30,732 research outputs found
New Two-Dimensional Models of Supernova Explosions by the Neutrino-Heating Mechanism: Evidence for Different Instability Regimes in Collapsing Stellar Cores
The neutrino-driven explosion mechanism for core-collapse supernovae in its
modern flavor relies on the additional support of hydrodynamical instabilities
in achieving shock revival. Two possible candidates, convection and the
so-called standing accretion shock instability (SASI), have been proposed for
this role. In this paper, we discuss new successful simulations of supernova
explosions that shed light on the relative importance of these two
instabilities. While convection has so far been observed to grow first in
self-consistent hydrodynamical models with multi-group neutrino transport, we
here present the first such simulation in which the SASI grows faster while the
development of convection is initially inhibited. We illustrate the features of
this SASI-dominated regime using an explosion model of a 27 solar mass
progenitor, which is contrasted with a convectively-dominated model of an 8.1
solar mass progenitor with subsolar metallicity, whose early post-bounce
behavior is more in line with previous 11.2 and 15 solar mass explosion models.
We analyze the conditions discriminating between the two different regimes,
showing that a high mass-accretion rate and a short advection time-scale are
conducive for strong SASI activity. We also briefly discuss some important
factors for capturing the SASI-driven regime, such as general relativity, the
progenitor structure, a nuclear equation of state leading to a compact
proto-neutron star, and the neutrino treatment. Finally, we evaluate possible
implications of our findings for 2D and 3D supernova simulations. Our results
show that a better understanding of the SASI and convection in the non-linear
regime is required.Comment: 12 pages, 13 figures; revised version accepted for publication in Ap
Resonant photon absorption in the low spin molecule V15
We report the first study of the micro-SQUID response of a molecular system
to electromagnetic radiation. The advantages of our micro-SQUID technique in
respect to pulsed electron paramagnetic resonance (EPR) techniques consist in
the possibility to perform time-resolved experiments (below 1 ns) on
submicrometer sizes samples (about 1000 spins) at low temperature (below 100
mK).
Resonant photon absorption in the GHz range was observed via low temperature
micro-SQUID magnetization measurements of the spin ground state S = 1/2 of the
molecular complex V15. The line-width essentially results from intra-molecular
hyperfine interaction. The results point out that observing Rabi oscillations
in molecular nanomagnets requires well isolated low spin systems and high
radiation power. Our first results open the way for time-resolved observations
of quantum superposition of spin-up and spin-down states in SMMs.Comment: 7 pages, 5 figure
Hamiltonian Dynamics of Yang-Mills Fields on a Lattice
We review recent results from studies of the dynamics of classical Yang-Mills
fields on a lattice. We discuss the numerical techniques employed in solving
the classical lattice Yang-Mills equations in real time, and present results
exhibiting the universal chaotic behavior of nonabelian gauge theories. The
complete spectrum of Lyapunov exponents is determined for the gauge group
SU(2). We survey results obtained for the SU(3) gauge theory and other
nonlinear field theories. We also discuss the relevance of these results to the
problem of thermalization in gauge theories.Comment: REVTeX, 51 pages, 20 figure
Gluon Distribution Functions for Very Large Nuclei at Small Transverse Momentum
We show that the gluon distribution function for very large nuclei may be
computed for small transverse momentum as correlation functions of an
ultraviolet finite two dimensional Euclidean field theory. This computation is
valid to all orders in the density of partons per unit area, but to lowest
order in . The gluon distribution function is proportional to ,
and the effect of the finite density of partons is to modify the dependence on
transverse momentum for small transverse momentum.Comment: TPI--MINN--93--52/T, NUC--MINN--93--28/T, UMN--TH--1224/93, LaTex, 11
page
Algorithm Developments for Discrete Adjoint Methods
This paper presents a number of algorithm developments for adjoint methods using the 'discrete' approach in which the discretisation of the non-linear equations is linearised and the resulting matrix is then transposed. With a new iterative procedure for solving the adjoint equations, exact numerical equivalence is maintained between the linear and adjoint discretisations. The incorporation of strong boundary conditions within the discrete approach is discussed, as well as a new application of adjoint methods to linear unsteady flow in turbomachinery
Classifying LEP Data with Support Vector Algorithms
We have studied the application of different classification algorithms in the
analysis of simulated high energy physics data. Whereas Neural Network
algorithms have become a standard tool for data analysis, the performance of
other classifiers such as Support Vector Machines has not yet been tested in
this environment. We chose two different problems to compare the performance of
a Support Vector Machine and a Neural Net trained with back-propagation:
tagging events of the type e+e- -> ccbar and the identification of muons
produced in multihadronic e+e- annihilation events.Comment: 7 pages, 4 figures, submitted to proceedings of AIHENP99, Crete,
April 199
Color Transparency at COMPASS energies
Pionic quasielastic knockout of protons from nuclei at 200 GeV show very
large effects of color transparency as -t increases from 0 to several GeV^2.
Similar effects are expected for quasielastic photoproduction of vector mesons.Comment: 9 pages, 4 figure
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