The Standard Model explains why some particles decay into other particles.

In nuclear decay, an atomic nucleus can split into smaller nuclei. This makes sense: a bunch of protons and neutrons divide into smaller bunches of protons and neutrons. But the decay of a fundamental particle cannot mean splitting into its constituents, because "fundamental" means it has no constituents. Here, particle decay refers to the transformation of a fundamental particle into other fundamental particles. This type of decay is strange, because the end products are not pieces of the starting particle, but totally new particles.

In this section we will discuss the types of decay, how they happen, and under what circumstances a decay will or will not happen.