© Wikimedia Commons, Public Domain. Author: Thymo, 6 April 2009.
The muon astonished physicists when they discovered it in 1936. It behaved like an electron, but had 200 times more mass and survived for a surprisingly long time before decaying. Experimenters eventually identified the muon's most common decay mode, shown here. The negatively charged muon () decays into a muon neutrino (), an electron (e-) and an anti-electron neutrino (). The interaction is mediated by a W- boson, a carrier of the weak force to be described in Unit 2. (Unit: 1)