The puzzle of the quark model: weak radiative hyperon decays

Weak radiative hyperon decays present us with a long-standing puzzle, namely the question of validity of a hadron-level theorem proved by Hara. We briefly discuss the conflict between expectation based on Hara's theorem and experiment as well as the way in which the quark model evades the theorem. Violation of Hara's theorem in the quark model is traced to the issue of hadron compositeness and the non equivalence of standard ways of imposing gauge-invariance condition at quark and hadron levels. This suggests our understanding of nonlocal composite nature of hadrons may require some important change. (author). 14 refs, 2 figs, 3 tabs