747 research outputs found
Radiative corrections and parity-violating electron-nucleon scattering
Radiative corrections to the parity-violating asymmetry measured in elastic
electron-proton scattering are analyzed in the framework of the Standard Model.
We include the complete set of one-loop contributions to one quark current
amplitudes. The contribution of soft photon emission to the asymmetry is also
calculated, giving final results free of infrared divergences. The one quark
radiative corrections, when combined with previous work on many quark effects
and recent SAMPLE experimental data, are used to place some new constraints on
electroweak form factors of the nucleon
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
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
Quark-hadron duality constraints on \gamma Z box corrections to parity-violating elastic scattering
We examine the interference \gamma Z box corrections to parity-violating
elastic electron--proton scattering in the light of the recent observation of
quark-hadron duality in parity-violating deep-inelastic scattering from the
deuteron, and the approximate isospin independence of duality in the
electromagnetic nucleon structure functions down to Q^2 \approx 1 GeV^2.
Assuming that a similar behavior also holds for the \gamma Z proton structure
functions, we find that duality constrains the \gamma Z box correction to the
proton's weak charge to be \Re e\, \square_{\gamma Z}^V = (5.4 \pm 0.4) \times
10^{-3} at the kinematics of the Q_{\text{weak}} experiment. Within the same
model we also provide estimates of the \gamma Z corrections for future
parity-violating experiments, such as MOLLER at Jefferson Lab and MESA at
Mainz.Comment: 10 pages, 3 figures. Final version to be published in Phys. Lett.
The Rarita-Schwinger spin-3/2 equation in a nonuniform, central potential
The equations of motion for a massive spin-3/2 Rarita-Schwinger field in a
finite-range, central, Lorentz scalar potential are developed. It is shown that
the resulting density may not be everywhere positive definite.Comment: 9 pages, RevTe
Light Front Nuclear Physics: Toy Models, Static Sources and Tilted Light Front Coordinates
The principles behind the detailed results of a light-front mean field theory
of finite nuclei are elucidated by deriving the nucleon mode equation using a
simple general argument, based on the idea that a static source in equal time
coordinates corresponds to a moving source in light front coordinates. This
idea also allows us to solve several simple toy model examples: scalar field in
a box, 1+1 dimensional bag model, three-dimensional harmonic oscillator and the
Hulth\'en potential. The latter provide simplified versions of momentum
distributions and form factors of relevance to experiments. In particular, the
relativistic correction to the mean square radius of a nucleus is shown to be
very small. Solving these simple examples suggests another more general
approach-- the use of tilted light front coordinates. The simple examples are
made even simpler.Comment: 19 pages, references adde
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