muon
/ MYOO-on /
A muon is, in a sentence, a heavy electron. It carries the same negative electric charge as the electron and behaves like it in almost every way, but it weighs about 207 times as much. When it was first spotted in cosmic rays in the 1930s, it was so unexpected that the physicist I. I. Rabi reportedly quipped, 'Who ordered that?' — nobody had asked for a second copy of the electron.
Being heavier makes the muon unstable. A muon at rest survives only about 2.2 microseconds before it decays, almost always into an electron plus two neutrinos. Muons are made in nature when cosmic rays crash into the upper atmosphere, and they rain down on us constantly — roughly one passes through your outstretched palm every second. They are also produced on demand at accelerators, where their relatively long lifetime (long by particle standards) makes them unusually useful experimental tools.
The muon is the second-generation charged lepton, sitting one rung above the electron in the lepton ladder. It is a workhorse of physics: muons that survive long enough to reach the ground are a classic demonstration of time dilation; cooled, stored muons are used to measure the muon's magnetic moment to astonishing precision (the famous 'g minus 2' experiments); and beams of muons can even act as gentle probes inside materials, archaeology, and volcanoes.
Muons created about 15 kilometres up should decay long before reaching the ground in their brief 2.2 microseconds — yet plenty arrive. Because they move near light speed, their internal clocks run slow from our point of view, and they live long enough to make the trip: time dilation caught in the act.
Cosmic-ray muons reaching the ground are everyday evidence of relativistic time dilation.
Despite being heavier, a muon is just as much a point particle as the electron — its extra mass comes from a stronger coupling to the Higgs field, not from any internal structure.