Particle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then transforms into other particles. If the particles created are not stable, the decay process can continue. The process of particle decay is distinct from radioactive decay, in which an unstable atomic nucleus is transformed into a smaller nucleus accompanied by the emission of particles or radiation.