442 research outputs found
Quark Cluster Model Study of Isospin-Two Dibaryons
Based on a quark cluster model for the non-strange sector that reproduces
reasonably well the nucleon-nucleon system and the excitation of the
isobar, we generate a nucleon- interaction and present the predictions
for the several isospin two channels. The only attractive channels are
and , but not attractive enough to generate a resonance. If a resonance is
artificially generated and is required to have the observed experimental mass,
then our model predicts a width that agrees with the experimental result.Comment: 12 pages, 5 poscript figures available under request. To appear in
Phys. Rev.
Three-body decay of the d* dibaryon
Under certain circumstances, a three-body decay width can be approximated by
an integral involving a product of two off-shell two-body decay widths. This
``angle-average'' approximation is used to calculate the decay width
of the dibaryon in a simple model for the most
important Feynman diagrams describing pion emissions with baryon-baryon recoil
and meson retardation. The decay width is found to be about 0.006 (0.07, 0.5)
MeV at the mass of 2065 (2100, 2150) MeV for input dynamics derived from
the Full Bonn potential. The smallness of this width is qualitatively
understood as the result of the three-body decay being ``third forbidden''. The
concept of forbiddenness and the threshold behavior of a three-body
decay are further studied in connection with the decay of the dibaryon
where the idea of unfavorness has to be introduced.
The implications of these results are briefly discussed.Comment: 15 pages, RevTeX, two-column journal style, six figure
Hepatic cell loss and proliferation induced by N-2-fluorenylacetamide, diethylnitrosamine, and aflatoxin B1 in relation to hepatoma induction.
Three hepatic carcinogens (aflatoxin B1, diethylnitrosamine (DEN) and N-2-fluorenylacetamide (FAA)) were compared for carcinogenicity, early cell toxicity and parenchymal cell proliferation. The carcinogens were administered to rats for 15 weeks as follows: aflatoxin B1, 1 in 10(6) in pelleted food; DEN, 2 in 10(5) in drinking water; FAA, 3 in 10(4) in pelleted food. The loss of prelabelled DNA and the [H3] TdR pulse-labelling indices (LI) of parenchymal and nonparenchymal cells were determined at various times during the period of carcinogen availability. On a molar basis, aflatoxin B1 was 90 times as carcinogenic as FAA and 24 times as carcinogenic as DEN. However, for about equal magnitudes of hepatic cell proliferation and loss, aflatoxin B1 was the least potent carcinogen. For a given level of carcinogenicity, FAA was more potent than DEN in causing loss of hepatic DNA and in increasing the parenchymal cell labelling index. DEN and aflatoxin B1 produced about the same degree of DNA loss and parenchymal cell labelling, but the former was a more potent carcinogen. When carcinogenicity was compared for approximately equal levels of early hepatic cell destruction and proliferation, the 3 chemicals in the present study could be ranked in descending order of potency as DEN, FAA and aflatoxin B1
The d' dibaryon in the quark-delocalization, color-screening model
We study the questions of the existence and mass of the proposed dibaryon in the quark-delocalization, color-screening model
(QDCSM). The transformation between physical and symmetry bases has been
extended to the cases beyond the SU(2) orbital symmetry. Using parameters fixed
by baryon properties and scattering, we find a mild attraction in the
channel, but it is not strong enough to form a deeply bound state
as proposed for the state. Nor does the (isospin) I=2 N
configuration have a deeply bound state. These results show that if a narrow
dibaryon state does exist, it must have a more complicated structure.Comment: 12 pp. latex, no figs., 2 tables, additional refs., Report-no was
adde
Possible Dibaryons with Strangeness s=-5
In the framework of , the binding energy of the six quark system with
strangeness s=-5 is systematically investigated under the SU(3) chiral
constituent quark model. The single channel calculation with
spins S=0 and 3 and the coupled and channel
calculation with spins S=1 and 2 are considered, respectively. The results show
following observations: In the spin=0 case, is a bound dibaryon
with the binding energy being . In the S=1 case,
is also a bound dibaryon. Its binding energy is ranged from to . In the S=2 and S=3 cases, no evidence of bound dibaryons
are found. The phase shifts and scattering lengths in the S=0 and S=1 cases are
also given.Comment: 10 pages, late
Effect of a nonuniform distribution of voids on the plastic response of voided materials: a computational and statistical analysis
This study investigates the overall and local response of porous media composed of a perfectly plastic matrix weakened by stress-free voids. Attention is focused on the specific role played by porosity fluctuations inside a representative volume element. To this end, numerical simulations using the Fast Fourier Transform (FFT) are performed on different classes of microstructure corresponding to different spatial distributions of voids. Three types of microstructures are investigated: random microstructures with no void clustering, microstructures with a connected cluster of voids and microstructures with disconnected void clusters. These numerical simulations show that the porosity fluctuations can have a strong effect on the overall yield surface of porous materials. Random microstructures without clusters and microstructures with a connected cluster are the hardest and the softest configurations, respectively, whereas microstructures with disconnected clusters lead to intermediate responses. At a more local scale, the salient feature of the fields is the tendency for the strain fields to concentrate in specific bands. Finally, an image analysis tool is proposed for the statistical characterization of the porosity distribution. It relies on the distribution of the ‘distance function’, the width of which increases when clusters are present. An additional connectedness analysis allows us to discriminate between clustered microstructures
Singlet-to-triplet ratio in the deuteron breakup reaction at 585 MeV
Available experimental data on the exclusive reaction at 585 MeV
show a narrow peak in the proton-neutron final-state interaction region. It was
supposed previously, on the basis of a phenomenological analysis of the shape
of this peak, that the final spin-singlet state provided about one third
of the observed cross section. By comparing the absolute value of the measured
cross section with that of elastic scattering using the F\"aldt-Wilkin
extrapolation theorem, it is shown here that the data can be
explained mainly by the spin-triplet final state with a singlet admixture of a
few percent. The smallness of the singlet contribution is compatible with
existing data and the one-pion exchange mechanism of the reaction.Comment: 10 pages, Latex, 2 Postscript figure
Does the KARMEN time anomaly originate from a beam-correlated background?
The time anomaly of neutrino events observed in the KARMEN detector at the
ISIS facility is discussed. We show that beam-correlated neutron events are not
completely suppressed by the KARMEN lower energy cut and could cause the
observed deviation of the measured time distribution from an exponential curve
with tau = 2.2 microseconds.Comment: 12 pages including figure captions, 2 figures, 1 tabl
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