1,385 research outputs found
The Kinetic Interpretation of the DGLAP Equation, its Kramers-Moyal Expansion and Positivity of Helicity Distributions
According to a rederivation - due to Collins and Qiu - the DGLAP equation can
be reinterpreted (in leading order) in a probabilistic way. This form of the
equation has been used indirectly to prove the bound
between polarized and unpolarized distributions, or positivity of the helicity
distributions, for any . We reanalize this issue by performing a detailed
numerical study of the positivity bounds of the helicity distributions. To
obtain the numerical solution we implement an x-space based algorithm for
polarized and unpolarized distributions to next-to-leading order in ,
which we illustrate. We also elaborate on some of the formal properties of the
Collins-Qiu form and comment on the underlying regularization, introduce a
Kramers-Moyal expansion of the equation and briefly analize its Fokker-Planck
approximation. These follow quite naturally once the master version is given.
We illustrate this expansion both for the valence quark distribution and
for the transverse spin distribution .Comment: 38 pages, 27 figures, Dedicated to Prof. Pierre Ramond for his 60th
birthda
Agriculture's contribution to national emissions
This info note offers an overview of the distribution of agricultural emissions among countries and the relative contribution of agriculture to national emissions. It is based on three data sources: the FAOSTAT database of greenhouse gas emissions from agriculture, United States Environmental Protection Agency (EPA) global emission estimates for 2010 and national reports to the United Nations Framework Convention on Climate Change (UNFCCC).
Polarized parton distributions from NLO QCD analysis of world DIS and SIDIS data
The combined analysis of polarized DIS and SIDIS data is performed in NLO
QCD. The new parametrization on polarized PDFs is constructed. The
uncertainties on PDFs and their first moments are estimated applying the
modified Hessian method.
The especial attention is paid to the impact of novel SIDIS data on the
polarized distributions of light sea and strange quarks. In particular, the
important question of polarized sea symmetry is studied in comparison with the
latest results on this subject
Interaction of small size wave packet with hadron target
We calculate in QCD the cross section for the scattering of an energetic
small-size wave packet off a hadron target. We use our results to study the
small- behaviour of , the distribution over cross
section for the pion, in the leading -order.Comment: Revised version of the report CEBAF-TH-96-0
Higher twists in the pion structure function
We calculate the QCD moments of the pion structure function using Drell-Yan
data on the quark distributions in the pion and a phenomenological model for
the resonance region. The extracted higher twist corrections are found to be
larger than those for the nucleon, contributing around 50% of the lowest moment
at Q^2=1 GeV^2.Comment: 8 pages, 3 figures, to appear in Phys. Rev.
Leading Chiral Contributions to the Spin Structure of the Proton
The leading chiral contributions to the quark and gluon components of the
proton spin are calculated using heavy-baryon chiral perturbation theory.
Similar calculations are done for the moments of the generalized parton
distributions relevant to the quark and gluon angular momentum densities. These
results provide useful insight about the role of pions in the spin structure of
the nucleon, and can serve as a guidance for extrapolating lattice QCD
calculations at large quark masses to the chiral limit.Comment: 8 pages, 2 figures; a typo in Ref. 7 correcte
Comment on the recent COMPASS data on the spin structure function g_1
We examine the recent COMPASS data on the spin structure function g_1
singlet. We show that it is rather difficult to use the data in the present
form in order to draw conclusions on the initial parton densities. However, our
tentative estimate is that the data better agree with positive rather than
negative initial gluon densities.Comment: 8 pages, 1 figur
The Non-Trapping Degree of Scattering
We consider classical potential scattering. If no orbit is trapped at energy
E, the Hamiltonian dynamics defines an integer-valued topological degree. This
can be calculated explicitly and be used for symbolic dynamics of
multi-obstacle scattering.
If the potential is bounded, then in the non-trapping case the boundary of
Hill's Region is empty or homeomorphic to a sphere.
We consider classical potential scattering. If at energy E no orbit is
trapped, the Hamiltonian dynamics defines an integer-valued topological degree
deg(E) < 2. This is calculated explicitly for all potentials, and exactly the
integers < 2 are shown to occur for suitable potentials.
The non-trapping condition is restrictive in the sense that for a bounded
potential it is shown to imply that the boundary of Hill's Region in
configuration space is either empty or homeomorphic to a sphere.
However, in many situations one can decompose a potential into a sum of
non-trapping potentials with non-trivial degree and embed symbolic dynamics of
multi-obstacle scattering. This comprises a large number of earlier results,
obtained by different authors on multi-obstacle scattering.Comment: 25 pages, 1 figure Revised and enlarged version, containing more
detailed proofs and remark
Cronin Effect in Hadron Production off Nuclei
Recent data from RHIC for high- hadrons in gold-gold collisions raised
again the long standing problem of quantitatively understanding the Cronin
effect, i.e. nuclear enhancement of high- hadrons due to multiple
interactions in nuclear matter. In nucleus-nucleus collisions this effect has
to be reliably calculated as baseline for a signal of new physics in high-
hadron production. The only possibility to test models is to compare with
available data for collisions, however, all existing models for the Cronin
effect rely on a fit to the data to be explained. We develop a phenomenological
description based on the light-cone QCD-dipole approach which allows to explain
available data without fitting to them and to provide predictions for
collisions at RHIC and LHC. We point out that the mechanism causing Cronin
effect drastically changes between the energies of fixed target experiments and
RHIC-LHC. High- hadrons are produced incoherently on different nucleons at
low energies, whereas the production amplitudes interfere if the energy is
sufficiently high.Comment: the final version to appear in Phys. Rev. Let
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