796 research outputs found
Variation in performance of beech saplings of 7 European provenances under shade and full light conditions
The use of beech seedlings from South-East European and North-West (NW) provenances for underplanting in coniferous forests in North-West Europe was investigated by means of experimental shading. The effects of this treatment on survival, morphology, phenology, physiology and growth were analysed by applying an individual plant growth model integrating these aspects. It was concluded that plant performance under full-light conditions are representative of shaded conditions, so good performing provenances can be selected in a field situation. It was further concluded that good performing South-East European seedlings can be used in North-West European conditions. The modelling results indicated an interesting trade-off between height growth and biomass increase and different provenances show different strategies. This allows selection of suitable provenances for specific situations, e.g. when beech seedlings need to compete with other plant species in the understorey
Chiral symmetry restoration in linear sigma models with different numbers of quark flavors
Chiral symmetry restoration at nonzero temperature is studied in the
framework of the O(4) linear sigma model and the U(N_f)_r x U(N_f)_l linear
sigma model with N_f=2,3, and 4 quark flavors. We investigate the temperature
dependence of the masses of the scalar and pseudoscalar mesons, and the
non-strange, strange, and charm condensates within the Hartree approximation as
derived from the Cornwall-Jackiw-Tomboulis formalism. We find that the masses
of the non-strange and strange mesons at nonzero temperature depend sensitively
on the particular symmetry of the model and the number of light quark flavors
N_f. On the other hand, due to the large charm quark mass, neither do charmed
mesons significantly affect the properties of the other mesons, nor do their
masses change appreciably in the temperature range around the chiral symmetry
restoration temperature. In the chiral limit, the transition temperatures for
chiral symmetry restoration are surprisingly close to those found in lattice
QCD.Comment: 28 pages, 8 figure
Microscopic description of light unstable nuclei with the stochastic variational method
The structure of the light proton and neutron rich nuclei is studied in a
microscopic multicluster model using the stochastic variational method. This
approach enables us to describe the weakly bound nature of these nuclei in a
consistent way. Applications for various nuclei Li, Be, B,
C, Be, B presented. The paper discusses the relation of
this model to other models as well as the possible extension for p and sd shell
nuclei.Comment: 11 pages, latex, no figures
Quantum state of a free spin-1/2 particle and the inextricable dependence of spin and momentum under Lorentz transformations
We revise the Dirac equation for a free particle and investigate Lorentz
transformations on spinors. We study how the spin quantization axis changes
under Lorentz transformations, and evince the interplay between spin and
momentum in this context.Comment: 14 pages, 3 figures, published as a Review in the IJQ
A T-Matrix Calculation for in-Medium Heavy-Quark Gluon Scattering
The interactions of charm and bottom quarks in a Quark-Gluon Plasma (QGP) are
evaluated using a thermodynamic 2-body T-matrix. We specifically focus on
heavy-quark (HQ) interactions with thermal gluons with an input potential
motivated by lattice-QCD computations of the HQ free energy. The latter is
implemented into a field-theoretic ansatz for color-Coulomb and (remnants of)
confining interactions. This, in particular, enables to discuss corrections to
the potential approach, specifically hard-thermal-loop corrections to the
vertices, relativistic corrections deduced from pertinent Feynman diagrams, and
a suitable projection on transverse thermal gluons. The resulting potentials
are applied to compute scattering amplitudes in different color channels and
utilized for a calculation of the corresponding HQ drag coefficient in the QGP.
A factor of ~2-3 enhancement over perturbative results is obtained, mainly
driven by the resummation in the attractive color-channels
Improving Orbit Estimates for Incomplete Orbits with a New Approach to Priors -- with Applications from Black Holes to Planets
We propose a new approach to Bayesian prior probability distributions
(priors) that can improve orbital solutions for low-phase-coverage orbits,
where data cover less than approximately 40% of an orbit. In instances of low
phase coverage such as with stellar orbits in the Galactic center or with
directly-imaged exoplanets, data have low constraining power and thus priors
can bias parameter estimates and produce under-estimated confidence intervals.
Uniform priors, which are commonly assumed in orbit fitting, are notorious for
this. We propose a new observable-based prior paradigm that is based on
uniformity in observables. We compare performance of this observable-based
prior and of commonly assumed uniform priors using Galactic center and
directly-imaged exoplanet (HR 8799) data. The observable-based prior can reduce
biases in model parameters by a factor of two and helps avoid under-estimation
of confidence intervals for simulations with less than about 40% phase
coverage. Above this threshold, orbital solutions for objects with sufficient
phase coverage such as S0-2, a short-period star at the Galactic center with
full phase coverage, are consistent with previously published results. Below
this threshold, the observable-based prior limits prior influence in regions of
prior dominance and increases data influence. Using the observable-based prior,
HR 8799 orbital analyses favor lower eccentricity orbits and provide stronger
evidence that the four planets have a consistent inclination around 30 degrees
to within 1-sigma. This analysis also allows for the possibility of
coplanarity. We present metrics to quantify improvements in orbital estimates
with different priors so that observable-based prior frameworks can be tested
and implemented for other low-phase-coverage orbits.Comment: Published in AJ. 23 pages, 14 figures. Monte Carlo chains are
available in the published article, or are available upon reques
Nonequilibrium evolution of Phi**4 theory in 1+1 dimensions in the 2PPI formalism
We consider the out-of-equilibrium evolution of a classical condensate field
and its quantum fluctuations for a Phi**4 model in 1+1 dimensions with a
symmetric and a double well potential. We use the 2PPI formalism and go beyond
the Hartree approximation by including the sunset term. In addition to the mean
field phi= the 2PPI formalism uses as variational parameter a time
dependent mass M**2(t) which contains all local insertions into the Green
function. We compare our results to those obtained in the Hartree
approximation. In the symmetric Phi**4 theory we observe that the mean field
shows a stronger dissipation than the one found in the Hartree approximation.
The dissipation is roughly exponential in an intermediate time region. In the
theory with spontaneous symmetry breaking, i.e., with a double well potential,
the field amplitude tends to zero, i.e., to the symmetric configuration. This
is expected on general grounds: in 1+1 dimensional quantum field theory there
is no spontaneous symmetry breaking for T >0, and so there should be none at
finite energy density (microcanonical ensemble), either. Within the time range
of our simulations the momentum spectra do not thermalize and display
parametric resonance bands.Comment: 14 pages, 18 encapsulated postscript figures; v2 minor changes, new
appendix, accepted for publication in Phys.Rev.
Pion damping width from SU(2) x SU(2) NJL model
Within the framework of the NJL model, we investigate the modification of the
pion damping width in a hot pion gas for temperatures ranging from 0 to 180
MeV. The pion is found to broaden noticeably at T > 60 MeV. Near the chiral
phase transition T ~ 180 MeV, the pion width is saturated and amounts to 70
MeV. The main contribution to the width comes from pion-pion collisions. Other
contributions are found negligibly small.Comment: LaTeX2e, 13 pages, 2 figure
Hard gluon damping in hot QCD
The gluon collisional width in hot QCD plasmas is discussed with emphasis on
temperatures near , where the coupling is large. Considering its effect on
the entropy, which is known from lattice calculations, it is argued that the
width, which in the perturbative limit is given by , should be sizeable at intermediate temperatures but has to be small close
to . Implications of these results for several phenomenologically relevant
quantities, such as the energy loss of hard jets, are pointed out.Comment: uses RevTex and graphic
Low mass dilepton radiation at RHIC
In this work we discuss the emission of low mass dilepton radiation from a
hydrodynamic evolution model of Au-Au collisions and make comparisons with
recent PHENIX measurements. The dilepton emission rates from the hadronic phase
are treated at finite temperature and baryon density and are completely
constrained by broken chiral symmetry in a density expansion. The rates are
expressed in terms of vacuum correlators which are measured in
annihilation, decays and photo-reactions on nucleons and nuclei. We
consider two possibilities for the hadronic phase: A chemical equilibrated an
off equilibrium hadronic gas. We find that while chemical off-equilibrium helps
explain part of the low mass (0.15 M GeV 0.7) enhancement seen in
the data there is still a large discrepancy.Comment: 8 pages, 12 figure
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