The term 'unstable subatomic particle' refers to two particles, the muon and the kaon. The muon is a lepton, a type of elementary particle with no net electric charge and a spin of 1/2. It has a mass of 105.7 MeV/c2 and a mean lifetime of 2.2 microseconds. The kaon is a meson, a type of particle composed of a quark and an antiquark. It has a mass of 493.677 MeV/c2 and a mean lifetime of 1.23×10−8 seconds. Both particles are unstable and decay into other particles.