82 research outputs found
Medium Effects in Coherent Pion Photo- and Electroproduction on 4He and 12C
Coherent pi0 photo- and electroproduction on 4He and 12C nuclei is
investigated in the framework of a distorted wave impulse approximation in
momentum space. The elementary process is described by the recently developed
unitary isobar model. Medium effects are considered by introducing a
phenomenological Delta self-energy. The recent experimental data for 4He and
12C can be well described over a wide range of energies and emission angles by
the assumption that the Delta-nuclear interaction saturates.Comment: 18 pages LaTeX including 7 postscript figure
Chiral unitary approach to the K^- deuteron scattering length
Starting from a recent model where the Kbar N amplitudes are evaluated from
the chiral Lagrangians using a coupled channel unitary method, we evaluate here
the scattering length for K^- deuteron scattering. We find that the double
scattering contribution is very large compared to the impulse approximation and
that the charge exchange contribution of this rescattering is as large as the
sequential K^- scattering on the two nucleons. Higher order rescattering
corrections are evaluated using coupled channels with K^- and Kbar^0 within the
integral form of the fixed centre approximation to the Faddeev equations. The
higher order corrections involving intermediate pions and hyperons are found
negligible.Comment: 16 pages, 4 figures, revised version to appear in Nucl. Phys.
Measurement of the 6Li(e,e'p) reaction cross sections at low momentum transfer
The triple differential cross sections for the 6Li(e,e'p) reaction have been
measured in the excitation energy region from 27 to 46 MeV in a search for
evidence of the giant dipole resonance (GDR) in 6Li. The cross sections have no
distinct structures in this energy region, and decrease smoothly with the
energy transfer. Angular distributions are different from those expected with
the GDR. Protons are emitted strongly in the momentum-transfer direction. The
data are well reproduced by a DWIA calculation assuming a direct proton
knockout process.Comment: 19 pages, 7 figures, revised text, to be published in Nucl. Phys.
Alternative evaluations of halos in nuclei
Data for the scattering of 6He, 8He, 9Li, and 11Li from hydrogen are analyzed
within a fully microscopic folding model of proton-nucleus scattering. Current
data suggest that of these only 11Li has a noticeable halo. For 6He, we have
also analysed the complementary reaction 6Li(gamma,pi)6He(gs). The available
data for that reaction support the hypothesis that 6He may not be a halo
nucleus. However, those data are scarce and there is clearly a need for more to
elicit the microscopic structure of 6He.Comment: 18 pages, 8 figures (added 4 figures), added reference. Version
accepted for publication in Phys. Rev.
Muon capture by 3He nuclei followed by proton and deuteron production
The paper describes an experiment aimed at studying muon capture by
nuclei in pure and mixtures at various densities. Energy distributions of
protons and deuterons produced via and are measured for the
energy intervals MeV and MeV, respectively. Muon capture
rates, and are obtained using two different analysis methods. The
least--squares methods gives , . The Bayes theorem
gives ,
. The experimental
differential capture rates, and , are compared with theoretical
calculations performed using the plane--wave impulse approximation (PWIA) with
the realistic NN interaction Bonn B potential. Extrapolation to the full energy
range yields total proton and deuteron capture rates in good agreement with
former results.Comment: 17 pages, 13 figures, accepted for publication in PR
Pion interaction with the trinucleon up to the eta production threshold
Pion elastic, charge exchange scattering and induced eta production on the
trinucleon systems are investigated in a coupled-channels approach in momentum
space with Fadeev wave functions. The channel is
included using an isobar model with S-, P-, and D-wave resonances. While the
coherent reactions like He(He can be reasonably well reproduced
up to =500 MeV, large discrepancies appear for the incoherent
processes, He(H and He(H at backward
angles and energies above -resonance. In the forward direction the
calculations underestimate the experimental measurements very
close to threshold but agreement with the data improves with increasing pion
energy. Predictions are made for the asymmetries of the various reactions on
polarized He.Comment: 40 pages, 12 figures (available from the authors), Mainz preprint
MKPH-T-92-1
Induced pseudoscalar coupling of the proton weak interaction
The induced pseudoscalar coupling is the least well known of the weak
coupling constants of the proton's charged--current interaction. Its size is
dictated by chiral symmetry arguments, and its measurement represents an
important test of quantum chromodynamics at low energies. During the past
decade a large body of new data relevant to the coupling has been
accumulated. This data includes measurements of radiative and non radiative
muon capture on targets ranging from hydrogen and few--nucleon systems to
complex nuclei. Herein the authors review the theoretical underpinnings of
, the experimental studies of , and the procedures and uncertainties
in extracting the coupling from data. Current puzzles are highlighted and
future opportunities are discussed.Comment: 58 pages, Latex, Revtex4, prepared for Reviews of Modern Physic
Coherent Compton scattering on light nuclei in the delta resonance region
Coherent Compton scattering on light nuclei in the delta resonance region is
studied in the impulse approximation and is shown to be a sensitive probe of
the in-medium properties of the delta resonance. The elementary amplitude on a
single nucleon is calculated from the unitary K-matrix approach developed
previously. Modifications of the properties of the delta resonance due to the
nuclear medium are accounted for through the self-energy operator of the delta,
calculated from the one-pion loop. The dominant medium effects such as the
Pauli blocking, mean-field modification of the nucleon and delta masses, and
particle-hole excitations in the pion propagator are consistently included in
nuclear matter.Comment: 30 pages, 11 figures, accepted for publication in Phys. Rev.
Nuclear astrophysics: the unfinished quest for the origin of the elements
Half a century has passed since the foundation of nuclear astrophysics. Since
then, this discipline has reached its maturity. Today, nuclear astrophysics
constitutes a multidisciplinary crucible of knowledge that combines the
achievements in theoretical astrophysics, observational astronomy,
cosmochemistry and nuclear physics. New tools and developments have
revolutionized our understanding of the origin of the elements: supercomputers
have provided astrophysicists with the required computational capabilities to
study the evolution of stars in a multidimensional framework; the emergence of
high-energy astrophysics with space-borne observatories has opened new windows
to observe the Universe, from a novel panchromatic perspective; cosmochemists
have isolated tiny pieces of stardust embedded in primitive meteorites, giving
clues on the processes operating in stars as well as on the way matter
condenses to form solids; and nuclear physicists have measured reactions near
stellar energies, through the combined efforts using stable and radioactive ion
beam facilities. This review provides comprehensive insight into the nuclear
history of the Universe and related topics: starting from the Big Bang, when
the ashes from the primordial explosion were transformed to hydrogen, helium,
and few trace elements, to the rich variety of nucleosynthesis mechanisms and
sites in the Universe. Particular attention is paid to the hydrostatic
processes governing the evolution of low-mass stars, red giants and asymptotic
giant-branch stars, as well as to the explosive nucleosynthesis occurring in
core-collapse and thermonuclear supernovae, gamma-ray bursts, classical novae,
X-ray bursts, superbursts, and stellar mergers.Comment: Invited Review. Accepted for publication in "Reports on Progress in
Physics" (version with low-resolution figures
- …