2,429 research outputs found
Non-perturbative model for the half-off-shell \gamma N N vertex
Form factors in the nucleon-photon vertex with one off-shell nucleon are
calculated by dressing the vertex with pion loops up to infinite order. Cutting
rules and dispersion relations are implemented in the model. Using the
prescription of minimal substitution we construct a \gamma \pi N N vertex and
show that it has to be included in the model in order that the Ward-Takahashi
identity for the \gamma N N vertex be fulfilled. The vertex is to be applied in
a coupled-channel K-matrix formalism for Compton scattering, pion
photoproduction and pion scattering. The form factors show a pronounced cusp
structure at the pion threshold. As an illustration of a consistent application
of the model, we calculate the cross section of Compton scattering. To provide
gauge invariance in Compton scattering, a four-point \gamma \gamma N N contact
term is constructed using minimal substitution.Comment: 51 pages, 12 Postscript figures, using REVTeX. Submitted to Phys.
Rev. C, added reference
Q^2-Dependence of the Drell-Hearn-Gerasimov integral
The energy and four-momentum (Q^2) dependence of the photo-absorption cross
section on the proton is calculated for helicity -1/2 and -3/2 states. An
effective Lagrangian model is used, formulated in terms of meson and baryon
degrees of freedom, which obeys crossing symmetry, unitarity, Lorentz and gauge
invariance. The difference in the cross sections for the two helicity states,
the Drell-Hearn-Gerasimov integral , is evaluated at different
. We find that at small momentum transfer the absolute value of
first increases to reach a maximum at
before decreasing at higher .Comment: 8 pages, 3 figures, revte
On electromagnetic off-shell effects in proton-proton bremsstrahlung
We study the influence of the off-shell structure of the nucleon
electromagnetic vertex on proton-proton bremsstrahlung observables. Realistic
choices for these off-shell effects are found to have considerable influences
on observables such as cross sections and analyzing powers. The rescattering
contribution diminishes the effects of off-shell modifications in
negative-energy states.Comment: 7 pages, 3 figure
A description of odd mass W-isotopes in the Interacting 2 Boson-Fermion Model
The negative and positive parity low-spin states of the even-odd Tungsten
isotopes, 183,185,187W are studied in the frame work of the Interacting
Boson-Fermion Approximation (IBFA) model. The fermion that is coupled to the
system of bosons is taken to be in the negative parity 2f_7|2, 2f_5\2, 3p_3\2,
3p_1\2 and in the positive parity 1i_13\2 single-particle orbits. The
calculated energies of low-spin energy levels of the odd isotopes are found to
agree well with the experimental data. Also B(E2) values and spectroscopic
factors for single-neutron transfer are calculated and found to be in good
agreement with experimental data
Channel coupling effects in -meson photoproduction
We investigate -meson photoproduction in a coupled-channels
formulation. It is shown that channel coupling effects are large and account
for discrepancies observed in several single-channel treatments.Comment: 8 pages, 5 figures; small textual corrections and notation change
K-Lambda and K-Sigma photoproduction in a coupled channels framework
A coupled channels analysis, based on the K-matrix approach, is presented for
photo-induced kaon production. It is shown that channel coupling effects are
large and should not be ignored. The importance of contact terms in the
analysis, associated with short range correlations, is pointed out. The
extracted parameters are compared with SU(3)-model predictions.Comment: 15 pages, 10 figures; Submitted to Physical Review C. v2: A figure
showing the effect of P13(1900) added v3: Small spelling corrections, one
reference update
Quasi dynamical symmetry in an interacting boson model phase transition
The oft-observed persistence of symmetry properties in the face of strong
symmetry-breaking interactions is examined in the SO(5)-invariant interacting
boson model. This model exhibits a transition between two phases associated
with U(5) and O(6) symmetries, respectively, as the value of a control
parameter progresses from 0 to 1. The remarkable fact is that, for intermediate
values of the control parameter, the model states exhibit the characteristics
of its closest symmetry limit for all but a relatively narrow transition region
that becomes progressively narrower as the particle number of the model
increases. This phenomenon is explained in terms of quasi-dynamical symmetry.Comment: 4 figure
Semi-inclusive cross sections for deep-inelastic neutrino scattering on hydrogen and deuterium
The production of low energy protons observed in recent deep-inelastic (anti-)neutrino scattering on hydrogen and deuteron targets is interpreted in terms of fragmentation of the spectator di-quark and emission of nuclear spectators
Photoproduction of eta meson within a coupled-channels K-matrix approach
We investigate photoproduction of eta mesons off protons and neutrons within
a coupled-channels effective-Lagrangian method which is based on the K-matrix
approach. The two-body final channels included are pi-N, eta-N, phi-N, rho-N,
gamma-N, K-Lambda, and K-Sigma. Non-resonant meson-baryon interactions are
included in the model via nucleon intermediate states in the s- and u-channels,
meson exchanges in the t-channel amplitude and the u-channel resonances. The
nucleon resonances S_{11}(1535), S_{11}(1650), S_{31}(1620), P_{11}(1440),
P_{11}(1710), P_{13}(1720), P_{33}(1232), P_{33}(1600), D_{13}(1520),
D_{13}(1700), and D_{33}(1700) are included explicitly in calculations. Our
model describes simultaneously the available data as well on total and
differential cross sections as on beam and target asymmetries. This holds for
the p(gamma,eta)p reaction for photon energies ranging from very close to
threshold to up to 3 GeV. The polarization observables show strong sensitivity
to resonances that otherwise contribute only weakly to the total cross section.
It is found that the pronounced bump-like structure seen in the excitation
function of the n(gamma,eta)n cross section at gamma energies around 1 GeV, can
be explained by the interference effects of S_{11}, P_{11} and P_{13} resonance
contributions.Comment: 36 pages, 14 figures, one new figure added, discussions revised,
version to appear in Phys. Rev.
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