2,674 research outputs found
Muon capture on light nuclei
This work investigates the muon capture reactions 2H(\mu^-,\nu_\mu)nn and
3He(\mu^-,\nu_\mu)3H and the contribution to their total capture rates arising
from the axial two-body currents obtained imposing the
partially-conserved-axial-current (PCAC) hypothesis. The initial and final A=2
and 3 nuclear wave functions are obtained from the Argonne v_{18} two-nucleon
potential, in combination with the Urbana IX three-nucleon potential in the
case of A=3. The weak current consists of vector and axial components derived
in chiral effective field theory. The low-energy constant entering the vector
(axial) component is determined by reproducting the isovector combination of
the trinucleon magnetic moment (Gamow-Teller matrix element of tritium
beta-decay). The total capture rates are 393.1(8) s^{-1} for A=2 and 1488(9)
s^{-1} for A=3, where the uncertainties arise from the adopted fitting
procedure.Comment: 6 pages, submitted to Few-Body Sys
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
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
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
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
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
Electrodisintegration of He below and above deuteron breakup threshold
Recent advances in the study of electrodisintegration of 3He are presented
and discussed. The pair-correlated hyperspherical harmonics method is used to
calculate the initial and final state wave functions, with a realistic
Hamiltonian consisting of the Argonne v18 two-nucleon and Urbana IX
three-nucleon interactions. The model for the nuclear current and charge
operators retains one- and many-body contributions. Particular attention is
made in the construction of the two-body current operators arising from the
momentum-dependent part of the two-nucleon interaction. Three-body current
operators are also included so that the full current operator is strictly
conserved. The present model for the nuclear current operator is tested
comparing theoretical predictions and experimental data of pd radiative capture
cross section and spin observables.Comment: 5 pages, 5 figures, submitted to Eur. Phys. J.
Electromagnetic transitions for A=3 nuclear systems
Recent advances in the study of pd radiative capture in a wide range of
center-of-mass energy below and above deuteron breakup threshold are presented
and discussed.Comment: Invited lead talk at the 19th European Conference on Few-Body
Problems in Physics, Groningen, The Netherlands, 8/23 - 8/27 2004, 5 pages, 4
figure
Electromagnetic structure of A=2 and 3 nuclei in chiral effective field theory
The objectives of the present work are twofold. The first is to address and
resolve some of the differences present in independent,
chiral-effective-field-theory (\chiEFT) derivations up to one loop, recently
appeared in the literature, of the nuclear charge and current operators. The
second objective is to provide a complete set of \chiEFT predictions for the
structure functions and tensor polarization of the deuteron, for the charge and
magnetic form factors of 3He and 3H, and for the charge and magnetic radii of
these few-nucleon systems. The calculations use wave functions derived from
high-order chiral two- and three-nucleon potentials and Monte Carlo methods to
evaluate the relevant matrix elements. Predictions based on conventional
potentials in combination with \chiEFT charge and current operators are also
presented. There is excellent agreement between theory and experiment for all
these observables for momentum transfers up to q< 2.0-2.5 (1/fm); for a subset
of them, this agreement extends to momentum transfers as high as q~5-6 (1/fm).
A complete analysis of the results is provided.Comment: 34 pages, Revte
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