145 research outputs found
Exchange Currents for Hypernuclear Magnetic Moments
The meson(K and ) exchange currents for the hypernuclear magnetic
moments are calculated using the effective Lagrangian method. The seagull
diagram, the mesonic diagram and the -excitation diagram are
considered. The -N exchange magnetic moments for the
, and are calculated
employing the harmonic oscillator shell model. It is found that the two-body
correction is about -9% of the single particle value for .
The exchange current, induced only in the -excitation diagram,
is found to give dominant contribution for the isovector magnetic moments of
hypernuclei with A=6.Comment: 11pp, LaTeX, 7 EPS figures, uses epsf.st
Isospin-mixing corrections for fp-shell Fermi transitions
Isospin-mixing corrections for superallowed Fermi transitions in {\it
fp}-shell nuclei are computed within the framework of the shell model. The
study includes three nuclei that are part of the set of nine accurately
measured transitions as well as five cases that are expected to be measured in
the future at radioactive-beam facilities. We also include some new
calculations for C. With the isospin-mixing corrections applied to the
nine accurately measured values, the conserved-vector-current hypothesis
and the unitarity condition of the Cabbibo-Kobayashi-Maskawa (CKM) matrix are
tested.Comment: 13 pages plus five tables. revtex macro
The Quenching of the Axial Coupling in Nuclear and Neutron-Star Matter
Using a chirally invariant effective Lagrangian, we calculate the density and
isospin dependences of the in-medium axial coupling, , in spatially
uniform matter present in core collapse supernovae and neutron stars. The
quenching of with density in matter with different proton fractions is
found to be similar. However, our results suggest that the quenching of the
nucleon's in matter with hyperons is likely to be significantly greater
than in matter with nucleons only.Comment: 4 pages revtex, 2 eps figure
Parity Mixed Doublets in A = 36 Nuclei
The -circular polarizations () and asymmetries
() of the parity forbidden M1 + E2 -decays: MeV) and MeV)
MeV) are investigated theoretically. We use the recently proposed
Warburton-Becker-Brown shell-model interaction. For the weak forces we discuss
comparatively different weak interaction models based on different assumptions
for evaluating the weak meson-hadron coupling constants. The results determine
a range of values from which we find the most probable values:
= for and = for .Comment: RevTeX, 17 pages; to appear in Phys. Rev.
Large-basis shell-model calculation of 10C->10B Fermi matrix element
We use a shell-model calculation with a two-body effective
interaction derived microscopically from the Reid93 potential to calculate the
isospin-mixing correction for the 10C->10B superallowed Fermi transition. The
effective interaction takes into account the Coulomb potential as well as the
charge dependence of T=1 partial waves. Our results suggest the isospin- mixing
correction , which is compatible with previous
calculations. The correction obtained in those calculations, performed in a
space, was dominated by deviation from unity of the radial
overlap between the converted proton and the corresponding neutron. In the
present calculation this effect is accommodated by the large model space. The
obtained correction is about a factor of four too small to obtain
unitarity of the Cabibbo-Kobayashi-Maskawa matrix with the present experimental
data.Comment: 14 pages. REVTEX. 3 PostScript figure
Parity nonconservation effects in the photodesintegration of polarized deuterons
P-odd correlations in the deuteron photodesintegration are considered. The
-meson exchange is not operative in the case of unpolarized deuterons. For
polarized deuterons a P-odd correlation due to the -meson exchange is
about . Short-distance P-odd contributions exceed essentially
than the contribution of the -meson exchange.Comment: 12 pages, Latex, 3 figure
The Effect of the Pairing Interaction on the Energies of Isobar Analog Resonances in Sb and Isospin Admixture in Sn Isotopes
In the present study, the effect of the pairing interaction and the isovector
correlation between nucleons on the properties of the isobar analog resonances
(IAR) in Sb isotopes and the isospin admixture in Sn
isotopes is investigated within the framework of the quasiparticle random phase
approximation (QRPA). The form of the interaction strength parameter is related
to the shell model potential by restoring the isotopic invariance of the
nuclear part of the total Hamiltonian. In this respect, the isospin admixtures
in the Sn isotopes are calculated, and the dependence of the
differential cross section and the volume integral for the
Sn(He,t)Sb reactions at E(He) MeV occurring by the excitation
of IAR on mass number A is examined. Our results show that the calculated value
for the isospin mixing in the Sn isotope is in good agreement with Colo
et al.'s estimates , and the obtained values for the volume integral
change within the error range of the value reported by Fujiwara et al.
(535 MeV fm). Moreover, it is concluded that although the
differential cross section of the isobar analog resonance for the (He,t)
reactions is not sensitive to pairing correlations between nucleons, a
considerable effect on the isospin admixtures in isotopes can be
seen with the presence of these correlations.Comment: 16 pages, 5 EPS figures and 2 tables, Late
Model Analysis of Time Reversal Symmetry Test in the Caltech Fe-57 Gamma-Transition Experiment
The CALTECH gamma-transition experiment testing time reversal symmetry via
the E2/M1 mulipole mixing ratio of the 122 keV gamma-line in Fe-57 has already
been performed in 1977. Extending an earlier analysis in terms of an effective
one-body potential, this experiment is now analyzed in terms of effective one
boson exchange T-odd P-even nucleon nucleon potentials. Within the model space
considered for the Fe-57 nucleus no contribution from isovector rho-type
exchange is possible. The bound on the coupling strength phi_A from effective
short range axial-vector type exchange induced by the experimental bound on
sin(eta) leads to phi_A < 10^{-2}.Comment: 5 pages, RevTex 3.
Towards a Statistical Theory of Finite Systems and Compound States: Random Two-Body Interaction Approach
The model of Fermi particles with random two-body interaction is
investigated. This model allows to study the origin and accuracy of statistical
laws in few-body systems, the role of interaction and chaos in thermalization,
Fermi-Dirac distribution for quasi-particles with spreading widths, matrix
elements of external field and enhancement of weak perturbation in chaotic
compound states.Comment: 4 pages in RevTex and 2 Postscript figures; to appear in Phys.Rev.
Partially conserved axial current constraints on pion production/absorption within nonrelativistic dynamics
We show the necessity of two-nucleon axial currents and associated pion
emission/ absorption operators for the partial conservation of the axial
current (PCAC) nuclear matrix elements with arbitrary nuclear dynamics
described by a nonrelativistic Schroedinger equation. As examples we construct
such nonrelativistic axial two-body currents in the linear- and heterotic (g_A
= 1.26) sigma models, with an optional isoscalar vector (omega) meson exchange.
The nuclear matrix elements obey PCAC only if the nuclear wave functions used
in the calculation are solutions to the Schroedinger equation with the static
one-meson-exchange potential constructed in the respective (sigma) model. The
same holds true for the nucler pion production amplitude, since it is
proportional to the divergence of the axial current matrix element, by virtue
of PCAC. Thus we found a new consistency condition between the pion
creation/absorption operator and the nuclear Hamiltonian. We present examples
drawn from our models and discuss implications for one-pion-two-nucleon
processes.Comment: 19 pages, 7 figures, submitted to Phys. Rev.
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