Today, they continue to present us with puzzles. The standard model of subatomic particles and interactions purports to provide an exact description of the muon decay spectrum. This is the same model which describes the interactions and indeed the existence of the quarks, and which has been incredibly successful in describing a wide variety of subatomic phenomena. In spite of this success, it is widely believed that the theory is only approximately correct. Indeed virtual armies have been deployed in an attempt to discover phenomena which reveal the shortcomings of the standard theory.
Muon decay is a particularly interesting phenomena.
In particular, the quarks are not involved in this decay. Furthermore, the decay is thought to respect certain symmetries. There is no a priori reason to expect that neutrinos violating this symmetry principle cannot exist. It is likely that a careful examination of the muon decay spectrum would be the best test for the existence of such right-handed interactions. It is remarkable that to date, a precise measurement of the entire decay distribution has not been undertaken.