1,459 research outputs found
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
On the non-vanishing of the Collins mechanism for single spin asymmetries
The Collins mechanism provides a non-perturbative explanation for the large
single spin asymmetries found in hard semi-inclusive reactions involving a
transversely polarized nucleon. However, there are seemingly convincing reasons
to suspect that the mechanism vanishes, and indeed it does vanish in the naive
parton model where a quark is regarded as an essentially 'free' particle. We
give an intuitive analysis which highlights the difference between the naive
picture and the realistic one, and shows how the Collins mechanism arises when
the quark is described as an off-shell particle by a field in interaction. A
typographical error is corrected in this version.Comment: 15 pages, 2 figure
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
Pathos: a web facility that uses metabolic maps to display experimental changes in metabolites identified by mass spectrometry
This work describes a freely available web-based facility which can be used to analyse raw or processed mass spectrometric data from metabolomics experiments and display the metabolites identified – and changes in their experimental abundance – in the context of the metabolic pathways in which they occur. The facility, Pathos (http://motif.gla.ac.uk/Pathos/), employs Java servlets and is underpinned by a relational database populated from the Kyoto Encyclopaedia of Genes and Genomes (KEGG). Input files can contain either raw m/z values from experiments conducted in different modes, or KEGG or MetaCyc IDs assigned by the user on the basis of the m/z values and other criteria. The textual output lists the KEGG pathways on an XHTML page according to the number of metabolites or potential metabolites that they contain. Filtering by organism is also available. For metabolic pathways of interest, the user is able to retrieve a pathway map with identified metabolites highlighted. A particular feature of Pathos is its ability to process relative quantification data for metabolites identified under different experimental conditions, and to present this in an easily comprehensible manner. Results are colour-coded according to the degree of experimental change, and bar charts of the results can be generated interactively from either the text listings or the pathway maps. The visual presentation of the output from Pathos is designed to allow the rapid identification of metabolic areas of potential interest, after which particular results may be examined in detail
A relativistic treatment of pion wave functions in the annihilation antiproton-proton -> pi^-pi^+
Quark model intrinsic wave functions of highly energetic pions in the
reaction \bar pp->\pi^-\pi^+ are subjected to a relativistic treatment. The
annihilation is described in a constituent quark model with A2 and R2
flavor-flux topology and the annihilated quark-antiquark pairs are in 3P_0 and
3S_1 states. We study the effects of pure Lorentz transformations on the
antiquark and quark spatial wave functions and their respective spinors in the
pion. The modified quark geometry of the pion has considerable impact on the
angular dependence of the annihilation mechanisms.Comment: 10 pages in revtex format, 3 figure
Can the polarization of the strange quarks in the proton be positive ?
Recently, the HERMES Collaboration at DESY, using a leading order QCD
analysis of their data on semi-inclusive deep inelastic production of charged
hadrons, reported a marginally positive polarization for the strange quarks in
the proton. We argue that a non-negative polarization is almost impossible.Comment: 6 pages, latex, minor changes in the discussion after Eq. (9
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