1,463,726 research outputs found
Quantum Environments: Spin Baths, Oscillator Baths, and applications to Quantum Magnetism
The low-energy physics of systems coupled to their surroundings is understood
by truncating to effective Hamiltonians; these tend to reduce to a few
canonical forms, involving coupling to "baths" of oscillators or spins. The
method for doing this is demonstrated using examples from magnetism,
superconductivity, and measurement theory, as is the way one then solves for
the low-energy dynamics. Finally, detailed application is given to the exciting
recent Quantum relaxation and tunneling work in naomagnets.Comment: Chapter in "Tunneling in Complex Systems" (World Sci., edited T.
Tomsovic); 97 pages. Published in June 199
Dynamics of a Pair of Interacting Spins Coupled to an Environmental Sea
We solve for the dynamics of a pair of spins, coupled to each other and also
to an environmental sea of oscillators. The environment mediates an indirect
interaction between the spins, causing both mutual coherence effects and
dissipation. This model describes a wide variety of physical systems, ranging
from 2 coupled microscopic systems (eg., magnetic impurities, bromophores,
etc), to 2 coupled macroscopic quantum systems. We obtain analytic results for
3 regimes, viz., (i) The locked regime, where the 2 spins lock together; (ii)
The correlated relaxation regime (mutually correlated incoherent relaxation);
and (iii) The mutual coherence regime, with correlated damped oscillations.
These results cover most of the parameter space of the system.Comment: 49 pages, To appear in Int J. Mod. Phys.
Can the James Webb Space Telescope detect isolated population III stars?
Isolated population III stars are postulated to exist at approximately
z=10-30 and may attain masses up to a few hundred solar masses. The James Webb
Space telescope (JWST) is the next large space based infrared telescope and is
scheduled for launch in 2014. Using a 6.5 meter primary mirror, it will
probably be able to detect some of the first galaxies forming in the early
Universe. A natural question is whether it will also be able to see any
isolated population III stars. Here, we calculate the apparent broadband
AB-magnitudes for 300 solar masses population III stars in JWST filters at
z=10-20. Our calculations are based on realistic stellar atmospheres and take
into account the potential flux contribution from the surrounding HII region.
The gravitational magnification boost achieved when pointing JWST through a
foreground galaxy cluster is also considered. Using this machinery, we derive
the conditions required for JWST to be able to detect population III stars in
isolation. We find that a detection of individual population III stars with
JWST is unlikely at these redshifts. However, the main problem is not
necessarily that these stars are too faint, once gravitational lensing is taken
into account, but that their surface number densities are too low.Comment: 6 pages, 3 figures, proceedings of CRF2010, DESY Hamburg, Nov 9-12
201
Bayesian networks for enterprise risk assessment
According to different typologies of activity and priority, risks can assume
diverse meanings and it can be assessed in different ways. In general risk is
measured in terms of a probability combination of an event (frequency) and its
consequence (impact). To estimate the frequency and the impact (severity)
historical data or expert opinions (either qualitative or quantitative data)
are used. Moreover qualitative data must be converted in numerical values to be
used in the model. In the case of enterprise risk assessment the considered
risks are, for instance, strategic, operational, legal and of image, which many
times are difficult to be quantified. So in most cases only expert data,
gathered by scorecard approaches, are available for risk analysis. The Bayesian
Network is a useful tool to integrate different information and in particular
to study the risk's joint distribution by using data collected from experts. In
this paper we want to show a possible approach for building a Bayesian networks
in the particular case in which only prior probabilities of node states and
marginal correlations between nodes are available, and when the variables have
only two states
Microscopic description of Li in the and elastic scattering at high energies
We employ a microscopic continuum-discretized coupled-channels reaction
framework (MCDCC) to study the elastic angular distribution of the
Li nucleus colliding with C and Si targets at
=350 MeV. In this framework, the Li projectile is described
in a microscopic cluster model and impinges on non-composite targets. The
diagonal and coupling potentials are constructed from nucleon-target
interactions and Li microscopic wave functions. We obtain a fair
description of the experimental data, in the whole angular range studied, when
continuum channels are included. The inelastic and breakup angular
distributions on the lightest target are also investigated. In addition, we
compute LiC MCDCC elastic cross sections at energies much higher
than the Coulomb barrier and we use them as reference calculations to test the
validity of multichannel eikonal cross sections.Comment: 9 Pages, 6 Figure
Coulomb breakup of 22C in a four-body model
Breakup cross sections are determined for the Borromean nucleus 22C by using
a four-body eikonal model, including Coulomb corrections. Bound and continuum
states are constructed within a 20C + n + n three-body model in hyperspherical
coordinates. We compute continuum states with the correct asymptotic behavior
through the R-matrix method. For the n+ n potential, we use the Minnesota
interaction. As there is no precise experimental information on 21C, we define
different parameter sets for the 20C + n potentials. These parameter sets
provide different scattering lengths, and resonance energies of an expected
3/2+ excited state. Then we analyze the 22C ground-state energy and rms radius,
as well as E1 strength distributions and breakup cross sections. The E1
strength distribution presents an enhancement at low energies. Its amplitude is
associated with the low binding energy, rather than with a three-body
resonance. We show that the shape of the cross section at low energies is
sensitive to the ground-state properties. In addition, we suggest the existence
of a low-energy 2+ resonance, which should be observable in breakup
experiments
Effect of endurance training on lung function: A one year study
The official published version can be accessed from the link below.Objective: To identify in a follow up study airway changes occurring during the course of a sport season in healthy endurance athletes training in a Mediterranean region.
Methods: Respiratory pattern and function were analysed in 13 healthy endurance trained athletes, either during a maximal exercise test, or at rest and during recovery through respiratory manoeuvres (spirometry and closing volume tests). The exercise test was conducted on three different occasions: during basic endurance training and then during the precompetition and competitive periods.
Results: During the competitive period, a slight but non-clinically significant decrease was found in forced vital capacity (−3.5%, p = 0.0001) and an increase in slope of phase III (+25%, p = 0.0029), both at rest and after exercise. No concomitant reduction in expiratory flow rates was noticed. During maximal exercise there was a tachypnoeic shift over the course of the year (mean (SEM) breathing frequency and tidal volume were respectively 50 (2) cycles/min and 3.13 (0.09) litres during basic endurance training v 55 (3) cycles/min and 2.98 (0.10) litres during the competitive period; p<0.05).
Conclusions: This study does not provide significant evidence of lung function impairment in healthy Mediterranean athletes after one year of endurance training
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