2,125 research outputs found
Coulomb effects in nucleon-deuteron polarization-transfer coefficients
Coulomb effects in the neutron-deuteron and proton-deuteron
polarization-transfer coefficients , ,
and are studied at energies above the deuteron breakup threshold.
Theoretical predictions for these observables are evaluated in the framework of
the Kohn Variational Principle using correlated basis functions to expand the
three-nucleon scattering wave function. The two-nucleon Argonne and
the three-nucleon Urbana IX potentials are considered. In the proton-deuteron
case, the Coulomb interaction between the two protons is included explicitly
and the results are compared to the experimental data available at
MeV. In the neutron-deuteron case, a comparison to a
recent measurement of by Hempen {\sl et al.} at MeV
evidences a contribution of the calculated Coulomb effects opposite to those
extracted from the experiment.Comment: 7 pages, 3 figure
The two-nucleon electromagnetic charge operator in chiral effective field theory (EFT) up to one loop
The electromagnetic charge operator in a two-nucleon system is derived in
chiral effective field theory (EFT) up to order (or N4LO), where
denotes the low-momentum scale and is the electric charge. The specific
form of the N3LO and N4LO corrections from, respectively, one-pion-exchange and
two-pion-exchange depends on the off-the-energy-shell prescriptions adopted for
the non-static terms in the corresponding potentials. We show that different
prescriptions lead to unitarily equivalent potentials and accompanying charge
operators. Thus, provided a consistent set is adopted, predictions for physical
observables will remain unaffected by the non-uniqueness associated with these
off-the-energy-shell effects.Comment: 16 pages, 10 figure
Electromagnetic processes in a EFT framework
Recently, we have derived a two--nucleon potential and consistent nuclear
electromagnetic currents in chiral effective field theory with pions and
nucleons as explicit degrees of freedom. The calculation of the currents has
been carried out to include NLO corrections, consisting of two--pion
exchange and contact contributions. The latter involve unknown low-energy
constants (LECs), some of which have been fixed by fitting the S- and
P-wave phase shifts up to 100 MeV lab energies. The remaining LECs entering the
current operator are determined so as to reproduce the experimental deuteron
and trinucleon magnetic moments, as well as the cross section. This
electromagnetic current operator is utilized to study the and He
radiative captures at thermal neutron energies. Here we discuss our results
stressing on the important role played by the LECs in reproducing the
experimental data.Comment: Invited talk at the 5th International Conference on Quarks and
Nuclear Physics, to appear in Chinese Physics
Electromagnetic Structure and Reactions of Few-Nucleon Systems in EFT
We summarize our recent work dealing with the construction of the
nucleon-nucleon potential and associated electromagnetic currents up to one
loop in chiral effective field theory (EFT). The magnetic dipole
operators derived from these currents are then used in hybrid calculations of
static properties and low-energy radiative capture processes in few-body
nuclei. A preliminary set of results are presented for the magnetic moments of
the deuteron and trinucleons and thermal neutron captures on , , and
He.Comment: Invited talk to the 19th International IUPAP Conference on Few-Body
Problems in Physic
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 structure of A=2 and 3 nuclei and the nuclear current operator
Different models for conserved two- and three-body electromagnetic currents
are constructed from two- and three-nucleon interactions, using either
meson-exchange mechanisms or minimal substitution in the momentum dependence of
these interactions. The connection between these two different schemes is
elucidated. A number of low-energy electronuclear observables, including (i)
radiative capture at thermal neutron energies and deuteron
photodisintegration at low energies, (ii) and radiative capture
reactions, and (iii) isoscalar and isovector magnetic form factors of H and
He, are calculated in order to make a comparative study of these models for
the current operator. The realistic Argonne two-nucleon and Urbana IX
or Tucson-Melbourne three-nucleon interactions are taken as a case study. For
=3 processes, the bound and continuum wave functions, both below and above
deuteron breakup threshold, are obtained with the correlated
hyperspherical-harmonics method. Three-body currents give small but significant
contributions to some of the polarization observables in the
H()He process and the H()H cross section at
thermal neutron energies. It is shown that the use of a current which did not
exactly satisfy current conservation with the two- and three-nucleon
interactions in the Hamiltonian was responsible for some of the discrepancies
reported in previous studies between the experimental and theoretical
polarization observables in radiative capture.Comment: 48 pages, 25 figures, 4 tables, revtex4. Submitted to Phys. Rev.
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
Chiral effective field theory predictions for muon capture on deuteron and 3He
The muon-capture reactions 2H(\mu^-,\nu_\mu)nn and 3He(\mu^-,\nu_\mu)3H are
studied with nuclear strong-interaction potentials and charge-changing weak
currents, derived in chiral effective field theory. The low-energy constants
(LEC's) c_D and c_E, present in the three-nucleon potential and (c_D)
axial-vector current, are constrained to reproduce the A=3 binding energies and
the triton Gamow-Teller matrix element. The vector weak current is related to
the isovector component of the electromagnetic current via the
conserved-vector-current constraint, and the two LEC's entering the contact
terms in the latter are constrained to reproduce the A=3 magnetic moments. The
muon capture rates on deuteron and 3He are predicted to be 399(3) sec^{-1} and
1494 (21) sec^{-1}, respectively, where the spread accounts for the cutoff
sensitivity as well as uncertainties in the LEC's and electroweak radiative
corrections. By comparing the calculated and precisely measured rates on 3He, a
value for the induced pseudoscalar form factor is obtained in good agreement
with the chiral perturbation theory prediction.Comment: 4 pages, 2 figures, revisited version accepted for publication on
Phys. Rev. Let
Proton-3He elastic scattering at low energies and the "A_y Puzzle"
The Kohn variational principle and the hyperspherical harmonic technique are
applied to study p-3He elastic scattering at low energies. Preliminary results
obtained using several interaction models are reported. The calculations are
compared to a recent phase shift analysis performed at the Triangle University
Nuclear Laboratory and to the available experimental data. Using a
three-nucleon interaction derived from chiral perturbation theory at N2LO, we
have found a noticeable reduction of the discrepancy observed for the A_y
observable.Comment: 9 pages, 9 figures, to be published in the Proceedings of the 19th
International IUPAP Conference on Few-Body Problems in Physics, Bonn, 200
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