Advanced Physics Laboratories

The muon is a heavy unstable version of the electron, with mass mm= 105.6 MeV/c2 and lifetime tm ~ 2.1 msec. The decay m g e ne nm, or "muon beta decay"  is one of the simplest manifestations of the weak interaction. Particle production from the rain of cosmic rays on the upper atmosphere creates a muon flux of approx. 100/m2-s at sea level. In this experiment we use a photomultiplier in a tank of liquid scintillator to detect cosmic ray muons, and to time the delay between stoppage of a cosmic ray muon and appearance of its decay electron. The distribution of muon lifetimes has an exponential form characteristic of radioactive decay; the lifetime is extracted and compared to expectations for the universal weak interaction.