2,268 research outputs found
Improved estimate of electron capture rates on nuclei during stellar core collapse
Electron captures on nuclei play an important role in the dynamics of the
collapsing core of a massive star that leads to a supernova explosion. Recent
calculations of these capture rates were based on microscopic models which
account for relevant degrees of freedom. Due to computational restrictions such
calculations were limited to a modest number of nuclei, mainly in the mass
range A=45-110. Recent supernova simulations show that this pool of nuclei,
however, omits the very neutron-rich and heavy nuclei which dominate the
nuclear composition during the last phase of the collapse before neutrino
trapping. Assuming that the composition is given by Nuclear Statistical
Equilibrium we present here electron capture rates for collapse conditions
derived from individual rates for roughly 2700 individual nuclei. For those
nuclei which dominate in the early stage of the collapse, the individual rates
are derived within the framework of microscopic models, while for the nuclei
which dominate at high densities we have derived the rates based on the Random
Phase Approximation with a global parametrization of the single particle
occupation numbers. In addition, we have improved previous rate evaluations by
properly including screening corrections to the reaction rates into account.Comment: 32 pages, 13 figures, 1 table; elsart; to appear in Nuclear Physics
Effects of Inelastic Neutrino-Nucleus Scattering on Supernova Dynamics and Radiated Neutrino Spectra
Based on the shell model for Gamow-Teller and the Random Phase Approximation
for forbidden transitions, we have calculated reaction rates for inelastic
neutrino-nucleus scattering (INNS) under supernova (SN) conditions, assuming a
matter composition given by Nuclear Statistical Equilibrium. The rates have
been incorporated into state-of-the-art stellar core-collapse simulations with
detailed energy-dependent neutrino transport. While no significant effect on
the SN dynamics is observed, INNS increases the neutrino opacities noticeably
and strongly reduces the high-energy tail of the neutrino spectrum emitted in
the neutrino burst at shock breakout. Relatedly the expected event rates for
the observation of such neutrinos by earthbound detectors are reduced by up to
about 60%.Comment: 4 pages, 2 figures, 1 tabl
Symmetries and Ambiguities in the linear sigma model with light quarks
We investigate the role of undetermined finite contributions generated by
radiative corrections in a linear sigma model with quarks.
Although some of such terms can be absorbed in the renormalization procedure,
one such contribution is left in the expression for the pion decay constant.
This arbitrariness is eliminated by chiral symmetry.Comment: 9 pages. Added references through the text; an author was added due
to an important contribution; corrected typos; the title also was changed.
Submitted to Modern Physics Letter
Cobertura morta de leguminosas no controle de ervas invasoras em sistema de cultivo em faixas (Alley cropping).
bitstream/item/32692/1/CPATU-BP137.pd
Skyrmion morphology in ultrathin magnetic films
Nitrogen-vacancy magnetic microscopy is employed in quenching mode as a
non-invasive, high resolution tool to investigate the morphology of isolated
skyrmions in ultrathin magnetic films. The skyrmion size and shape are found to
be strongly affected by local pinning effects and magnetic field history.
Micromagnetic simulations including static disorder, based on a physical model
of grain-to-grain thickness variations, reproduce all experimental observations
and reveal the key role of disorder and magnetic history in the stabilization
of skyrmions in ultrathin magnetic films. This work opens the way to an
in-depth understanding of skyrmion dynamics in real, disordered media.Comment: 9 pages, 8 figures, including supplementary information
Implicit Regularization and Renormalization of QCD
We apply the Implicit Regularization Technique (IR) in a non-abelian gauge
theory. We show that IR preserves gauge symmetry as encoded in relations
between the renormalizations constants required by the Slavnov-Taylor
identities at the one loop level of QCD. Moreover, we show that the technique
handles divergencies in massive and massless QFT on equal footing.Comment: (11 pages, 2 figures
Space Shuttle program communication and tracking systems interface analysis
The Space Shuttle Program Communications and Tracking Systems Interface Analysis began April 18, 1983. During this time, the shuttle communication and tracking systems began flight testing. Two areas of analysis documented were a result of observations made during flight tests. These analyses involved the Ku-band communication system. First, there was a detailed analysis of the interface between the solar max data format and the Ku-band communication system including the TDRSS ground station. The second analysis involving the Ku-band communication system was an analysis of the frequency lock loop of the Gunn oscillator used to generate the transmit frequency. The stability of the frequency lock loop was investigated and changes to the design were reviewed to alleviate the potential loss of data due the loop losing lock and entering the reacquisition mode. Other areas of investigation were the S-band antenna analysis and RF coverage analysis
Casimir energy and a cosmological bounce
We review different computation methods for the renormalised energy momentum
tensor of a quantised scalar field in an Einstein Static Universe. For the
extensively studied conformally coupled case we check their equivalence; for
different couplings we discuss violation of different energy conditions. In
particular, there is a family of masses and couplings which violate the weak
and strong energy conditions but do not lead to spacelike propagation. Amongst
these cases is that of a minimally coupled massless scalar field with no
potential. We also point out a particular coupling for which a massless scalar
field has vanishing renormalised energy momentum tensor. We discuss the
backreaction problem and in particular the possibility that this Casimir energy
could both source a short inflationary epoch and avoid the big bang singularity
through a bounce.Comment: 13 pages, LaTeX, 8 figure
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