14,848 research outputs found
Efeito do período pós-parto na fertilidade de vacas de corte submetidas à re-sincronização do estro.
Um grupo de 118 vacas dividido segundo o período pós-parto em G1 (<42 dias pós-parto) e G2 (3 42 dias) foi submetido a sincronização do estro e inseminação artificial em momento pre-estabelecido (IAME). Doze dias depois procedeu-se a re-sincronização com um implante de 6 mg de norgestomet, que foi retirado nove dias depois. Uma segunda IAME foi feita 48 horas depois, apenas nas vacas nao-prenhes a 1 IAME. As vacas do G2 tiveram taxa de prenhez maior na IAME inicial. As taxas de prenhez a 2 IA e cumulativa não deferiram entre grupos
Ecosystems with mutually exclusive interactions self-organize to a state of high diversity
Ecological systems comprise an astonishing diversity of species that
cooperate or compete with each other forming complex mutual dependencies. The
minimum requirements to maintain a large species diversity on long time scales
are in general unknown. Using lichen communities as an example, we propose a
model for the evolution of mutually excluding organisms that compete for space.
We suggest that chain-like or cyclic invasions involving three or more species
open for creation of spatially separated sub-populations that subsequently can
lead to increased diversity. In contrast to its non-spatial counterpart, our
model predicts robust co-existence of a large number of species, in accordance
with observations on lichen growth. It is demonstrated that large species
diversity can be obtained on evolutionary timescales, provided that
interactions between species have spatial constraints. In particular, a phase
transition to a sustainable state of high diversity is identified.Comment: 4 pages, 4 figure
Zero Temperature Chiral Phase Transition in (2+1)-Dimensional QED with a Chern-Simons Term
We investigate the zero temperature chiral phase transition in
(2+1)-dimensional QED in the presence of a Chern-Simons term, changing the
number of fermion flavors. In the symmetric phase, there are no light degrees
of freedom even at the critical point. Unlike the case without a Chern-Simons
term, the phase transition is first-order.Comment: 7 pages, RevTeX, no figure
Hot dense capsule implosion cores produced by z-pinch dynamic hohlraum radiation
Hot dense capsule implosions driven by z-pinch x-rays have been measured for
the first time. A ~220 eV dynamic hohlraum imploded 1.7-2.1 mm diameter
gas-filled CH capsules which absorbed up to ~20 kJ of x-rays. Argon tracer atom
spectra were used to measure the Te~ 1keV electron temperature and the ne ~ 1-4
x10^23 cm-3 electron density. Spectra from multiple directions provide core
symmetry estimates. Computer simulations agree well with the peak compression
values of Te, ne, and symmetry, indicating reasonable understanding of the
hohlraum and implosion physics.Comment: submitted to Phys. Rev. Let
The Zero Temperature Chiral Phase Transition in SU(N) Gauge Theories
We investigate the zero temperature chiral phase transition in an SU(N) gauge
theory as the number of fermions is varied. We argue that there exists a
critical number of fermions , above which there is no chiral symmetry
breaking or confinement, and below which both chiral symmetry breaking and
confinement set in. We estimate and discuss the nature of the phase
transition.Comment: 13 pages, LaTeX, version published in PR
2+1 Dimensional QED and a Novel Phase Transition
We investigate the chiral phase transition in 2+1 dimensional QED. Previous
gap equation and lattice Monte-Carlo studies of symmetry breaking have found
that symmetry breaking ceases to occur when the number of fermion flavors
exceeds a critical value. Here we focus on the order of the transition. We find
that there are no light scalar degrees of freedom present as the critical
number of flavors is approached from above (in the symmetric phase). Thus the
phase transition is not second order, rendering irrelevant the renormalization
group arguments for a fluctuation induced transition. However, the order
parameter vanishes continuously in the broken phase, so this transition is also
unlike a conventional first order phase transition.Comment: 11 pages, Late
The Phase Structure of an SU(N) Gauge Theory with N_f Flavors
We investigate the chiral phase transition in SU(N) gauge theories as the
number of quark flavors, , is varied. We argue that the transition takes
place at a large enough value of so that it is governed by the infrared
fixed point of the function. We study the nature of the phase
transition analytically and numerically, and discuss the spectrum of the theory
as the critical value of is approached in both the symmetric and broken
phases. Since the transition is governed by a conformal fixed point, there are
no light excitations on the symmetric side. We extend previous work to include
higher order effects by developing a renormalization group estimate of the
critical coupling.Comment: 34 pages, 1 figure. More references adde
A simple method for finite range decomposition of quadratic forms and Gaussian fields
We present a simple method to decompose the Green forms corresponding to a
large class of interesting symmetric Dirichlet forms into integrals over
symmetric positive semi-definite and finite range (properly supported) forms
that are smoother than the original Green form. This result gives rise to
multiscale decompositions of the associated Gaussian free fields into sums of
independent smoother Gaussian fields with spatially localized correlations. Our
method makes use of the finite propagation speed of the wave equation and
Chebyshev polynomials. It improves several existing results and also gives
simpler proofs.Comment: minor correction for t<
Effect of retardation on dynamical mass generation in two-dimensional QED at finite temperature
The effect of retardation on dynamical mass generation in is studied, in the
imaginary time formalism. The photon porarization tensor is evaluated to
leading order in 1/N (N is the number of flavours), and simple closed form
expressions are found for the fully retarded longitudinal and transverse
propagators, which have the correct limit when T goes to zero. The resulting
S-D equation for the fermion mass (at order 1/N) has an infrared divergence
associated with the contribution of the transverse photon propagator; only the
longitudinal contribution is retained, as in earlier treatments. For solutions
of constant mass, it is found that the retardation reduces the value of the
parameter r (the ratio of twice the mass to the critical temperature) from
about 10 to about 6. The gap equation is then solved allowing for the mass to
depend on frequency. It was found that the r value remained close to 6.
Possibilities for including the transverse propagator are discussed.Comment: 26 pages 8 figure
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