1,269 research outputs found
Some Remarks on Methods of QCD Analysis of Polarized DIS Data
The results on polarized parton densities (PDFs) obtained using different
methods of QCD analysis of the present polarized DIS data are discussed. Their
dependence on the method used in the analysis, accounting or not for the
kinematic and dynamic 1/Q^2 corrections to spin structure function g_1, is
demonstrated. It is pointed out that the precise data in the preasymptotic
region require a more careful matching of the QCD predictions to the data in
this region in order to determine the polarized PDFs correctly.Comment: 14 pages, 8 figure
A New Study of the Polarized Parton Densities in the Nucleon
We present a new next-to-leading order QCD analysis of the world data on
inclusive polarized deep inelastic lepton-nucleon scattering adding to the old
set of data the final SMC results, the HERMES proton and very recent SLAC/E155
deuteron data. We find an excellent fit to the data and present results for the
polarized parton densities in different factorization schemes. These results
are in a good agreement with what follows from the theory. We have also found
that the main effect of the newly incorporated data is a better determination
of the polarized gluon density.Comment: 9 pages, latex, 5 ps figures Presented at the 7th International
Workshop on Deep Inelastic Scattering and QCD, Zeuthen, Germany, April 19-23,
199
On Combining High and Low Q**2 Information on the Polarized Parton Densities
We draw attention to some problems in the combined use of high-Q^2 deep
inelastic scattering (DIS) data and low-Q^2 hyperon \beta-decay data in the
determination of the polarized parton densities. We explain why factorization
schemes like the JET or AB schemes are the simplest in which to study the
implications of the DIS parton densities for the physics of the low-Q^2 region.Comment: 10 pages, LaTeX, minor corrections made in Section 4, a reference
added, to be published in JHE
On the controversy concerning the definition of quark and gluon angular momentum
A major controversy has arisen in QCD as to how to split the total angular
momentum into separate quark and gluon contributions, and as to whether the
gluon angular momentum can itself be split, in a gauge invariant way, into a
spin and orbital part. Several authors have proposed various answers to these
questions and offered a variety of different expressions for the relevant
operators. I argue that none of these is acceptable and suggest that the
canonical expression for the momentum and angular momentum operators is the
correct and physically meaningful one. It is then an inescapable fact that the
gluon angular momentum operator cannot, in general, be split in a gauge
invariant way into a spin and orbital part. However, the projection of the
gluon spin onto its direction of motion i.e. its helicity is gauge invariant
and is measured in deep inelastic scattering on nucleons. The Ji sum rule,
relating the quark angular momentum to generalized parton distributions, though
not based on the canonical operators, is shown to be correct, if interpreted
with due care. I also draw attention to several interesting aspects of QED and
QCD, which, to the best of my knowledge, are not commented upon in the standard
textbooks on Field Theory.Comment: 41 pages; Some incorrect statements have been rectified and a
detailed discussion has been added concerning the momentum carried by quarks
and the Ji sum rule for the angular momentu
Determination of the fragmentation functions from an NLO QCD analysis of the HERMES data on pion multiplicities
An NLO QCD analysis of the final HERMES data on pion multiplicities is
presented and a new set of pion fragmentation functions is extracted from the
best fit to the data. We have studied the so-called [x,z] and [Q^2,z]
presentations of their data, as given by HERMES, which, in principle, should
simply be two different ways of presenting the experimental data. We have based
our extraction on an excellent fit to the [Q^2,z] presentation of the data. We
also draw attention to what appears to be a problem with the [x,z] presentation
of the HERMES data.Comment: 12 pages, 7 figures; This updated version corresponds to the paper
accepted for publication in Phys. Rev.
The Role of Higher Twist in Determining Polarized Parton Densities from DIS data
Different methods to extract the polarized parton densities from the world
polarized DIS data are considered. The higher twist corrections to
the spin dependent proton and neutron structure functions are found to be
non-negligible and important in the QCD analysis of the present experimental
data. Their role in determining the polarized parton densities in the framework
of the different approaches is discussed.Comment: To appear in the Proceedings of the Spin2004 Symposium, Trieste,
11-16 Oct 200
Higher Twist Effects in Polarized DIS
The size of higher twist corrections to the spin proton and neutron g1
structure functions and their role in determining the polarized parton
densities in the nucleon is discussed.Comment: 6 pages, 2 figures, to appear in the proceedings of First Workshop on
Quark-Hadron Duality and the Transition to pQCD, Frascati, June 6-8, 200
Polarized Parton Densities in the Nucleon
We present a new NLO QCD analysis of the world data on inclusive polarized
deep inelastic scattering. Comparing to our previous analysis: i) the values of
g_A and a_8=3F-D are updated ii) the MRST'99 instead of the MRST'98
parametrization for the input unpolarized parton densities is used and iii) the
recent SLAC E155 proton data on the spin asymmetry A1 are included in the
analysis. A new set of polarized parton densities is extracted from the data
and the sensitivity of the results to different positivity constraints is
discussed.Comment: 4 pages, 1 figure, LaTeX, typing errors in the numbers for \Delta
\Sigma are corrected, talk presented at the 9th International Workshop on
Deep Inelastic Scattering and QCD (DIS2001), Bologna, Italy, 27 Apr-1 May
200
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