1,409 research outputs found
Testing the Meson Cloud Model in Inclusive Meson Production
We have applied the Meson Cloud Model to calculate inclusive momentum spectra
of pions and kaons produced in high energy proton-proton and proton-nucleus
collisions. For the first time these data are used to constrain the cloud
cut-off parameters. We show that it is possible to obtain a reasonable
description of data, especially the large () part of the
spectrum and at the same time describe (partially) the E866 data on and . We also discuss the relative strength of the
and vertices. We find out that the corresponding cut-off
parameters should be both soft and should not differ by more than 200 MeV from
each other. An additional source (other than the meson cloud) of sea antiquark
asymmetry, seems to be necessary to completely explain the data. A first
extension of the MCM to proton nucleus collisions is discussed.Comment: 14 pages, Latex, 6 ps figures. Submitted to Phys. Rev.
Chiral extrapolation of lattice moments of proton quark distributions
We present the resolution of a long-standing discrepancy between the moments
of parton distributions calculated from lattice QCD and their experimental
values. We propose a simple extrapolation formula for the moments of the
nonsinglet quark distribution u-d, as a function of quark mass, which embodies
the general constraints imposed by the chiral symmetry of QCD. The inclusion of
the leading nonanalytic behavior leads to an excellent description of both the
lattice data and the experimental values of the moments.Comment: 9 pages, 1 figure, to appear in Physical Review Letter
Dynamics of Light Antiquarks in the Proton
We present a comprehensive analysis of the recent data from the E866
experiment at Fermilab on Drell-Yan production in pD and pp collisions, which
indicates a non-trivial x-dependence for the asymmetry between u-bar and d-bar
quark distributions in the proton. The relatively fast decrease of the
asymmetry at large x suggests the important role played by the chiral structure
of the nucleon, in particular the pi-N and pi-Delta components of the nucleon
wave function. At small x the data require an additional non-chiral component,
which may be attributed to the Pauli exclusion principle as first suggested by
Field and Feynman.Comment: version to appear in Phys. Rev.
Parton Distributions for the Octet and Decuplet Baryons
We calculate the parton distributions for both polarized and unpolarized
octet and decuplet baryons, using the MIT bag, dressed by mesons. We show that
the hyperfine interaction responsible for the and splittings leads to large deviations from SU(3) and SU(6) predictions.
For the we find significant polarized, non-strange parton
distributions which lead to a sizable polarization in polarized,
semi-inclusive scattering. We also discuss the flavour symmetry violation
arising from the meson-cloud associated with the chiral structure of baryons.Comment: 29 pages, 15 figure
Flavor and Charge Symmetry in the Parton Distributions of the Nucleon
Recent calculations of charge symmetry violation(CSV) in the valence quark
distributions of the nucleon have revealed that the dominant symmetry breaking
contribution comes from the mass associated with the spectator quark
system.Assuming that the change in the spectator mass can be treated
perturbatively, we derive a model independent expression for the shift in the
parton distributions of the nucleon. This result is used to derive a relation
between the charge and flavor asymmetric contributions to the valence quark
distributions in the proton, and to calculate CSV contributions to the nucleon
sea. The CSV contribution to the Gottfried sum rule is also estimated, and
found to be small
Parton energy loss limits and shadowing in Drell-Yan dimuon production
A precise measurement of the ratios of the Drell-Yan cross section per
nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is
reported. The behavior of the Drell-Yan ratios at small target parton momentum
fraction is well described by an existing fit to the shadowing observed in
deep-inelastic scattering. The cross section ratios as a function of the
incident parton momentum fraction set tight limits on the energy loss of quarks
passing through a cold nucleus
Next-to-Leading Order Constituent Quark Structure and Hadronic Structure Functions
We calculate the partonic structure of a constituent quark in the
Next-to-Leading Order framework. The structure of any hadron can be obtained
thereafter using a convolution method. Such a procedure is used to generate the
structure function of proton and pion in NLO, neglecting certain corrections to
. It is shown that while the constituent quark structure is
generated purely perturbatively and accounts for the most part of the hadronic
structure, there is a few percent contributions coming from the nonperturbative
sector in the hadronic structure. This contribution plays the key role in
explaining the SU(2) symmetry breaking of the nucleon sea and the observed
violation of Gottfried sum rule. These effects are calculated. We obtained an
Excellent agreement with the experimental data in a wide range of and for the proton structure function. We
have also calculated Pion structure and compared it with the existing data.
Again, the model calculations agree rather well with the data from experiment.Comment: 32 pages,10 figures, Accepted to publish in Phys. Rev.
Parton model versus color dipole formulation of the Drell-Yan process
In the kinematical region where the center of mass energy is much larger than
all other scales, the Drell-Yan process can be formulated in the target rest
frame in terms of the same color dipole cross section as low Bjorken-x deep
inelastic scattering. Since the mechanisms for heavy dilepton production appear
very different in the dipole approach and in the conventional parton model, one
may wonder whether these two formulations really represent the same physics. We
perform a comparison of numerical calculations in the color dipole approach
with calculations in the next-to-leading order parton model. For proton-proton
scattering, the results are very similar at low x_2 from fixed target to RHIC
energies, confirming the close connection between these two very different
approaches. We also compare the transverse momentum distributions of Drell-Yan
dileptons predicted in both formulations. The range of applicability of the
dipole formulation and the impact of future Drell-Yan data from RHIC for
determining the color dipole cross section are discussed. A detailed derivation
of the dipole formulation of the Drell-Yan process is also included.Comment: 20 pages, 5 figure
dbar/ubar Asymmetry and the Origin of the Nucleon Sea
The Drell-Yan cross section ratios, , measured in
Fermilab E866, have led to the first determination of ,
, and the integral of for the
proton over the range . The E866 results are compared
with predictions based on parton distribution functions and various theoretical
models. The relationship between the E866 results and the NMC measurement of
the Gottfried integral is discussed. The agreement between the E866 results and
models employing virtual mesons indicates these non-perturbative processes play
an important role in the origin of the , asymmetry in the
nucleon sea.Comment: 5 pages, 3 figures, ReVTe
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