Unstable leptons are subatomic particles that have a short lifetime and decay quickly. The two most common unstable leptons are the TAU (TAU) and the muon (MUON). The TAU is an elementary particle with a negative electric charge and a lifetime of 2.9 x 10^-13 seconds. The muon is also an elementary particle with a negative electric charge and a lifetime of 2.2 x 10^-6 seconds. Both particles are important components of the Standard Model of particle physics.