Mesons, unstable elementary particles belonging to the class of hadrons; unlike baryons, mesons carry no baryon charge and have zero or integer spin (they are bosons). The name “mesons” (from the Greek mesos — middle, intermediate) reflects the fact that the masses of the first mesons to be discovered — the pi-meson and the K-meson — have values intermediate between the masses of the proton and the electron. (Muons, originally called mu-mesons, are not mesons, since they have spin 1/2 and do not take part in the strong interaction.) Many other mesons with very short lifetimes were discovered later (the so-called boson resonances), and the mass of some of them exceeds the mass of the proton. There are neutral and charged mesons (with a positive or negative elementary electric charge), with zero (for example the π-meson) and non-zero (for example the K-meson) strangeness, “charm” and so on. According to the quark model of hadrons, a meson consists of a quark and an antiquark.