Sixty-five years of muons

18 January 2002

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In 1937, Carl Anderson and Seth Neddermeyer working at Caltech discovered evidence for a charged particle about 200 times heavier than an electron. This particle we now know as the muon - a heavier version of the electron. Anderson and Neddermeyer discovered the muon through the tracks of cosmic rays in a cloud chamber. Since then particle physicists have learned how to make beams of muons to probe inside the proton, and how to study materials using the phenomenon of muon spin rotation. They can detect muons in a variety of ways - this image shows the pattern of Cerenkov light produced by a high-energy muon in the water of the Super-Kamiokande detector. But the question first posed by physicist Isidor Rabi, remains. "Who ordered that?". We still do not fully understand why "heavy electrons" are a necessary part of Nature's set of particle building bricks.

Credit: Super-Kamiokande/Tomasz Barszczak

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