The muon was discovered accidentally in 1936-1937 in the hunt for
Yukawa's
pion. At the time
it seemed to serve no purpose in the scheme of
particle physics.
I.I. Rabi
asked, "Who ordered that?", when he was told of it. It proved to be only the
first of a soon to be
huge number of
anomalous particles that forced many
new
theories to be developed.
Physicists learned that many of the particles
previously considered
fundamental, such as the
proton and
neutron, are actually
composite objects. However, when the dust finally settled and the
Standard Model was born, the muon retained it's status as fundamental, as the
charged lepton in the
second generation of elementary particles.
[Ed. note: Corrected muon discovery date 10/19/2001]