327 research outputs found
Two-photon exchange and elastic electron-proton scattering
Two-photon exchange contributions to elastic electron-proton scattering cross
sections are evaluated in a simple hadronic model including the finite size of
the proton. The corrections are found to be small in magnitude, but with a
strong angular dependence at fixed . This is significant for the
Rosenbluth technique for determining the ratio of the electric and magnetic
form factors of the proton at high , and partly reconciles the apparent
discrepancy with the results of the polarization transfer technique.Comment: 4 pages, 4 figures, v2: additional references and minor
clarifications in text, accepted in Physical Review Letter
resonance contribution to two-photon exchange in electron-proton scattering
We calculate the effects on the elastic electron-proton scattering cross
section of the two-photon exchange contribution with an intermediate
resonance. The two-photon exchange contribution is found to be smaller
in magnitude than the previously evaluated nucleon contribution, with an
opposite sign at backward scattering angles. The sum of the nucleon and
two-photon exchange corrections has an angular dependence compatible
with both the polarisation transfer and the Rosenbluth methods of measuring the
nucleon electromagnetic form factors.Comment: 9 pages, 3 figures, RevTeX4; more complete discussion of results,
conclusions unchanged; to be published in Physical Review Letter
Polarization transfer observables for quasielastic proton-nucleus scattering in terms of a complete Lorentz invariant representation of the NN scattering matrix
For the calculation of polarization transfer observables for quasielastic
scattering of protons on nuclei, a formalism in the context of the Relativistic
Plane Wave Impulse Approximation is developed, in which the interaction matrix
is expanded in terms of a complete set of 44 independent invariant amplitudes.
A boson-exchange model is used to predict the 39 amplitudes which were omitted
in the formerly used five-term parameterization(the SPVAT form) of the
nucleon-nucleon scattering matrix. Use of the complete set of amplitudes
eliminates the arbitrariness of the five-term representation.Comment: 29 pages, 2 figure
Form Factors from a Relativistic Dynamical Model of Pion Electroproduction
We obtain the electromagnetic form factors of the transition
by analyzing recent pion-electroproduction data using a fully relativistic
dynamical model. Special care is taken to satisfy Ward-Takahashi identities for
the Born term in the presence of form factors thereby allowing the use of
realistic electromagnetic form factors of the nucleon and pion. We parametrize
the dependence of the {\it bare} form factors by a
three-parameter form which is consistent with the asymptotic behavior inferred
from QCD. The parameters of the bare form factors are the
only free parameters of the model and are fitted to the differential
cross-section and multipole-analysis data up to (GeV/c) in the
-resonance region. This analysis emphasizes the significance of
the pion-cloud effects in the extraction of the resonance parameters.Comment: 9 pages, 5 figures, several small corrections, to be published in
Phys. Rev.
Two-photon exchange in elastic electron-nucleon scattering
A detailed study of two-photon exchange in unpolarized and polarized elastic
electron--nucleon scattering is presented, taking particular account of nucleon
finite size effects. Contributions from nucleon elastic intermediate states are
found to have a strong angular dependence, which leads to a partial resolution
of the discrepancy between the Rosenbluth and polarization transfer
measurements of the proton electric to magnetic form factor ratio, G_E/G_M. The
two-photon exchange contribution to the longitudinal polarization transfer P_L
is small, whereas the contribution to the transverse polarization transfer P_T
is enhanced at backward angles by several percent, increasing with Q^2. This
gives rise to a small, ~3% suppression of G_E/G_M obtained from the
polarization transfer ratio P_T/P_L at large Q^2. We also compare the
two-photon exchange effects with data on the ratio of e^+ p to e^- p cross
sections, which is predicted to be enhanced at backward angles. Finally, we
evaluate the corrections to the form factors of the neutron, and estimate the
elastic intermediate state contribution to the ^3He form factors
The Four-Boson System with Short-Range Interactions
We consider the non-relativistic four-boson system with short-range forces
and large scattering length in an effective quantum mechanics approach. We
construct the effective interaction potential at leading order in the large
scattering length and compute the four-body binding energies using the
Yakubovsky equations. Cutoff independence of the four-body binding energies
does not require the introduction of a four-body force. This suggests that two-
and three-body interactions are sufficient to renormalize the four-body system.
We apply the equations to 4He atoms and calculate the binding energy of the 4He
tetramer. We observe a correlation between the trimer and tetramer binding
energies similar to the Tjon line in nuclear physics. Over the range of binding
energies relevant to 4He atoms, the correlation is approximately linear.Comment: 23 pages, revtex4, 5 PS figures, discussion expanded, results
unchange
Relativistic quasipotential equations with u-channel exchange interactions
Various quasipotential two-body scattering equations are studied at the
one-loop level for the case of - and -channel exchange potentials. We
find that the quasipotential equations devised to satisfy the one-body limit
for the -channel exchange potential can be in large disagreement with the
field-theoretical prediction in the case of -channel exchange interactions.
Within the spectator model, the description of the -channel case improves if
another choice of the spectator particle is made. Since the appropriate choice
of the spectator depends strongly on the type of interaction used, one faces a
problem when both types of interaction are contained in the potential.
Equal-time formulations are presented, which, in the light-heavy particle
system corresponding to the mass situation of the system, approximate
in a reasonable way the field-theoretical result for both types of
interactions.Comment: Revtex, 20 pages, 12 PostScript figures, to appear in Phys. Rev.
Relativistic calculation of the triton binding energy and its implications
First results for the triton binding energy obtained from the relativistic
spectator or Gross equation are reported. The Dirac structure of the nucleons
is taken into account. Numerical results are presented for a family of
realistic OBE models with off-shell scalar couplings. It is shown that these
off-shell couplings improve both the fits to the two-body data and the
predictions for the binding energy.Comment: 5 pages, RevTeX 3.0, 1 figure (uses epsfig.sty
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