Isovector Spin States Oberved in Radiative Pion Capture in Flight and at Rest: Recent Results from SIN

Abstract

Photopion processes on first sight seem like a poor choice, compared to other charge exchange probes like (n, p) or (p,n) reactions or the traditional electromagnetic probe of nuclear structure at intermediate momentum transfers, electron scattering. They can not compete in final state resolution nor in precision. Their usefulness is limited to nuclear targets with mass below A = 40. The cross-sections for (π^±, γ) reactions in flight, where the momentum transfer can be varied, are typically 1μb/sr and the requirement to detect a high-energy photon with good resolution adds further difficulty. If negative pions are captured at rest, data are easier to obtain and hence exist for a variety of nuclei, but the momentum transfer is restricted to q ≅ m_π. Level spins therefore have to be inferred from other data or from comparison with theory. Despite all these shortcomings radiative pion absorption has contributed to the central topic of this conference, the study of spin-isospin states, since the late sixties when the sensitivity to this particular mode of nuclear excitation was first theoretically predicted and quickly experimentally demonstrated. ^1,2