25,726 research outputs found
Partial wave contributions to the antikaon potential at finite momentum
The momentum dependence of the antikaon optical potential in nuclear matter
is obtained from a microscopic and self-consistent calculation using the
meson-exchange J\"ulich interaction. Two self-consistent schemes are
discussed, which would lead to substantially different predictions for the
width of nuclear bound states. The effect of higher partial waves of
the interaction, beyond the L=0 component, is studied and found to
have moderate but non-negligible effects on the nuclear potential at
zero momentum. At momenta as large as 500 MeV/c, relevant in the analysis of
heavy-ion collisions, the higher partial partial waves modify the
optical potential by nearly a factor of two.Comment: 20 pages, 7 figure
Cascade production in heavy-ion collisions at SIS energies
Production of the doubly strange baryon in heavy-ion collisions at
\textrm{SIS} energies is studied in a relativistic transport model that
includes perturbatively the strangeness-exchange reactions and . Taking the cross sections for these
reactions from the predictions of a hadronic model, we find that the
yield is about in central collisions of Ni + Ni at
\textrm{GeV}. The yield is further found to be more sensitive
to the magnitude of the cross sections for strangeness-exchange reactions than
to the medium effects due to modified kaon properties. We have also made
predictions for production in Au+Au collisions at energies from 1 to 2
GeV/nucleon.Comment: 13 pages, 5 figures, typos fixed and discussions added, to appear in
PL
Extreme Value Theory and the Solar Cycle
We investigate the statistical properties of the extreme events of the solar
cycle as measured by the sunspot number. The recent advances in the methodology
of the theory of extreme values is applied to the maximal extremes of the time
series of sunspots. We focus on the extreme events that exceed a carefully
chosen threshold and a generalized Pareto distribution is fitted to the tail of
the empirical cumulative distribution. A maximum likelihood method is used to
estimate the parameters of the generalized Pareto distribution and confidence
levels are also given to the parameters. Due to the lack of an automatic
procedure for selecting the threshold, we analyze the sensitivity of the fitted
generalized Pareto distribution to the exact value of the threshold. According
to the available data, that only spans the previous ~250 years, the cumulative
distribution of the time series is bounded, yielding an upper limit of 324 for
the sunspot number. We also estimate that the return value for each solar cycle
is ~188, while the return value for a century increases to ~228. Finally, the
results also indicate that the most probable return time for a large event like
the maximum at solar cycle 19 happens once every ~700 years and that the
probability of finding such a large event with a frequency smaller than ~50
years is very small. In spite of the essentially extrapolative character of
these results, their statistical significance is very large.Comment: 6 pages, 4 figures, accepted for publication in A&
The antikaon nuclear potential in hot and dense matter
The antikaon optical potential in hot and dense nuclear matter is studied
within the framework of a coupled-channel self-consistent calculation taking,
as bare meson-baryon interaction, the meson-exchange potential of the J\"ulich
group. Typical conditions found in heavy-ion collisions at GSI are explored. As
in the case of zero temperature, the angular momentum components larger than
L=0 contribute significantly to the finite temperature antikaon optical
potential at finite momentum. It is found that the particular treatment of the
medium effects has a strong influence on the behavior of the antikaon potential
with temperature. Our self-consistent model, in which antikaons and pions are
dressed in the medium, gives a moderately temperature dependent antikaon
potential which remains attractive at GSI temperatures, contrary to what one
finds if only nuclear Pauli blocking effects are included.Comment: 30 pages, 8 figures, references added. Accepted for publication in
PR
Markovian properties of continuous group actions: algebraic actions, entropy and the homoclinic group
We provide a unifying approach which links results on algebraic actions by
Lind and Schmidt, Chung and Li, and a topological result by Meyerovitch that
relates entropy to the set of asymptotic pairs. In order to do this we
introduce a series of Markovian properties and, under the assumption that they
are satisfied, we prove several results that relate topological entropy and
asymptotic pairs (the homoclinic group in the algebraic case). As new
applications of our method, we give a characterization of the homoclinic group
of any finitely presented expansive algebraic action of (1) any elementary
amenable group with an upper bound on the orders of finite subgroups or (2) any
left orderable amenable group, using the language of independence entropy
pairs.Comment: Minor changes. 37 pages. Adv. Math. to appea
Design of switching damping controllers for power systems based on a Markov jump parameter system approach
The application of a new technique, based on the theory of Markov Jump Parameter Systems (MJPS), to the problem of designing controllers to damp power system oscillations is presented in this paper. This problem is very difficult to address, mainly because these controllers are required to have an output feedback decentralized structure. The technique relies on the statistical knowledge about the system operating conditions to provide less conservative controllers than other modern robust control approaches. The influence of the system interconnections over its modes of oscillation is reduced by means of a proper control design formulation involving Integral Quadratic Constraints. The discrete nature of some typical events in power systems (such as line tripping or load switching) is adequately modeled by the MJPS approach, therefore allowing the controller to withstand such abrupt changes in the operating conditions of the system, as shown in the results. © 2006 IEEE
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