1,211 research outputs found
Vector mesons in a relativistic point-form approach
We apply the point form of relativistic quantum mechanics to develop a
Poincare invariant coupled-channel formalism for two-particle systems
interacting via one-particle exchange. This approach takes the exchange
particle explicitly into account and leads to a generalized eigenvalue equation
for the Bakamjian-Thomas type mass operator of the system. The coupling of the
exchange particle is derived from quantum field theory. As an illustrative
example we consider vector mesons within the chiral constituent quark model in
which the hyperfine interaction between the confined quark-antiquark pair is
generated by Goldstone-boson exchange. We study the effect of retardation in
the Goldstone-boson exchange by comparing with the commonly used instantaneous
approximation. As a nice physical feature we find that the problem of a too
large - splitting can nearly be avoided by taking the dynamics of
the exchange meson explicitly into account.Comment: 14 pages, 1 figur
Electroweak properties of baryons in a covariant chiral quark model
The proton and neutron electromagnetic form factors and the nucleon axial
form factor have been calculated in the Goldstone-boson exchange
constituent-quark model within the point-form approach to relativistic quantum
mechanics. The results, obtained without any adjustable parameter nor quark
form factors, are, due to the dramatic effects of the boost required by the
covariant treatment, in striking agreement with the data.Comment: Proceedings of the Conference N*2001, Mainz; 4 pages, 3 figures
included in eps format; World Scientific style file include
Effects of large herbivores on grassland arthropod diversity
Both arthropods and large grazing herbivores are important components and drivers of biodiversity in grassland ecosystems, but a synthesis of how arthropod diversity is affected by large herbivores has been largely missing. To fill this gap, we conducted a literature search, which yielded 141 studies on this topic of which 24 simultaneously investigated plant and arthropod diversity. Using the data from these 24 studies, we compared the responses of plant and arthropod diversity to an increase in grazing intensity. This quantitative assessment showed no overall significant effect of increasing grazing intensity on plant diversity, while arthropod diversity was generally negatively affected. To understand these negative effects, we explored the mechanisms by which large herbivores affect arthropod communities: direct effects, changes in vegetation structure, changes in plant community composition, changes in soil conditions, and cascading effects within the arthropod interaction web. We identify three main factors determining the effects of large herbivores on arthropod diversity: (i) unintentional predation and increased disturbance, (ii) decreases in total resource abundance for arthropods (biomass) and (iii) changes in plant diversity, vegetation structure and abiotic conditions. In general, heterogeneity in vegetation structure and abiotic conditions increases at intermediate grazing intensity, but declines at both low and high grazing intensity. We conclude that large herbivores can only increase arthropod diversity if they cause an increase in (a)biotic heterogeneity, and then only if this increase is large enough to compensate for the loss of total resource abundance and the increased mortality rate. This is expected to occur only at low herbivore densities or with spatio-temporal variation in herbivore densities. As we demonstrate that arthropod diversity is often more negatively affected by grazing than plant diversity, we strongly recommend considering the specific requirements of arthropods when applying grazing management and to include arthropods in monitoring schemes. Conservation strategies aiming at maximizing heterogeneity, including regulation of herbivore densities (through human interventions or top-down control), maintenance of different types of management in close proximity and rotational grazing regimes, are the most promising options to conserve arthropod diversity
Nucleon electromagnetic and axial form factors in point-form relativistic quantum mechanics
Results for the proton and neutron electric and magnetic form factors as well
as the nucleon axial form factor are presented for constituent quark models,
based on either one-gluon-exchange and Goldstone-boson-exchange dynamics. The
calculations are performed in a covariant framework using the point-form
approach to relativistic quantum mechanics. The only input to the calculations
is the nucleon wave function of the corresponding constituent quark model. A
comparison is given to results of the instanton-induced constituent quark model
treated with the Bethe-Salpeter equation.Comment: 4 pages, 6 figures, contribution to XVIII European Conference on
Few-Body Problems in Physics, September 2002, Bled, Sloveni
Covariant axial form factor of the nucleon in a chiral constituent quark model
The axial form factor G_A of the nucleon is investigated for the
Goldstone-boson-exchange constituent quark model using the point-form approach
to relativistic quantum mechanics. The results, being covariant, show large
contributions from relativistic boost effects. The predictions are obtained
directly from the quark-model wave functions, without any further input such as
vertex or constituent-quark form factors, and fall remarkably close to the
available experimental data.Comment: 10 pages, 1 figure in .eps format, typeset with Elsevier elsart style
files included. Revised version with a newly added section about discussion
of results. To appear in Phys. Lett.
Covariant electroweak nucleon form factors in a chiral constituent quark model
Results for the proton and neutron electric and magnetic form factors as well
asthe nucleon axial and induced pseudoscalar form factors are presented for the
chiral constituent quark model based on Goldstone-boson-exchange dynamics. The
calculations are performed in a covariant framework using the point-form
approach to relativistic quantum mechanics. The direct predictions of the model
yield a remarkably consistent picture of the electroweak nucleon structure.Comment: 4 pages, 3 figures revte
Spectator-model operators in point-form relativistic quantum mechanics
We address the construction of transition operators for electromagnetic,
weak, and hadronic reactions of relativistic few-quark systems along the
spectator model. While the problem is of relevance for all forms of
relativistic quantum mechanics, we specifically adhere to the point form, since
it preserves the spectator character of the corresponding transition operators
in any reference frame. The conditions imposed on the construction of
point-form spectator-model operators are discussed and their implications are
exemplified for mesonic decays of baryon resonances within a relativistic
constituent quark model.Comment: 10 pages, 4 figures, updated version accepted for publication in
Europ. Phys. J.
Relativistic Quantum Mechanics - Particle Production and Cluster Properties
This paper constructs relativistic quantum mechanical models of particles
satisfying cluster properties and the spectral condition which do not conserve
particle number. The treatment of particle production is limited to systems
with a bounded number of bare-particle degrees of freedom. The focus of this
paper is about the realization of cluster properties in these theories.Comment: 36 pages, Late
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