191 research outputs found
The Flavor Asymmetry of the Nucleon Sea
We re-examine the effects of anti-symmetry on the anti-quarks in the nucleon
sea arising from gluon exchange and pion exchange between confined quarks.
While the effect is primarily to suppress anti-down relative to anti-up quarks,
this is numerically insignificant for the pion terms.Comment: To appear in Phys. Rev.
Perturbative and nonperturbative contributions to the strange quark asymmetry in the nucleon
There are two mechanisms for the generation of an asymmetry between the
strange and anti-strange quark distributions in the nucleon: nonperturbative
contributions originating from nucleons fluctuating into virtual baryon-meson
pairs such as and , and perturbative contributions
arising from gluons splitting into strange and anti-strange quark pairs. While
the nonperturbative contributions are dominant in the large- region, the
perturbative contributions are more significant in the small- region. We
calculate this asymmetry taking into account both nonperturbative and
perturbative contributions, thus giving a more accurate evaluation of this
asymmetry over the whole domain of . We find that the perturbative
contributions are generally a few times larger in magnitude than the
nonperturbative contributions, which suggests that the best region to detect
this asymmetry experimentally is in the region . We find that
the asymmetry may have more than one node, which is an effect that should be
taken into account, e.g. for parameterizations of the strange and anti-strange
quark distributions used in global analysis of parton distributions.Comment: 14 pages, 4 figures, figures comparing theoretical calculations with
NNPDF global analysis added, accepted for publication in EPJ
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
Non-perturbative structure of the polarized nucleon sea
We investigate the flavour and quark-antiquark structure of the polarized
nucleon by calculating the parton distribution functions of the nucleon sea
using the meson cloud model. We find that the SU(2) flavor symmetry in the
light antiquark sea and quark-antiquark symmetry in the strange quark sea are
broken, {\it i.e.} \Delta\ubar < \Delta \dbar and \Delta s < \Delta \sbar.
The polarization of the strange sea is found to be positive, which is in
contradiction to previous analyses. We predict a much larger quark-antiquark
asymmetry in the polarized strange quark sea than that in the unpolarized
strange quark sea. Our results for both polarized light quark sea and polarized
strange quark sea are consistent with the recent HERMES data.Comment: RevTex, 17 pages plus 8 PS figure
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.
Chiral Symmetry and the Nucleon Structure Functions
The isospin asymmetry of the sea quark distribution as well as the
unexpectedly small quark spin fraction of the nucleon are two outstanding
discoveries recently made in the physics of deep-inelastic structure functions.
We evaluate here the corresponding quark distribution functions within the
framework of the chiral quark soliton model, which is an effective quark model
of baryons maximally incorporating the most important feature of low energy
QCD, i.e. the chiral symmetry and its spontaneous breakdown. It is shown that
the model can explain qualitative features of the above-mentioned nucleon
structure functions within a single framework, thereby disclosing the
importance of chiral symmetry in the physics of high energy deep-inelastic
scatterings.Comment: 20pages, LaTex, 5 Postscript figures A numerical error of the
original version was corrected. The discussion on the regularization
dependence of distribution functions has been added. A comparison with the
low energy-scale parametrization of Gloeck, Reya and Vogt has been mad
A Study of Off-Forward Parton Distributions
An extensive theoretical analysis of off-forward parton distributions (OFPDs)
is presented. The OFPDs and the form factors of the quark energy-momentum
tensor are estimated at a low-energy scale using a bag model. Relations among
the second moments of OFPDs, the form factors, and the fraction of the nucleon
spin carried by quarks are discussed.Comment: 29 pages revtex, 12 postscript figures, minor corrections, references
update
Charge symmetry violation in the parton distributions of the nucleon
We point out that charge symmetry violation in both the valence and sea quark
distributions of the nucleon has a non-perturbative source. We calculate this
non-perturbative charge symmetry violation using the meson cloud model, which
has earlier been successfully applied to both the study of SU(2) flavour
asymmetry in the nucleon sea and quark-antiquark asymmetry in the nucleon. We
find that the charge symmetry violation in the valence quark distribution is
well below 1%, which is consistent with most low energy tests but significantly
smaller than the quark model prediction about 5%-10%. Our prediction for the
charge symmetry violation in the sea quark distribution is also much smaller
than the quark model calculation.Comment: RevTex, 26 pages, 6 PostScript figure
Audio-based identification of beehive states
The absence of the queen in a beehive is a very strong indicator of the need for beekeeper intervention. Manually searching for the queen is an arduous recurrent task for beekeepers that disrupts the normal life cycle of the beehive and can be a source of stress for bees. Sound is an indicator for signalling different states of the beehive, including the absence of the queen bee. In this work, we apply machine learning methods to automatically recognise different states in a beehive using audio as input. We investigate both support vector machines and convolutional neural networks for beehive state recognition, using audio data of beehives collected from the NU-Hive project. Results indicate the potential of machine learning methods as well as the challenges of generalizing the system to new hives
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
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