Study of the proton deformation using the Out-Of-Plane Spectrometers

Abstract

Nucleon deformation can be studied through electro-excitation to the first nucleon resonance, the Δ+(1232), and quantified through the quadrupole amplitudes in the γ*Ν --> Δ transition. A search for these small amplitudes has been the fo­ cus of a series of measurements undertaken at Bates-MIT. A first set of in-plane data show evidence for strong resonant and non-resonant ("background") ampli­ tudes in the longitudinal-transverse interference, which is sensitive to leading order to quadrupole contributions. Using beams of polarized electrons and the technique of out-of-plane detection, a second data set was collected on the two hadronic decay channels. It contains the first measurement of the fifth structure function on the nucléon, which provides an important theoretical constraint on the "background" amplitudes. Planned future measurements will focus on refining the resonance- background decomposition which is absolutely necessary before conclusions on the issue of nucléon deformation can be drawn.