42 research outputs found
Relativistic Effects and the Role of Heavy Meson Exchange in Deuteron Photodisintegration
Relativistic effects and the role of heavy meson exchange in deuteron
photodisintegration are studied systematically for photon energies below the
pion production threshold. In a (p/M)-expansion, all leading order relativistic
one-body and pi-exchange as well as all static heavy meson exchange currents
consistent with the Bonn OBEPQ model are included. In addition, one- and
two-body boost effects have been investigated. Sizeable effects from the
various two-body contributions beyond pi-exchange have been found in almost
every observable considered, i.e., differential cross section and single
polarization observables.Comment: 14 pages revtex including 8 postscript figure
Meson retardation in deuteron photodisintegration above pi-threshold
Photodisintegration of the deuteron above pi-threshold is studied in a
coupled channel approach including N Delta- and pi d-channels with pion
retardation in potentials and exchange currents. A much improved description of
total and differential cross sections in the energy region between pi-threshold
and 400-450 MeV is achieved.Comment: 12 pages revtex including 5 postscript figure
Deuteron Compton Scattering in Effective Field Theory: Spin-Dependent Cross Sections and Asymmetries
Polarized Compton scattering on the deuteron is studied in nuclear effective
field theory. A set of tensor structures is introduced to define 12 independent
Compton amplitudes. The scalar and vector amplitudes are calculated up to
in low-energy power counting. Significant
contribution to the vector amplitudes is found to come from the spin-orbit type
of relativistic corrections. A double-helicity dependent cross section
is calculated to the same
order, and the effect of the nucleon isoscalar spin-dependent polarizabilities
is found to be smaller than the effect of isoscalar spin-independent ones.
Contributions of spin-independent polarizabilities are investigated in various
asymmetries, one of which has as large as 12 (26) percent effect at the
center-of-mass photon energy 30 (50) MeV.Comment: 22 pages, 8 figures included, replaced with the version submitted to
PR
Chiral perturbation theory for electroweak reactions on deuterium
I summarize two recent applications of chiral perturbation theory to
electromagnetic reactions on deuterium: elastic electron-deuteron scattering,
and Compton scattering on deuterium. Both calculations have now been carried
out to three orders in the chiral expansion. The expansion shows good
convergence and is able to reproduce data for q < 600 MeV in e-d and for
omega=55-95 MeV in gamma-d. These results demonstrate that ChiPT can be used to
reliably compute operators and wave functions for low-momentum-transfer
reactions in light nuclear systems.Comment: 10 pages, 6 figures. Write-up of invited talk at INT Workshop on
"Nuclear Forces and the Quantum Many-Body Problem", October 4-8, 200
Longitudinal response functions of 3H and 3He
Trinucleon longitudinal response functions R_L(q,omega) are calculated for q
values up to 500 MeV/c. These are the first calculations beyond the threshold
region in which both three-nucleon (3N) and Coulomb forces are fully included.
We employ two realistic NN potentials (configuration space BonnA, AV18) and two
3N potentials (UrbanaIX, Tucson-Melbourne). Complete final state interactions
are taken into account via the Lorentz integral transform technique. We study
relativistic corrections arising from first order corrections to the nuclear
charge operator. In addition the reference frame dependence due to our
non-relativistic framework is investigated. For q less equal 350 MeV/c we find
a 3N force effect between 5 and 15 %, while the dependence on other theoretical
ingredients is small. At q greater equal 400 MeV/c relativistic corrections to
the charge operator and effects of frame dependence, especially for large
omega, become more important. In comparison with experimental data there is
generally a rather good agreement. Exceptions are the responses at excitation
energies close to threshold, where there exists a large discrepancy with
experiment at higher q. Concerning the effect of 3N forces there are a few
cases, in particular for the R_L of 3He, where one finds a much improved
agreement with experiment if 3N forces are included.Comment: 26 pages, 9 figure
Local gauge invariance implies Siegert's hypothesis
The nonrelativistic Ward-Takahashi identity, a consequence of local gauge
invariance in quantum mechanics, shows the necessity of exchange current
contributions in case of nonlocal and/or isospin-dependent potentials. It also
implies Siegert's hypothesis: in the nonrelativistic limit, two-body charge
densities identically vanish. Neither current conservation, which follows from
global gauge invariance, nor the constraints of (lowest order) relativity are
sufficient to arrive at this result. Furthermore, a low-energy theorem for
exchange contributions is established.Comment: 5 pages, REVTE
On the extraction of electromagnetic properties of the Delta(1232) excitation from pion photoproduction
Several methods for the treatment of pion photoproduction in the region of
the Delta(1232) resonance are discussed, in particular the effective Lagrangian
approach and the speed plot analysis are compared to a dynamical treatment. As
a main topic, we discuss the extraction of the genuine resonance parts of the
magnetic dipole and electric quadrupole multipoles of the electromagnetic
excitation of the resonance. To this end, we try to relate the various values
for the ratio R_{EM} of the E2 to M1 multipole excitation strengths for the
Delta(1232) resonance as extracted by the different methods to corresponding
ratios of a dynamical model. Moreover, it is confirmed that all methods for
extracting resonance properties suffer from an unitary ambiguity which is due
to some phenomenological contributions entering the models.Comment: 22 pages revtex including 7 postscript figure
Consistent Treatment of Relativistic Effects in Electrodisintegration of the Deuteron
The influence of relativistic contributions to deuteron electrodisintegration
is systematically studied in various kinematic regions of energy and momentum
transfer. As theoretical framework the equation-of-motion and the unitarily
equivalent S-matrix approaches are used. In a (p/M)-expansion, all leading
order relativistic -exchange contributions consistent with the Bonn OBEPQ
model are included. In addition, static heavy meson exchange currents including
boost terms, -currents, and -isobar contributions
are considered. Sizeable effects from the various relativistic two-body
contributions, mainly from -exchange, have been found in inclusive form
factors and exclusive structure functions for a variety of kinematic regions.Comment: 41 pages revtex including 15 postscript figure
The relativistic impulse approximation for the exclusive electrodisintegration of the deuteron
The electrodisintegration of the deuteron in the frame of the Bethe-Salpeter
approach with a separable kernel of the nucleon-nucleon interaction is
considered. This conception keeps the covariance of a description of the
process. A comparison of relativistic and nonrelativistic calculations is
presented. The factorization of the cross section of the reaction in the
impulse approximation is obtained by analytical calculations. It is shown that
the photon-neutron interaction plays an important role.Comment: 31 pages, 14 figures, 1 tabl
Nucleon-Nucleon Correlations and Two-Nucleon Currents in Exclusive () Reactions
The contributions of short-range nucleon-nucleon (NN) correlations, various
meson exchange current (MEC) terms and the influence of isobar
excitations (isobaric currents, IC) on exclusive two-nucleon knockout reactions
induced by electron scattering are investigated. The nuclear structure
functions are evaluated for nuclear matter. Realistic NN interactions derived
in the framework of One-Boson-Exchange model are employed to evaluate the
effects of correlations and MEC in a consistent way. The correlations
correlations are determined by solving the Bethe-Goldstone equation. This
yields significant contributions to the structure functions W_L and W_T of the
(e,e'pn) and (e,e'pp) reactions. These contributions compete with MEC
corrections originating from the and exchange terms of the same
interaction. Special attention is paid to the so-called 'super parallel'
kinematics at momentum transfers which can be measured e.g. at MAMI in Mainz.Comment: 14 pages, 8 figures include