96,397 research outputs found
Five-quark components in decay
Five-quark components in the are shown to
contribute significantly to decay through
quark-antiquark annihilation transitions. These involve the overlap between the
and components and may be triggered by the confining
interaction between the quarks. With a 10% admixture of five-quark
components in the the decay width can be larger by factors 2 - 3
over that calculated in the quark model with 3 valence quarks, depending on the
details of the confining interaction. The effect of transitions between the
components themselves on the calculated decay width is however
small. The large contribution of the quark-antiquark annihilation transitions
thus may compensate the underprediction of the width of the by
the valence quark model, once the contains
components with 10% probability.Comment: accepted versio
Sea-quark effects in the pion charge form factor
It is shown that the data on the pion charge form factor admit the
possibility for a substantial sea-quark components in the pion wave function.
If the charge form factor is calculated with instant form kinematics in a
constituent quark model that is extended to include explicit
components in the pion wave function, that component will give the dominant
contribution to the calculated charge form factor at large values of
momentum transfer. The present experimental values can be described well
with component admixtures of up to 50%. The sensitivity of the
calculated charge form factor to whether one of the quarks or one of
the antiquarks is taken to be in the P-state is small.Comment: 14 page
Medium modifications of the nucleon-nucleon elastic cross section in neutron-rich intermediate energy HICs
Several observables of unbound nucleons which are to some extent sensitive to
the medium modifications of nucleon-nucleon elastic cross sections in
neutron-rich intermediate energy heavy ion collisions are investigated. The
splitting effect of neutron and proton effective masses on cross sections is
discussed. It is found that the transverse flow as a function of rapidity, the
as a function of momentum, and the ratio of halfwidths of the
transverse to that of longitudinal rapidity distribution are very
sensitive to the medium modifications of the cross sections. The transverse
momentum distribution of correlation functions of two-nucleons does not yield
information on the in-medium cross section.Comment: 14 pages, 5 figure
Out of plane effect on the superconductivity of Sr2-xBaxCuO3+y with Tc up to 98K
A series of new Sr2-xBaxCuO3+y (0 x 0.6) superconductors were prepared using
high-pressure and high-temperature synthesis. A Rietveld refinement based on
powder x-ray diffraction confirms that the superconductors crystallize in the
K2NiF4-type structure of a space group I4/mmm similar to that of La2CuO4 but
with partially occupied apical oxygen sites. It is found that the
superconducting transition temperature Tc of this Ba substituted Sr2CuO3+y
superconductor with constant carrier doping level, i.e., constant d, is
controlled not only by order/disorder of apical-O atoms but also by Ba content.
Tcmax =98 K is achieved in the material with x=0.6 that reaches the record
value of Tc among the single-layer copper oxide superconductors, and is higher
than Tc=95K of Sr2CuO3+y with optimally ordered apical-O atoms. There is
Sr-site disorder in Sr2-xBaxCuO3+y which might lead to a reduction of Tc. The
result indicates that another effect surpasses the disorder effect that is
related either to the increased in-plane Cu-O bond length or to elongated
apical-O distance due to Ba substitution with larger cation size. The present
experiment demonstrates that the optimization of local geometry out of the Cu-O
plane can dramatically enhance Tc in the cuprate superconductors.Comment: 23 Pages, 1 Table, 5 Figure
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