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Who ordered that? exclaimed the physicist I.I. Rabi over 70 years ago when scientists discovered the muon. A muon is an elementary particle of matter, a heavier copy of the electron, the particle we all know from the electricity that lights our homes. Can a muon change into an electron? The answer tells us if we have the laws of physics right.
Elementary particles come in families: quarks, neutrinos and charged leptons. Members of the quark and neutrino families can morph from one family member to another from one kind of quark to another, for example. But the charged leptons, the electron, muon and tau, seem different. No one has ever observed them switching identities.
If the charged lepton family really is different, if the electron, muon and tau do not change from one to another, it means there is no sensible model to unify the different forces of nature and there is something very wrong with theories of the fundamental laws of physics. So physicists need to look as hard as possible to see if muons and electrons do very rarely interchange identities, or truly not at all.
By producing huge numbers of muons in a controlled environment, physicists of the mu2e experiment hope to observe the direct conversion of muons into electrons. Catching them in the act would finally answer Rabi's question: the laws of physics ordered the muon.
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