93 research outputs found
Effective Hamiltonians with Relativistic Corrections I: The Foldy--Wouthuysen transformation versus the direct Pauli reduction
Two different methods of obtaining ``effective Hamiltonians''
which include relativistic corrections to nonrelativistic calculations are
discussed, the standard Foldy--Wouthuysen transformation and what we call the
``direct Pauli reduction''. We wish to investigate under which circumstances
the two approaches yield the same result. Using a generic interaction with
harmonic time dependence we show that differences in the corresponding
effective S--matrices do arise beyond first--order perturbation theory. We
attribute them to the fact that the use of the direct reduction effective
Hamiltonian involves the additional approximation of neglecting contributions
from the negative--energy intermediate states, an approximation which is
unnecessary in the Foldy--Wouthuysen case as there the Hamiltonian
does not connect positive-- and negative--energy states. We conclude that at
least in the cases where the relativistic Hamiltonian is known, using the
direct Pauli reduction effective Hamiltonian introduces spurious relativistic
effects and therefore the Foldy--Wouthuysen reduction should be preferred.Comment: TRIUMF preprint TRI-PP-93-1
Neutron beta decay in effective field theory
Radiative corrections to the lifetime and angular correlation coefficients of
neutron beta-decay are evaluated in effecitive field theory. We also evaluate
the lowest order nucleon recoil corrections, including weak-magnetism. Our
results agree with those of the long-range and model-independent part of
previous calculations. In an effective theory the model-dependent radiative
corrections are replaced by well-defined low-energy constants. The effective
field theory allows a systematic evaluation of higher order corrections to our
results to the extent that the relevant low-energy constants are known.Comment: 13 pages, 1 figure; two references added, minor correctio
The equivalence theorem and the Bethe-Salpeter equation
We solve the Bethe-Salpeter equation for two-particle scattering in a
field-theoretical model using two lagrangians related by a field
transformation. The kernel of the equation consists of the sum of all
tree-level diagrams for each lagrangian. The solutions differ even if all four
external particles are put on the mass shell, which implies that observables
calculated by solving the Bethe-Salpeter equation depend on the representation
of the theory. We point out that this violation of the equivalence theorem has
a simple explanation and should be expected for any Bethe-Salpeter equation
with a tree-level kernel. Implications for dynamical models of hadronic
interactions are discussed.Comment: 10 pages, 4 figures, using REVTeX. Fig. 2 corrected, results
unchanged, to be published in Phys. Lett.
Global regionalized characterization factors for phosphorus and nitrogen impacts on freshwater fish biodiversity
Inefficient global nutrient (i.e., phosphorus (P) and nitrogen (N)) management leads to an increase in nutrient delivery to freshwater and coastal ecosystems and induces eutrophication in these aquatic environments. This process threatens the various species inhabiting these ecosystems. In this study, we developed regionalized characterization factors (CFs) for freshwater eutrophication at 0.5 Ă 0.5-degree resolution, considering different fates for direct emissions to freshwater, diffuse emissions, and increased erosion due to agricultural land use. The CFs were provided for global and regional species loss of freshwater fish. CFs for global species loss were quantified by integrating global extinction probabilities. Results showed that the CFs for P and N impacts on freshwater fish are higher in densely populated regions that encompass either large lakes or the headwaters of large rivers. Focusing on nutrient-limited areas increases country-level CFs in 51.9 % of the countries for P and 49.5 % of the countries for N compared to not considering nutrient limitation. This study highlights the relevance of considering freshwater eutrophication impacts via both P and N emissions and identifying the limiting nutrient when performing life cycle impact assessments
Effects of Nitrogen Emissions on Fish Species Richness across the Worldâs Freshwater Ecoregions
The increasing application of synthetic fertilizer has tripled nitrogen (N) inputs over the 20th century. N enrichment decreases water quality and threatens aquatic species such as fish through eutrophication and toxicity. However, the impacts of N on freshwater ecosystems are typically neglected in life cycle assessment (LCA). Due to the variety of environmental conditions and species compositions, the response of species to N emissions differs among ecoregions, requiring a regionalized effect assessment. Our study tackled this issue by establishing regionalized species sensitivity distributions (SSDs) of freshwater fish against N concentrations for 367 ecoregions and 48 combinations of realms and major habitat types globally. Subsequently, effect factors (EFs) were derived for LCA to assess the effects of N on fish species richness at a 0.5 degree Ă 0.5 degree resolution. Results show good SSD fits for all of the ecoregions that contain sufficient data and similar patterns for average and marginal EFs. The SSDs highlight strong effects on species richness due to high N concentrations in the tropical zone and the vulnerability of cold regions. Our study revealed the regional differences in sensitivities of freshwater ecosystems against N content in great spatial detail and can be used to assess more precisely and comprehensively nutrient-induced impacts in LCA
Chiral effective field theories of the strong interactions
Effective field theories of the strong interactions based on the approximate
chiral symmetry of QCD provide a model-independent approach to low-energy
hadron physics. We give a brief introduction to mesonic and baryonic chiral
perturbation theory and discuss a number of applications. We also consider the
effective field theory including vector and axial-vector mesons.Comment: 22 pages, 9 figures, proceedings of "Many-Body Structure of Strongly
Interacting Systems", Mainz, Germany, Feb. 23-25 201
Dressing the nucleon in a dispersion approach
We present a model for dressing the nucleon propagator and vertices. In the
model the use of a K-matrix approach (unitarity) and dispersion relations
(analyticity) are combined. The principal application of the model lies in
pion-nucleon scattering where we discuss effects of the dressing on the phase
shifts.Comment: 17 pages, using REVTeX, 6 figure
Relativistic Heavy--Ion Collisions in the Dynamical String--Parton Model
We develop and extend the dynamical string parton model. This model, which is
based on the salient features of QCD, uses classical Nambu-Got\=o strings with
the endpoints identified as partons, an invariant string breaking model of the
hadronization process, and interactions described as quark-quark interactions.
In this work, the original model is extended to include a phenomenological
quantization of the mass of the strings, an analytical technique for treating
the incident nucleons as a distribution of string configurations determined by
the experimentally measured structure function, the inclusion of the gluonic
content of the nucleon through the introduction of purely gluonic strings, and
the use of a hard parton-parton interaction taken from perturbative QCD
combined with a phenomenological soft interaction. The limited number of
parameters in the model are adjusted to and -- data. Utilizing
these parameters, the first calculations of the model for -- and
-- collisions are presented and found to be in reasonable agreement with
a broad set of data.Comment: 26 pages of text with 23 Postscript figures placed in tex
Electromagnetic transitions in an effective chiral Lagrangian with the eta-prime and light vector mesons
We consider the chiral Lagrangian with a nonet of Goldstone bosons and a
nonet of light vector mesons. The mixing between the pseudoscalar mesons eta
and eta-prime is taken into account. A novel counting scheme is suggested that
is based on hadrogenesis, which conjectures a mass gap in the meson spectrum of
QCD in the limit of a large number of colors. Such a mass gap would justify to
consider the vector mesons and the eta-prime meson as light degrees of freedom.
The complete leading order Lagrangian is constructed and discussed. As a first
application it is tested against electromagnetic transitions of light vector
mesons to pseudoscalar mesons. Our parameters are determined by the
experimental data on photon decays of the omega, phi and eta-prime meson. In
terms of such parameters we predict the corresponding decays into virtual
photons with either dielectrons or dimuons in the final state.Comment: 17 pages, extended discussion on mixin
Epidemiological impact of waning immunization on a vaccinated population
This is an epidemiological SIRV model based study that is de- signed to analyze the impact of vaccination in containing infection spread, in a 4-tiered population compartment comprised of susceptible, infected, recov- ered and vaccinated agents. While many models assume a lifelong protection through vaccination, we focus on the impact of waning immunization due to conversion of vaccinated and recovered agents back to susceptible ones. Two asymptotic states exist, the \disease-free equilibrium" and the \endemic equi- librium" and we express the transitions between these states as function of the vaccination and conversion rates and using the basic reproduction number. We nd that the vaccination of newborns and adults have dierent consequences on controlling an epidemic. Also, a decaying disease protection within the re- covered sub-population is not sucient to trigger an epidemic on the linear level. We perform simulations for a parameter set modelling a disease with waning immunization like pertussis. For a diusively coupled population, a transition to the endemic state can proceed via the propagation of a traveling infection wave, described successfully within a Fisher-Kolmogorov framework
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