60 research outputs found
The Valence Quark Distribution of the Pion
The pion structure function is investigated in a simple model, where the pion
and its constituent quark fields are coupled through the simplest pseudoscalar
coupling. The imaginary part of the forward gamma* pi -> gamma* pi scattering
amplitude is evaluated and related to the structure functions. It is shown that
the introduction of non-perturbative effects, linked to the size of the pion,
allows a connection with the quark distribution. It is predicted that
higher-twist terms become negligible for Q^2 larger than ~2 GeV^2, that quarks
in the pion have a momentum fraction smaller than in the proton case, and that
the momentum sum rule is violated for the pion.Comment: Presented by J. P. Lansberg at the XXII Physics in Collision
Conference (PIC02), Stanford, California, June 20-22, 2002, 3 pages, 4
figures, LaTeX, uses pic02.sty (included
A toy model for generalised parton distributions
We give the results of a simple model for the diagonal and off-diagonal
valence quark distributions of a pion. We show that structure can be
implemented in a gauge-invariant manner. This explicit model questions the
validityof the momentum sum rule, and gives an explicit counter-example to the
Wandzura-Wilczek ansatz for twist-3 GPD's.Comment: 11 pages, presented at HADRONIC PHYSICS (HLPR 2004): Joint
Meeting-Heidelberg-Liege-Paris-Rostock, Spa, Belgium, 16-18 December 2004, 11
pages, 11 figures, LaTe
Pion Structure Function and Violation of the Momentum Sum Rule
We present a method to evaluate the pion structure functions from a box
diagram calculation. Pion and constituent quark fields are coupled through the
simplest pseudoscalar coupling. The gamma^* pi -> q \bar q cross-section is
evaluated and related to the structure functions. We then show that the
introduction of non-perturbative effects, related to the pion size and
preserving gauge invariance, provides us with a straighforward relation with
the quark distribution. It is predicted that higher-twist terms become
negligible for Q^2 larger than about 2 GeV^2 and that quarks in the pion have a
momentum fraction smaller than in the proton. We enlarge the discussion
concerning this violation of the momentum sum rule, emphasizing that the sum
rule is recovered in the chiral limit and also when the finite size condition
is not imposed.Comment: Presented by J.P. Lansberg at the II International Workshop on Hadron
Physics, 25-29 September, 2002, Coimbra, Portugal, 14 pages, 9 figures,
LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included
Confinement and cut-off: a model for the pion quark distribution function
The pion structure function is investigated in a simple pseudo-scalar
coupling model of pion and constituent quark fields. The imaginary part of the
forward Compton scattering amplitude is evaluated. We show that the
introduction of non-perturbative effects, linked through a cut-off to the size
of the pion, allows the reproduction of important features of the pion quark
distribution function.Comment: 3 pages, 1 figur
A Model for the Pion Structure Function
The pion structure function is investigated in a simple model, where pion and
constituent quark fields are coupled through the simplest pseudoscalar
coupling. The imaginary part of the forward gamma* pi-> gamma* pi scattering
amplitude is evaluated and related to the structure functions. It is shown that
the introduction of non-perturbative effects, linked to the size of the pion
and preserving gauge invariance, allows a connection with the quark
distribution. It is predicted that higher-twist terms become negligible for Q2
larger than about 2 GeV2 and that quarks in the pion have a momentum fraction
smaller than in the proton case.Comment: 14 pages, 6 figures, LaTeX, elsart clas
Proposal of a principle cum scale analytical framework for analyzing agroecological development projects
Because agroecology has different meanings, it may be used in an arbitrary and potentially abusive way when deployed by development cooperation actors conceiving “agroecology-based” development projects. To make the appropriation of agroecology more transparent, we first review the recent attempts in academia to clarify the concept and identify two main trends: a principle-based agroecology and a series of different agroecologies. Based on a critical assessment of these attempts, we then propose a new framework to program, implement and analyze agroecological development projects: it distinguishes different agroecologies with their corresponding categories of principles and their scales of intervention. Further, we argue in favor of a specific category of methodological principles.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Dilepton production in proton-proton collisions at BEVALAC energies
The dilepton production in elementary reactions at
BEVALAC energies GeV is investigated. The calculations
include direct decays of the vector mesons , , and , Dalitz decays of the -, -, -, -, and -mesons, and of the baryon resonances
. The subthreshold vector meson production cross sections in
collisions are treated in a way sufficient to avoid double counting with the
inclusive vector meson production. The vector meson dominance model for the
transition form factors of the resonance Dalitz decays is
used in an extended form to ensure correct asymptotics which are in agreement
with the quark counting rules. Such a modification gives an unified and
consistent description of both radiative decays and meson decays.
The effect of multiple pion production on the experimental efficiency for the
detection of the dilepton pairs is studied. We find the dilepton yield in
reasonable agreement with the experimental data for the set of intermediate
energies whereas at the highest energy GeV the number of
dilepton pairs is likely to be overestimated experimentally in the mass range
MeV.Comment: 25 pages (IOP style), 5 figures, revised manuscript accepted for
publication in JP
The Lore of Low Methane Livestock:Co-Producing Technology and Animals for Reduced Climate Change Impact
Methane emissions from sheep and cattle production have gained increasing profile in the context of climate change. Policy and scientific research communities have suggested a number of technological approaches to mitigate these emissions. This paper uses the concept of co-production as an analytical framework to understand farmers’ evaluation of a 'good animal’. It examines how technology and sheep and beef cattle are co-produced in the context of concerns about the climate change impact of methane. Drawing on 42 semi-structured interviews, this paper demonstrates that methane emissions are viewed as a natural and integral part of sheep and beef cattle by farmers, rather than as a pollutant. Sheep and beef cattle farmers in the UK are found to be an extremely heterogeneous group that need to be understood in their specific social, environmental and consumer contexts. Some are more amenable to appropriating methane reducing measures than others, but largely because animals are already co-constructed from the natural and the technical for reasons of increased production efficiency
What is the structure of the Roper resonance?
We investigate the structure of the nucleon resonance N^*(1440) (Roper)
within a coupled-channel meson exchange model for pion-nucleon scattering. The
coupling to pipiN states is realized effectively by the coupling to the sigmaN,
piDelta and rhoN channels. The interaction within and between these channels is
derived from an effective Lagrangian based on a chirally symmetric Lagrangian,
which is supplemented by well known terms for the coupling of the Delta isobar,
the omega meson and the 'sigma', which is the name given here to the strong
correlation of two pions in the scalar-isoscalar channel. In this model the
Roper resonance can be described by meson-baryon dynamics alone; no genuine
N^*(1440) (3 quark) resonance is needed in order to fit piN phase shifts and
inelasticities.Comment: 55 pages, 14 figure
Sigma photoproduction in the resonance region
A study of p(gamma,K)Sigma processes in an isobar model at tree level is
reported. By comparing model calculations to the published SAPHIR data, we
explore the possible role of different isospin I=1/2 (N*) and I=3/2 (Delta*)
resonances in the reaction dynamics. In our analysis, the inclusion of the
``missing'' D_{13}(1895) resonance does only slightly improve the global
description of the Sigma photoproduction data. More convincing signals for the
presence of such a ``missing'' resonance emerged in the analysis of the isospin
related p(gamma,K+)Lambda reaction. Various implementations of the nonresonant
part of the Sigma photoproduction amplitude are presented. The sensitivity of
the computed observables and extracted resonance parameters to the
uncertainties inherent to the treatment of the nonresonant (background)
diagrams are discussed.Comment: subsection and table added, to appear in Phys. Rev.
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