A class of elementary particle. Mesons are a subdivision of the hadrons, and are composed of two quarks. Mesons are involved in the processes that hold atomic nuclei together, and are also a major component of secondary cosmic-ray showers. They are unstable, with half-lives in the range 10−8 to 10−16 s, and decay into stable particles. They can be positively or negatively charged, or electrically neutral. Mesons have masses intermediate between those of the electron and proton, and include the kaon, pion, and psi particles. The mu-meson was an old name for a muon; it is not a meson at all, but a lepton.