1,850 research outputs found
Breakup of three particles within the adiabatic expansion method
General expressions for the breakup cross sections in the lab frame for
reactions are given in terms of the hyperspherical adiabatic basis. The
three-body wave function is expanded in this basis and the corresponding
hyperradial functions are obtained by solving a set of second order
differential equations. The -matrix is computed by using two recently
derived integral relations. Even though the method is shown to be well suited
to describe processes, there are nevertheless particular configurations
in the breakup channel (for example those in which two particles move away
close to each other in a relative zero-energy state) that need a huge number of
basis states. This pathology manifests itself in the extremely slow convergence
of the breakup amplitude in terms of the hyperspherical harmonic basis used to
construct the adiabatic channels. To overcome this difficulty the breakup
amplitude is extracted from an integral relation as well. For the sake of
illustration, we consider neutron-deuteron scattering. The results are compared
to the available benchmark calculations
The proton-proton weak capture in chiral effective field theory
The astrophysical S-factor for proton-proton weak capture is calculated in
chiral effective field theory over the center-of-mass relative-energy range
0--100 keV. The chiral two-nucleon potential derived up to
next-to-next-to-next-to leading order is augmented by the full electromagnetic
interaction including, beyond Coulomb, two-photon and vacuum-polarization
corrections. The low-energy constants (LEC's) entering the weak current
operators are fixed so as to reproduce the A=3 binding energies and magnetic
moments, and the Gamow-Teller matrix element in tritium beta decay.
Contributions from S and P partial waves in the incoming two-proton channel are
retained. The S-factor at zero energy is found to be S(0)=(4.030 +/- 0.006) x
10^{-23} MeV fm^2, with a P-wave contribution of 0.020 x 10^{-23} MeV fm^2. The
theoretical uncertainty is due to the fitting procedure of the LEC's and to the
cutoff dependence.Comment: 4 pages, 3 figures; revisited version accepted for publication on
Phys. Rev. Lett. A misprint in Table II has been correcte
Neutron electromagnetic form factors and inclusive scattering of polarized electrons by polarized He and H targets
The electromagnetic inclusive responses of polarized He and H are
thoroughly investigated at the quasielastic peak for squared momentum transfers
up to , within the plane wave impulse approximation. Great
emphasys is put on the effects in the bound-state due to different two- and
three-body nuclear forces, and to the Coulomb interaction as well. A careful
analysis of the polarized responses allows to select possible experiments for
minimizing the model dependence in the extraction of the neutron
electromagnetic form factors. In particular, the relevant role played by the
proton in the transverse-longitudinal response of polarized He, at low
momentum transfer, can be utilized for obtaining valuable information on the
proton contribution to the total polarized response and eventually on the
neutron charge form factor.Comment: 27 pages, Latex, 9 Postscript figures. To appear in Phys. Rev. C
(July '97
Effect of three nucleon forces in p-3He scattering
The effect of the inclusion of different models of three nucleon (3N) forces
in p-3He elastic scattering at low energies is studied. Two models have been
considered: one derived from effective field theory at next-to-next-to-leading
order and one derived from a more phenomenological point of view -- the
so-called Illinois model. The four nucleon scattering observables are
calculated using the Kohn variational principle and the hyperspherical harmonic
technique and the results are compared with available experimental data. We
have found that with the inclusion of both 3N force models the agreement with
the experimental data is improved, in particular for the proton vector
analyzing power A_y.Comment: 8 pages, 4 figures, talk presented at the 20th International IUPAP
Conference on Few-Body Problems in Physics, 20 - 25 August, 2012, Fukuoka,
Japa
Integral relations and the adiabatic expansion method for 1+2 reactions above the breakup threshold: Helium trimers with soft-core potentials
The integral relations formalism introduced in \cite{bar09,rom11}, and
designed to describe 1+ reactions, is extended here to collision energies
above the threshold for the target breakup. These two relations are completely
general, and in this work they are used together with the adiabatic expansion
method for the description of 1+2 reactions. The neutron-deuteron breakup, for
which benchmark calculations are available, is taken as a test of the method.
The s-wave collision between the He atom and He dimer above the
breakup threshold and the possibility of using soft-core two-body potentials
plus a short-range three-body force will be investigated. Comparisons to
previous calculations for the three-body recombination and collision
dissociation rates will be shown.Comment: To be published in Physical Review
Effect of three-nucleon interaction in p-3He elastic scattering
We present a detailed study of the effect of different three-nucleon
interaction models in p-3He elastic scattering at low energies. In particular,
two models have been considered: one derived from effective field theory at
next-to-next-to-leading order and one derived from a more phenomenological
point of view -- the so-called Illinois model. The four-nucleon scattering
observables are calculated using the Kohn variational principle and the
hyperspherical harmonics technique and the results are compared with available
experimental data. We have found that the inclusion of either one of the other
force model improves the agreement with the experimental data, in particular
for the proton vector analyzing power.Comment: 4 pages, 3 figure
Comparative study of three-nucleon force models in systems
Using modern nucleon-nucleon interactions in the description of the
nuclei, it is not possible to reproduce both the three- and four-nucleon
binding energies simultaneously. This is one manifestation of the necessity of
including a three-nucleon force in the nuclear Hamiltonian. In this paper we
will perform a comparative study of some, widely used, three-nucleon force
models. We will analyze their capability to describe the aforementioned binding
energies as well as the doublet scattering length. A correct description
of these quantities can be considered a stringent requirement for a nuclear
Hamiltonian containing two- and three-nucleon interaction terms. As we will
show, this requirement is not fulfilled by several of the models available in
the literature. To satisfy it, we propose modifications in the parametrization
of the three-nucleon forces and we study their effects on few selected
low energy scattering observables.Comment: 30 pages, 10 figure
Implication of the proton-deuteron radiative capture for Big Bang Nucleosynthesis
The astrophysical -factor for the radiative capture He in
the energy-range of interest for Big Bang Nucleosynthesis (BBN) is calculated
using an {\it ab-initio} approach. The nuclear Hamiltonian retains both two-
and three-nucleon interactions - the Argonne and the Urbana IX,
respectively. Both one- and many-body contributions to the nuclear current
operator are included. The former retain for the first time, besides the
leading order contribution ( is the nucleon mass), also the next-to-leading
order term, proportional to . The many-body currents are constructed in
order to satisfy the current conservation relation with the adopted Hamiltonian
model. The hyperspherical harmonics technique is applied to solve the
bound and scattering states. A particular attention is used in this second case
in order to obtain, in the energy range of BBN, an uncertainty on the
astrophysical -factor of the order or below 1 %. Then, in this energy
range, the -factor is found to be 10 % larger than the currently
adopted values.Part of this increase (1-3 %) is due to the one-body
operator, while the remaining is due to the new more accurate scattering wave
functions. We have studied the implication of this new determination for the
He -factor on deuterium primordial abundance. We find that
the predicted theoretical value for H/H is in excellent agreement with its
experimental determination, using the most recent determination of baryon
density of Planck experiment, and with a standard number of relativistic
degrees of freedom during primordial nucleosynthesis.Comment: 5 pages, 2 figures, submitted to Phys. Rev. Let
General integral relations for the description of scattering states using the hyperspherical adiabatic basis
In this work we investigate 1+2 reactions within the framework of the
hyperspherical adiabatic expansion method. To this aim two integral relations,
derived from the Kohn variational principle, are used. A detailed derivation of
these relations is shown. The expressions derived are general, not restricted
to relative partial waves, and with applicability in multichannel
reactions. The convergence of the -matrix in terms of the adiabatic
potentials is investigated. Together with a simple model case used as a test
for the method, we show results for the collision of a He atom on a \dimer
dimer (only the elastic channel open), and for collisions involving a Li
and two He atoms (two channels open).Comment: Accepted for publication in Physical Review
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