The puzzle of the quark model: Weak radiative hyperon decays

Abstract

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 expectations 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 back to the issue of hadron compositeness and the nonequivalence of standard ways of imposing gauge-invariance condition at quark and hadron levels. This suggests that our understanding of nonlocal composite nature of hadrons may require some important change.