).
The final part of the project evolved into component studies to determine the effect of
supplementary feed on the performance of grazing bulls (Experiment 3), and the
consequences of weanling cattle grazing pasture in winter as an alternative to housing them in winter (Experiments 4 to 7)
Suppression of Bremsstrahlung at Non-Zero Temperature
The first-order bremsstrahlung emission spectrum is
at zero temperature. If the radiation is emitted into a region that contains a
thermal distribution of photons, then the rate is increased by a factor
where is the Bose-Einstein function. The stimulated
emission changes the spectrum to for . If this were correct, an infinite amount of energy would be radiated in the
low frequency modes. This unphysical result indicates a breakdown of
perturbation theory. The paper computes the bremsstrahlung rate to all orders
of perturbation theory, neglecting the recoil of the charged particle. When the
perturbation series is summed, it has a different low-energy behavior. For
, the spectrum is independent of and has a value
proportional to .Comment: 16 pages (plain TeX), figures available on reques
THERMAL EFFECTS ON THE CATALYSIS BY A MAGNETIC FIELD
We show that the formation of condensates in the presence of a constant
magnetic field in 2+1 dimensions is extremely unstable. It disappears as soon
as a heat bath is introduced with or without a chemical potential. We point out
some new nonanalytic behavior that develops in this system at finite
temperature.Comment: 10 pages, plain Te
Color, Spin and Flavor Diffusion in Quark-Gluon Plasmas
In weakly interacting quark-gluon plasmas diffusion of color is found to be
much slower than the diffusion of spin and flavor because color is easily
exchanged by the gluons in the very singular forward scattering processes. If
the infrared divergence is cut off by a magnetic mass, ,
the color diffusion is , a
factor smaller than spin and flavor diffusion. A similar effect is
expected in electroweak plasmas above due to exchanges. The color
conductivity in quark-gluon plasmas and the electrical conductivity in
electroweak plasmas are correspondingly small in relativistic heavy ion
collisions and the very early universe.Comment: 5 pages, no figure
How fast can the wall move? A study of the electroweak phase transition dynamics
We consider the dynamics of bubble growth in the Minimal Standard Model at
the electroweak phase transition and determine the shape and the velocity of
the phase boundary, or bubble wall. We show that in the semi-classical
approximation the friction on the wall arises from the deviation of massive
particle populations from thermal equilibrium. We treat these with Boltzmann
equations in a fluid approximation. This approximation is reasonable for the
top quarks and the light species while it underestimates the friction from the
infrared bosons and Higgs particles. We use the two-loop finite temperature
effective potential and find a subsonic bubble wall for the whole range of
Higgs masses GeV. The result is weakly dependent on : the wall
velocity falls in the range , while the wall thickness is
in the range . The wall is thicker than the phase equilibrium
value because out of equilibrium particles exert more friction on the back than
on the base of a moving wall. We also consider the effect of an infrared gauge
condensate which may exist in the symmetric phase; modelling it simplemindedly,
we find that the wall may become supersonic, but not ultrarelativistic.Comment: 42 pages, plain latex, with three figures. Minor editing August 1 (we
figured out how to do analytically some integrals we previously did
numerically, made corresponding (slight) changes to numerical results, and
corrected some typos.
Photon Dispersion in a Supernova Core
While the photon forward-scattering amplitude on free magnetic dipoles (e.g.
free neutrons) vanishes, the nucleon magnetic moments still contribute
significantly to the photon dispersion relation in a supernova (SN) core where
the nucleon spins are not free due to their interaction. We study the frequency
dependence of the relevant spin susceptibility in a toy model with only
neutrons which interact by one-pion exchange. Our approach amounts to
calculating the photon absorption rate from the inverse bremsstrahlung process
gamma n n --> n n, and then deriving the refractive index n_refr with the help
of the Kramers-Kronig relation. In the static limit (omega --> 0) the
dispersion relation is governed by the Pauli susceptibility chi_Pauli so that
(n_refr)^2-1 approx chi_Pauli > 0. For omega somewhat above the neutron
spin-relaxation rate Gamma_sigma we find (n_refr)^2-1>
Gamma_sigma the photon dispersion relation acquires the form
omega^2-k^2=(m_gamma)^2. An exact expression for the "transverse photon mass"
m_gamma is given in terms of the f-sum of the neutron spin autocorrelation
function; an estimate is (m_gamma)^2 approx chi_Pauli T Gamma_sigma. The
dominant contribution to n_refr$ in a SN core remains the electron plasma
frequency so that the Cherenkov processes gamma nu nu remain forbidden for
all photon frequencies.Comment: 8 pages, REVTEX, 7 postscript figures include
The graviton self-energy in thermal quantum gravity
We show generally that in thermal gravity, the one-particle irreducible
2-point function depends on the choice of the basic graviton fields. We derive
the relevant properties of a physical graviton self-energy, which is
independent of the parametrization of the graviton field. An explicit
expression for the graviton self-energy at high-temperature is given to
one-loop order.Comment: 13 pages, 2 figure
Fermion and Anti-Fermion Effective Masses in High Temperature Gauge Theories in -Asymmetric Background
We calculate the splitting between fermion and anti-fermion effective masses
in high temperature gauge theories in the presence of a non-vanishing chemical
potential due to the -asymmetric fermionic background. In particular we
consider the case of left-handed leptons in the theory when
the temperature is above GeV and the gauge symmetry is restored.Comment: 13 pages, TIPAC-93001
Structure of the Quark Propagator at High Temperature
In the high temperature, chirally invariant phase of QCD, the quark
propagator is shown to have two sets of poles with different dispersion
relations. A reflection property in momentum space relates all derivatives at
zero-momentum of the particle and hole energies, the particle and hole damping
rates, and the particle and hole residues. No use is made of perturbation
theory.Comment: 8 pages, Latex twocolum
Electroweak Baryogenesis and Standard Model CP Violation
We analyze the mechanism of electroweak baryogenesis proposed by Farrar and
Shaposhnikov in which the phase of the CKM mixing matrix is the only source of
violation. This mechanism is based on a phase separation of baryons via
the scattering of quasiparticles by the wall of an expanding bubble produced at
the electroweak phase transition. In agreement with the recent work of Gavela,
Hern\'andez, Orloff and P\`ene, we conclude that QCD damping effects reduce the
asymmetry produced to a negligible amount. We interpret the damping as quantum
decoherence. We compute the asymmetry analytically. Our analysis reflects the
observation that only a thin, outer layer of the bubble contributes to the
coherent scattering of the quasiparticles. The generality of our arguments
rules out any mechanism of electroweak baryogenesis that does not make use of a
new source of violation.Comment: 36 pages, in LaTeX, one LaTeX figure included, 5 others available
upon request, SLAC-PUB-647
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