11 research outputs found
Theory of Weak Hypernuclear Decay
The weak nonmesonic decay of Lambda-hypernuclei is studied in the context of
a one-meson-exchange model. Predictions are made for the decay rate, p/n
stimulation ratio and the asymmetry in polarized hypernuclear decay.Comment: Standard 41 page Latex fil
Relativistic Treatment of Hypernuclear Decay
We compute for the first time the decay width of lambda-hypernuclei in a
relativistic mean-field approximation to the Walecka model. Due to the small
mass difference between the lambda-hyperon and its decay products---a nucleon
and a pion---the mesonic component of the decay is strongly Pauli blocked in
the nuclear medium. Thus, the in-medium decay becomes dominated by the
non-mesonic, or two-body, component of the decay. For this mode, the
lambda-hyperon decays into a nucleon and a spacelike nuclear excitation. In
this work we concentrate exclusively on the pion-like modes. By relying on the
analytic structure of the nucleon and pion propagators, we express the
non-mesonic component of the decay in terms of the spin-longitudinal response
function. This response has been constrained from precise quasielastic (p,n)
measurements done at LAMPF. We compute the spin-longitudinal response in a
relativistic random-phase-approximation model that reproduces accurately the
quasielastic data. By doing so, we obtain hypernuclear decay widths that are
considerably smaller---by factors of two or three---relative to existing
nonrelativistic calculations.Comment: Revtex: 18 pages and 4 postscript figure
The Axial-Vector Current in Nuclear Many-Body Physics
Weak-interaction currents are studied in a recently proposed effective field
theory of the nuclear many-body problem. The Lorentz-invariant effective field
theory contains nucleons, pions, isoscalar scalar () and vector
() fields, and isovector vector () fields. The theory exhibits a
nonlinear realization of chiral symmetry and has three
desirable features: it uses the same degrees of freedom to describe the
axial-vector current and the strong-interaction dynamics, it satisfies the
symmetries of the underlying theory of quantum chromodynamics, and its
parameters can be calibrated using strong-interaction phenomena, like hadron
scattering or the empirical properties of finite nuclei. Moreover, it has
recently been verified that for normal nuclear systems, it is possible to
systematically expand the effective lagrangian in powers of the meson fields
(and their derivatives) and to reliably truncate the expansion after the first
few orders. Here it is shown that the expressions for the axial-vector current,
evaluated through the first few orders in the field expansion, satisfy both
PCAC and the Goldberger--Treiman relation, and it is verified that the
corresponding vector and axial-vector charges satisfy the familiar chiral
charge algebra. Explicit results are derived for the Lorentz-covariant,
axial-vector, two-nucleon amplitudes, from which axial-vector meson-exchange
currents can be deduced.Comment: 32 pages, REVTeX 4.0 with 12pt.rtx, aps.rtx, revsymb.sty,
revtex4.cls, plus 14 figures; two sentences added in Summary; two references
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