203 research outputs found
Possible S-wave Dibaryons in SU(3) Chiral Quark Model
In the framework of the SU(3) chiral quark model, the wave baryon-baryon
bound states are investigated. It is found that according to the symmetry
character of the system and the contributions from chiral fields, there are
three types of bound states. The states of the first type, such as
and are deeply bound
dibaryon with narrow widths. The second type states, ,,
and are also bound states, but with broad widths.
, , and are third type states. They, like {\em d}, are weakly bound
only if the chiral fields can provide attraction between baryons.Comment: Latex files, 1 figur
The role of five-quark components in gamma decay of the
An admixture of 10-20 % of qqqq\bar q components in the Delta(1232) resonance
is shown to reduce the well known underprediction for the decay width for
Delta(1232)->N gamma decay by about half and that of the corresponding helicity
amplitudes from a factor ~ 1.7 to ~ 1.5. The main effect is due to the
quark-antiquark annihilation transitions qqqq\bar q -> qqq gamma, the
consideration of which brings the ratio A_{3/2}/A_{1/2} and consequently the
E2/M1 ratio R_{EM} into agreement with the empirical value. Transitions between
qqqq\bar q components in the resonance and the nucleon qqqq\bar q->qqqq\bar q
gamma are shown to enhance the calculated width by only a few percent, as long
as the probability of the qqqq\bar q component of the Delta(1232) and the
proton is at most ~ 20 %. The transitions qqqq\bar q->qqqq\bar q gamma between
the qqqq\bar q components in the Delta(1232) and the proton do not lead to a
nonzero value for R_{EM}
Bound states of in nuclei
We study the binding energy and the width of the in nuclei,
associated to the and components. The first one leads to
negligible contributions while the second one leads to a sizeable attraction,
enough to bind the in nuclei. Pauli blocking and binding effects on
the decay reduce considerably the decay width in nuclei and
medium effects associated to the component also lead to a very small
width, as a consequence of which one finds separation between the bound levels
considerably larger than the width of the states.Comment: Presentation in the 10th International Baryon Conference BARYON0
The Role of 5-quark Components on the Nucleon Form Factors
The covariant quark model is shown to allow a phenomenological description of
the neutron electric form factor, G_E^n(Q^2), in the impulse approximation,
provided that the wave function contains minor (~ 3 %) admixtures of the lowest
sea-quark configurations. While that form factor is not very sensitive to
whether the \bar q in the qqqq\bar q component is in the P-state or in the
S-state, the calculated nucleon magnetic form factors are much closer to the
empirical values in the case of the former configuration. In the case of the
electric form factor of the proton, G_E^p(Q^2), a zero appears in the impulse
approximation close to 9 GeV^2, when the \bar q is in the P-state. That
configuration, which may be interpreted as a pion loop ("cloud") fluctuation,
also leads to a clearly better description of the nucleon magnetic moments.
When the amplitude of the sea-quark admixtures are set so as to describe the
electric form factor of the neutron, the qqqq\bar q admixtures have the
phenomenologically desirable feature, that the electric form factor of the
proton falls at a more rapid rate with momentum transfer than the magnetic form
factor.Comment: To appear in Nuclear Physics
Genetic engineering of E-coli SE5000 and its potential for Ni2+ bioremediation
A genetically engineered Escherichia coli SE5000 strain simultaneously expressing nickel transport system and metallothionein was constructed to accumulate Ni2+ from aqueous solution. Bioaccumulation was fast and followed linearized Langmuir isotherm. Compared with 1.62 mg/g of Ni2+ uptake capacity by original host E. coli cells, genetically engineered E. coli could bind 7.14 mg/g Ni2+, and it accumulated Ni2+ effectively over a broad range of pH (4-10) and the optimal pH was 8.6. The presence of 1000mg/l Na+ and Ca2+ or 50 mg/l Cd2+ and Pb2+ did not decrease Ni2+ bioaccumulation significantly, but Mg2+, Hg2+, Cr3+ and Cu2+ posed severe deleterious influences on Ni2+ uptake by genetically engineered E. coli. Furthermore, the presence of EDTA inhibited nickel bioaccumulation. (C) 2004 Elsevier Ltd. All rights reserved
Measurement of Trace I-129 Concentrations in CsI Powder and Organic Liquid Scintillator with Accelerator Mass Spectrometry
Levels of trace radiopurity in active detector materials is a subject of
major concern in low-background experiments. Procedures were devised to measure
trace concentrations of I-129 in the inorganic salt CsI as well as in organic
liquid scintillator with Accelerator Mass Spectrometry (AMS) which leads to
improvement in sensitivities by several orders of magnitude over other methods.
No evidence of their existence in these materials were observed. Limits of < 6
X 10^{-13} g/g and < 2.6 X 10^{-17} g/g on the contaminations of I-129 in CsI
and liquid scintillator, respectively, were derived.These are the first results
in a research program whose goals are to develop techniques to measure trace
radioactivity in detector materials by AMS.Comment: Proceedings of 10th International Conference on Accelerator Mass
Spectrometr
Effects of momentum conservation on the analysis of anisotropic flow
We present a general method for taking into account correlations due to
momentum conservation in the analysis of anisotropic flow, either by using the
two-particle correlation method or the standard flow vector method. In the
latter, the correlation between the particle and the flow vector is either
corrected through a redefinition (shift) of the flow vector, or subtracted
explicitly from the observed flow coefficient. In addition, momentum
conservation contributes to the reaction plane resolution. Momentum
conservation mostly affects the first harmonic in azimuthal distributions,
i.e., directed flow. It also modifies higher harmonics, for instance elliptic
flow, when they are measured with respect to a first harmonic event plane such
as one determined with the standard transverse momentum method. Our method is
illustrated by application to NA49 data on pion directed flow.Comment: RevTeX 4, 10 pages, 1 eps figure. Version accepted for publication in
Phys Rev
Study of five quark system with three kinds of quark-quark hyperfine interaction
The low-lying energy spectra of five quark systems (I=1/2, S=0)
and (I=0, S=-1) are investigated with three kinds of schematic
interactions: the chromomagnetic interaction, the flavor-spin dependent
interaction and the instanton-induced interaction. In all the three models, the
lowest five quark state ( or ) has an orbital angular
momentum L=0 and the spin-parity ; the mass of the lowest
state is heavier than the lowest state
RQM description of the charge form factor of the pion and its asymptotic behavior
The pion charge and scalar form factors, and , are first
calculated in different forms of relativistic quantum mechanics. This is done
using the solution of a mass operator that contains both confinement and
one-gluon-exchange interactions. Results of calculations, based on a one-body
current, are compared to experiment for the first one. As it could be expected,
those point-form, and instant and front-form ones in a parallel momentum
configuration fail to reproduce experiment. The other results corresponding to
a perpendicular momentum configuration (instant form in the Breit frame and
front form with ) do much better. The comparison of charge and scalar
form factors shows that the spin-1/2 nature of the constituents plays an
important role. Taking into account that only the last set of results
represents a reasonable basis for improving the description of the charge form
factor, this one is then discussed with regard to the asymptotic QCD-power-law
behavior . The contribution of two-body currents in achieving the right
power law is considered while the scalar form factor, , is shown to
have the right power-law behavior in any case. The low- behavior of the
charge form factor and the pion-decay constant are also discussed.}Comment: 30 pages, 10 figure
The ARGO-YBJ Experiment Progresses and Future Extension
Gamma ray source detection above 30TeV is an encouraging approach for finding
galactic cosmic ray origins. All sky survey for gamma ray sources using wide
field of view detector is essential for population accumulation for various
types of sources above 100GeV. To target the goals, the ARGO-YBJ experiment has
been established. Significant progresses have been made in the experiment. A
large air shower detector array in an area of 1km2 is proposed to boost the
sensitivity. Hybrid detection with multi-techniques will allow a good
discrimination between different types of primary particles, including photons
and protons, thus enable an energy spectrum measurement for individual specie.
Fluorescence light detector array will extend the spectrum measurement above
100PeV where the second knee is located. An energy scale determined by balloon
experiments at 10TeV will be propagated to ultra high energy cosmic ray
experiments
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