266 research outputs found
Liquid Xenon Detectors for Positron Emission Tomography
PET is a functional imaging technique based on detection of annihilation
photons following beta decay producing positrons. In this paper, we present the
concept of a new PET system for preclinical applications consisting of a ring
of twelve time projection chambers filled with liquid xenon viewed by avalanche
photodiodes. Simultaneous measurement of ionization charge and scintillation
light leads to a significant improvement to spatial resolution, image quality,
and sensitivity. Simulated performance shows that an energy resolution of <10%
(FWHM) and a sensitivity of 15% are achievable. First tests with a prototype
TPC indicate position resolution <1 mm (FWHM).Comment: Paper presented at the International Nuclear Physics Conference,
Vancouver, Canada, 201
Decisive test of color transparency in exclusive electroproduction of vector mesons
The exclusive production of vector mesons in deep inelastic scattering is a
hard scattering process with the well controlled size of quark configurations
which dominate the production amplitude. This allows an unambiguous prediction
of color transparency effects in the coherent and incoherent production of
vector mesons on nuclei. We demonstrate how the very mechanism of color
transparency leads to a belated onset of color transparency effects as a
function of . We conclude that the dependence of the exclusive
-meson production on nuclei and nucleons observed in the Fermilab
E665 experiment gives a solid evidence for the onset of color transparency. We
propose the scaling relation between the and the
production, which further tests the mechanism of color transparency in
exclusive (virtual) photoproduction.Comment: 11 pages, 4 figures on the request from
[email protected], Juelich preprint KFA-IKP(Th)-1993-27.
\phantom{.}\hspace{9cm}{\sl 8 November 1993
The Quark/Antiquark Asymmetry of the Nucleon Sea
Although the distributions of sea quarks and antiquarks generated by
leading-twist QCD evolution through gluon splitting
are necessarily CP symmetric, the distributions of nonvalence quarks and
antiquarks which are intrinsic to the nucleon's bound state wavefunction need
not be identical. In this paper we investigate the sea quark/antiquark
asymmetries in the nucleon wavefunction which are generated by a light-cone
model of energetically-favored meson-baryon fluctuations. The model predicts
striking quark/antiquark asymmetries in the momentum and helicity distributions
for the down and strange contributions to the proton structure function: the
intrinsic and quarks in the proton sea are predicted to be negatively
polarized, whereas the intrinsic and antiquarks give zero
contributions to the proton spin. Such a picture is supported by experimental
phenomena related to the proton spin problem and the violation of the
Ellis-Jaffe sum rule. The light-cone meson-baryon fluctuation model also
suggests a structured momentum distribution asymmetry for strange quarks and
antiquarks which could be relevant to an outstanding conflict between two
different determinations of the strange quark sea in the nucleon. The model
predicts an excess of intrinsic pairs over pairs, as
supported by the Gottfried sum rule violation. We also predict that the
intrinsic charm and anticharm helicity and momentum distributions are not
identical.Comment: LaTex 18 pages, 4 figures. To obtain a copy, send e-mail to
[email protected]
Flavor Asymmetry of the Nucleon Sea: Consequences for Dilepton Production
Parton distributions derived from a chiral quark model that generates an
excess of down quarks and antiquarks in the proton's sea satisfactorily
describe the measured yields of muon pairs produced in proton-nucleus
collisions. Comparison of dilepton yields from hydrogen and deuterium targets
promises greater sensitivity to the predicted flavor asymmetry.Comment: 11 pages, REVTEX, (Three PostScript figures available by anonymous
ftp from fnth06.fnal.gov in directory /pub/Fermilab-Pub/92.264.)
FERMILAB-PUB-92/264--T LBL-3298
The sigma term and the quark number operator in QCD
We discuss the relationship of the forward matrix element of the operator
, related to the so-called sigma term, to the quark number. We
show that in the naive quark model in the canonical formalism these quantities
coincide in the limit of small average quark momenta. In the QCD parton model
defined through light-front quantization this result is preserved at leading
perturbative order but it receives radiative corrections. We analyze the
theoretical and phenomenological consequences of this result, which provides a
bridge between a current algebra quantity, the sigma term, and a deep-inelastic
quantity, the parton number.Comment: 30 pages, 1 figure, DFTT-92-6 (April 1993
Estimation of Collision Impact Parameter
We demonstrate that the nuclear collision geometry (i.e. impact parameter)
can be determined with 1.5 fm accuracy in an event-by-event analysis by
measuring the transverse energy flow in the pseudorapidity region with a minimal dependence on collision dynamics details at the LHC
energy scale. Using the HIJING model we have illustrated our calculation by a
simulation of events of nucleus-nucleus interactions at the c.m.s energy from 1
up to 5.5 TeV per nucleon and various type of nuclei.Comment: 6 pages, 3 figure
Octet Baryon Magnetic Moments in the Chiral Quark Model with Configuration Mixing
The Coleman-Glashow sum-rule for magnetic moments is always fulfilled in the
chiral quark model, independently of SU(3) symmetry breaking. This is due to
the structure of the wave functions, coming from the non-relativistic quark
model. Experimentally, the Coleman-Glashow sum-rule is violated by about ten
standard deviations. To overcome this problem, two models of wave functions
with configuration mixing are studied. One of these models violates the
Coleman-Glashow sum-rule to the right degree and also reproduces the octet
baryon magnetic moments rather accurately.Comment: 22 pages, RevTe
Flavor and Spin Contents of the Nucleon in the Quark Model with Chiral Symmetry
A simple calculation in the framework of the chiral quark theory of Manohar
and Georgi yields results that can account for many of the ''failures'' of the
naive quark model: significant strange quark content in the nucleon as
indicated by the value of the -
asymmetry in the nucleon as measured by the deviation from Gottfried sum rule
and by the Drell-Yan process, as well as the various quark contributions to the
nucleon spin as measured by the deep inelastic polarized lepton-nucleon
scatterings.Comment: figure has been separated from tex file. No other changes. Preprint
CMU-HEP94-3
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