234 research outputs found
Longitudinal response function of 4He with a realistic force
The longitudinal response function of 4He is calculated with the Argonne V18
potential. The comparison with experiment suggests the need of a three-body
force. When adding the Urbana IX three-body potential in the calculation of the
lower longitudinal multipoles, the total strength is suppressed in the
quasi-elastic peak, towards the trend of the experimental data.Comment: 3 pages, 3 figures, proceedings of the 20th European Conference on
Few-Body Problems in Physics (EFB20
Neutrino processes in partially degenerate neutron matter
We investigate neutrino processes for conditions reached in simulations of
core-collapse supernovae. Where neutrino-matter interactions play an important
role, matter is partially degenerate, and we extend earlier work that addressed
the degenerate regime. We derive expressions for the spin structure factor in
neutron matter, which is a key quantity required for evaluating rates of
neutrino processes. We show that, for essentially all conditions encountered in
the post-bounce phase of core-collapse supernovae, it is a very good
approximation to calculate the spin relaxation rates in the nondegenerate
limit. We calculate spin relaxation rates based on chiral effective field
theory interactions and find that they are typically a factor of two smaller
than those obtained using the standard one-pion-exchange interaction alone.Comment: 41 pages, 9 figures, NORDITA-2011-116; added comparison figures and
fit function for use in simulations, to appear in Astrophys.
Case study: calculation of a narrow resonance with the LIT method
The possibility to resolve narrow structures in reaction cross sections in
calculations with the Lorentz integral transform (LIT) method is studied. To
this end we consider a fictitious two-nucleon problem with a low-lying and
narrow resonance in the nucleon-nucleon partial wave and calculate the
corresponding ``deuteron photoabsorption cross section''. In the LIT method the
use of continuum wave functions is avoided and one works instead with a
localized function \tilde\Psi. In this case study it is investigated how far
into the asymptotic region \tilde\Psi has to be determined in order to obtain a
precise resolution of the artificially introduced E1 resonance. Comparing with
the results of a conventional calculation with explicit neutron-proton
continuum wave functions it is shown that the LIT approach leads to an
excellent reproduction of the cross section in the resonance region and of
further finer cross section details at higher energies. To this end, however,
for \tilde\Psi one has to take into account two-nucleon distances up to at
least 30 fm.Comment: 18 pages, 11 figure
Photonuclear sum rules and the tetrahedral configuration of He
Three well known photonuclear sum rules (SR), i.e. the Thomas-Reiche-Kuhn,
the bremsstrahlungs and the polarizability SR are calculated for 4He with the
realistic nucleon-nucleon potential Argonne V18 and the three-nucleon force
Urbana IX. The relation between these sum rules and the corresponding energy
weighted integrals of the cross section is discussed. Two additional
equivalences for the bremsstrahlungs SR are given, which connect it to the
proton-neutron and neutron-neutron distances. Using them, together with our
result for the bremsstrahlungs SR, we find a deviation from the tetrahedral
symmetry of the spatial configuration of 4He. The possibility to access this
deviation experimentally is discussed.Comment: 13 pages, 1 tabl
Effects of three-nucleon forces and two-body currents on Gamow-Teller strengths
We optimize chiral interactions at next-to-next-to leading order to
observables in two- and three-nucleon systems, and compute Gamow-Teller
transitions in carbon-14, oxygen-22 and oxygen-24 using consistent two-body
currents. We compute spectra of the daughter nuclei nitrogen-14, fluorine-22
and fluorine-24 via an isospin-breaking coupled-cluster technique, with several
predictions. The two-body currents reduce the Ikeda sum rule, corresponding to
a quenching factor q^2 ~ 0.84-0.92 of the axial-vector coupling. The half life
of carbon-14 depends on the energy of the first excited 1+ state, the
three-nucleon force, and the two-body current
The nuclear electric polarizability of 6He
We present an estimate of the nuclear electric polarizability of the 6He halo
nucleus based on six-body microscopic calculations. Wave functions are obtained
from semi-realistic two-body interactions using the hyperspherical harmonics
expansion method. The polarizability is calculated as a sum rule of the dipole
response function using the Lanczos algorithm and also by integrating the
photo-absorption cross section calculated via the Lorentz integral transform
method. We obtain alpha_E=1.00(14) fm^3, which is much smaller than the
published value 1.99(40) fm^3 extracted from experimental data. This points
towards a potential disagreement between microscopic theories and experimental
observations.Comment: 8 pages, 8 figures, minor changes, added error analysi
Three-body Force Effects on the Longitudinal Response Function of 4He
In this contribution we summarize recent results on the longitudinal response function of 4He. It is intended to give an important contribution to one of the most interesting and much discussed topics in nuclear physics at present, i.e. the nuclear many-body forces. The longitudinal response is considered as a possible observable, involving many body scattering states, sensitive to the three-nucleon force (3NF). Such a sensitivity is predicted by ab initio calculations performed using the Lorentz Integral transform (LIT) method. The kinematics that are more interesting to measure are discussed
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