726 research outputs found
Contribution of spin 1/2 and 3/2 resonances to two-photon exchange effects in elastic electron-proton scattering
We calculate contributions of hadron resonances to two-photon exchange
effects in electron-proton scattering. In addition to the nucleon and P33
resonance, the following heavier resonances are included as intermediate states
in the two-photon exchange diagrams: D13, D33, P11, S11 and S31. We show that
the corrections due to the heavier resonances are smaller that the dominant
nucleon and P33 contributions. We also find that there is a partial
cancellation between the contributions from the spin 1/2 and spin 3/2
resonances, which results in a further suppression of their aggregate
two-photon exchange effect.Comment: 6 pages, 1 figure; additional comparison with data, results
unchanged; to be published in Phys. Rev.
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
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
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
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
Mechano-responsive polymer solutions based on CO2 supersaturation: shaking-induced phase transitions and self-assembly or dissociation of polymeric nanoparticles
Mechanical stimulation of supersaturated aqueous CO2 solutions is accompanied by a pH increase within seconds. In solutions of tailored homo- and AB diblock copolymers this is exploited to induce micelle formation, or, taking advantage of an aqueous upper critical solution temperature transition, nanoparticle disassembly
Nucleon Sigma Term and In-medium Quark Condensate in the Modified Quark-Meson Coupling Model
We evaluate the nucleon sigma term and in-medium quark condensate in the
modified quark-meson coupling model which features a density-dependent bag
constant. We obtain a nucleon sigma term consistent with its empirical value,
which requires a significant reduction of the bag constant in the nuclear
medium similar to those found in the previous works. The resulting in-medium
quark condensate at low densities agrees well with the model independent linear
order result. At higher densities, the magnitude of the in-medium quark
condensate tends to increase, indicating no tendency toward chiral symmetry
restoration.Comment: 9 pages, modified version to be publishe
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