Liquid hydrogen bubble chamber photograph of a negatively-charged pi meson interacting with a proton (hydrogen nucleus) to produce a positive K meson, a neutral meson, shown by the dotted line, a negative meson, and a neutron, whose path is now shown (neutral or uncharged particles do not leave tracks). In particle physics, mesons are hadronic subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Mesons are not produced by radioactive decay, but appear in nature only as short-lived products of very high-energy interactions in matter, between particles made of quarks. A bubble chamber is a vessel filled with a superheated transparent liquid (most often liquid hydrogen) used to detect electrically charged particles moving through it.