This picture shows the annihilation of an electron and positron into a muon and an anti-muon. The muon is a particle in all ways identical to the electron, except that its mass is about 200 times greater. Because of their greater mass, muons are more penetrating than electrons and are not stopped by calorimeters, and reach the muon chambers, right on the outside of the detector, where they are visible as small squares. Muon pair events are therefore easily distinguished from electron - positron pairs.