11530731
Predictions of elastic alpha particle scattering from theoretical nuclear densities.
Data for the diffraction region of the elastic scattering of 42--140 MeV ..cap alpha.. particles from /sup 42,44,48/Ca have been compared to calculations based on Hartree-Fock nuclear densities. The ..cap alpha..-particle optical potential was obtained from the densities using a double folding model where the effective interaction was adjusted to the scattering from /sup 40/Ca. This study is insensitive to the details of the reaction calculation because it is restricted to the diffraction region and is only for the purpose of comparing relative sizes. The predicted cross section is in good agreement with data indicating that the theoretical densities are correct in the vicinity of 10% of central density to approximately +- 0.1 fm. This uncertainty is dominated by the approximations in the reaction calculation.
17840235
Unexpected impact of D waves in low-energy neutral pion photoproduction from the proton and the extraction of multipoles
Contributions of D waves to physical observables for neutral pion photoproduction from the proton in the near-threshold region are studied and means to isolate them are proposed. Various approaches to describe the multipoles are employed--a phenomenological one, a unitary one, and heavy baryon chiral perturbation theory. The results of these approaches are compared and found to yield essentially the same answers. D waves are seen to enter together with S waves in a way that any means which attempt to obtain the E{sub 0+} multipole accurately must rely on knowledge of D waves and that consequently the latter cannot be dismissed in analyses of low-energy pion photoproduction. It is shown that D waves have a significant impact on double-polarization observables that can be measured. This importance of D waves is due to the soft nature of the S wave and is a direct consequence of chiral symmetry and the Nambu-Goldstone nature of the pion. F-wave contributions are shown to be negligible in the near-threshold region.
7799394
A Status Report on Chiral Dynamics: Theory and Experiment
This paper gives an overview of a Workshop on Chiral Dynamics: Theory and Experiment which was held at MIT July 25-29, 1994; its unique feature was the equal mixture of theory and experiment. The purpose of the workshop was to bring together many of the active participants to assess the status of the field and to explore fruitful future directions. The foundations and present status of the theory were reviewed as well as the experimental status of the field. To facilitate the discussion of future directions working groups were organized on Threshold Photo/Electropion Production, Two-Pion Threshold and the Chiral Anomaly, Nucleon Polarizabilities, Pion and Sigma Polarizabilities, $\pi-\pi$ Interactions, and $\pi$-N Interactions; these subjects were the focus of the experimental aspects of the workshop.
119421635
Light Quark Mass Difference and Isospin Breaking In Electromagnetic Pion Production
It is demonstrated that there is a dynamic isospin breaking effect in the near threshold $\gamma^{*} N\to \pi N$ reaction due to the mass difference of the up and down quarks, which also causes isospin breaking in the $\pi N$ system. The photopion reaction is affected through final state $\pi N$ interactions (formally implemented by unitarity and time reversal invariance). It is also demonstrated that the near threshold $\gamma \vec{N} \to \pi N$ reaction is a practical reaction to measure isospin breaking in the $\pi N$ system, which was first predicted by Weinberg about 20 years ago but has never been experimentally tested.
116982551
Precise electro-pion production experiments in the delta region
High precision, exclusive measurements of the γ*N→Δ transition have been pursued at the MIT/Bates laboratory during in the last several years. These involve polarized beams, out-of-plane detection and focal plane polarimetry in the H(e,e′p)π0 and H(e,e′π+)n channels. The goal is a precise determination of the quadrupole amplitudes in the γ*N→Δ transition due to non-spherical interquark interactions and the isolation of competing background mechanisms such as Born terms and tails of higher resonances. Similar studies are being pursed intensively at many laboratories. The recent data from Bates are compared to theoretical calculations.
14938403
Measurements of the generalized electric and magnetic polarizabilities of the proton at low Q2 using the virtual-compton-scattering reaction.
The mean square polarizability radii of the proton have been measured for the first time in a virtual-Compton-scattering experiment performed at the MIT-Bates out-of-plane scattering facility. Response functions and polarizabilities obtained from a dispersion analysis of the data at Q2 = 0.057 GeV2/c2 are in agreement with O(p3) heavy baryon chiral perturbation theory. The data support the dominance of mesonic effects in the polarizabilities.
36903378
Terrestrial long-baseline neutrino-oscillation experiments.
We present a systematic study of potential long-baseline (distances {gt}300 km) neutrino-oscillation experiments performed with {nu}{sub {mu}} and {bar {nu}}{sub {mu}} beams from the Fermilab Main Injector ({l angle}{ital E}{sub {nu}}{r angle}{approx}10--20 GeV). The effects of matter enhancement are included where appropriate. We find that there are three key variables for such an experiment: the length of the baseline, the muon energy threshold, and the minimum measurable oscillation probability. An advantage in one of these variables can easily be neglected by a disadvantage in one of the others. Finally, for any long-baseline experiment at these energies to conclusively confirm or refute the interpretation of the atmospheric neutrino deficit as neutrino oscillations it must have a low energy threshold and a low minimum measurable oscillation probabilty.