Unitary ambiguity in the extraction of theE2/M1 ratio for the γN⇆Δ transition

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Abstract
The resonant electric quadrupole amplitude in the transition γN⇆Δ(1232) is of great interest for the understanding of baryon structure. Various dynamical models have been developed to extract it from the corresponding photoproduction multipole of pions on nucleons. It is shown that once such a model is specified, a whole class of unitarily equivalent models can be constructed, all of them providing exactly the same fit to the experimental data. However, they may predict quite different resonant amplitudes. Therefore, the extraction of the E2/M1(γN⇆Δ) ratio (bare or dressed), which is based on a dynamical model using a largely phenomenological πN interaction, is not unique. © 1996 The American Physical Society.
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