 In 1932, Paul Coons discovered the muon. Using both the direction of the curvature and the thickness of the bubble track, he calculated that it was a positively charged particle that was lighter than a proton but heavier than an electron. Later on it was found that this was the anti-muon. Here's another look at a muon created by an event that created two visible particles. A muon and an unknown particle. The muon displayed a new particle property that had not been seen before. It was unstable. Unlike protons and electrons, the muon would only exist for a short period of time before it decayed into other particles. One was an electron and the other was unknown. On average it lasted only 2.2 microseconds. That's 2.2 millionths of a second. Muons are light, elementary particles like the electron and the positron. These particles are called leptons, meaning light rather than heavy like a proton. The particle that was created at the start, along with the muon, turned out to be the long sought after particle called the pion. In 1947, using cosmic rays at high altitudes, this particle was found. Here we see an event that kicks a proton into motion and creates a muon and a pion. Then we see the pion decay into an anti-muon that itself decays into a positron. It lasted only 26 nanoseconds. Now that is a very short life. The muon lasted almost a thousand times longer than that. Pions are spin-zero particles with around 14-hundredths of a proton's mass. Although that's small, it's a good deal more massive than the muon. Pions came in three flavors, one with a positive charge, one with a negative charge called an anti-pion, and one with no charge at all. Also in 1947 another particle called the kion was discovered by George Rochester and Clifford Butler. They also used cosmic rays. Here's a look at their bubble chamber picture. Just below the lead plate, in the lower right hand quadrant, you see an inverted V that extends to the lower right. Measuring the momentum in charges they determined that they were a pion and an anti-pion. Rochester and Butler concluded that this event had to be a photographic record of a novel phenomenon. The decay of a previously unknown, neutral, heavy particle later called the kion. The discovery of kions represented the first time we deduced the existence of a particle from its decay components.