meson
/ MEZ-on or MEE-zon /
Quarks can pair up in two ways. Three of them locked together make a baryon, like the proton. But a single quark can also pair with a single antiquark, and that two-body bundle is called a meson. Mesons are the lighter, more fleeting cousins of the proton and neutron. None of them is stable; every meson decays, often in a tiny fraction of a second.
A meson is a hadron made of one quark and one antiquark, held together by the strong force. Because a quark has half-integer spin and an antiquark has half-integer spin, their spins combine to a whole number, which makes every meson a boson — a particle that, unlike matter particles, does not obey the exclusion principle. The lightest meson is the pion; a slightly heavier one carrying a strange quark is the kaon. The name meson means middle, because the first ones found had masses between the light electron and the heavy proton.
Mesons matter for two reasons. First, they are the messengers of the residual strong force that holds protons and neutrons together inside a nucleus: in an older but still useful picture, nucleons exchange pions the way two skaters exchange a thrown ball. Second, mesons containing heavy quarks, especially the neutral kaons and B mesons, are the best laboratories for studying the tiny difference between matter and antimatter known as CP violation.
A positive pion is an up quark plus an anti-down antiquark; a positive kaon is an up quark plus an anti-strange antiquark. Both decay before traveling far.
A meson is one quark bound to one antiquark; the lightest are the pions and kaons.
A common confusion: a meson is matter plus antimatter bound together, yet it does not instantly annihilate. The quark and antiquark are usually different flavors, so they cannot simply cancel; even when they share a flavor, the binding holds them as a real particle for a short but measurable time.