A muon is a subatomic particle, like an electron but for two minor differences: it’s 207 times heavier, and radioactive. They are often referred to by the Greek letter mu, or µ.
They come in two types, matter and antimatter. The matter muon, like the matter electron, is negatively charged (so it’s known as a µ–), whilst the antimatter muon, like a positron, is positive (µ+).
Muons’ longevity is quite impressive for a subatomic particle—they live an average of two millionths of a second. Protons and electrons live forever and neutrons, when not safely tucked up in an atomic nucleus, can only last fifteen minutes, but every other particle is quite substantially more fleeting even than the muon.
Muons were first observed in cosmic rays. High-energy protons are constantly bombarding the Earth’s upper atmosphere, and the nuclear reactions they induce in atmospheric gases result in a fountain of different particles. The only ones which live long enough to make it to the Earth’s surface are the muons. In fact, about 10,000 muons hit every square metre of the Earth every minute, so there are a few going through you every second as you read this!
Muons are also crucial to particle physics. The Compact Muon Solenoid at CERN is looking for the Higgs boson—the most elusive particle in modern physics—specifically by looking for the muons it would produce when it decays. One way to find the Higgs would be to catch it decaying into four muons, which is something no other particle yet known can do.
The reason I use muons is because of their magnetic properties. Every muon is like a tiny compass and, if you implant them in a magnetic or superconducting material, you can find out interesting things about the material from which way the muon is pointing when it dies.