quarkonium (charmonium and bottomonium)
/ kwor-KOH-nee-um /
Imagine a heavy quark bound to its own antiquark — a particle and its exact opposite circling each other, held by the strong force, like a tiny solar system that has not yet annihilated itself. This special kind of meson is called quarkonium. When the quark is a charm, it is charmonium; when it is a bottom, it is bottomonium.
Quarkonium is a meson made of a heavy quark and its antiquark of the same flavor. Because the two partners are so heavy and move relatively slowly, quarkonium behaves much more simply than ordinary hadrons — almost like a textbook atom, with the quark and antiquark sitting in well-defined energy levels. Just as an atom has a ladder of excited states, quarkonium has a ladder of related particles: a ground state, a first excited state, and so on, each a slightly heavier version of the same quark-antiquark pair.
The discovery of charmonium in 1974 — the famous J/psi particle, found simultaneously by two groups — was a watershed. It was so narrow and long-lived for its mass that it could only be explained by a new, fourth quark: the charm. This November Revolution convinced the last doubters that quarks were real. Today quarkonium spectra are a precision laboratory for the strong force: because the system is so clean, comparing its measured energy levels to theory and to lattice calculations is one of the sharpest tests we have of quantum chromodynamics.
Charmonium = charm + anti-charm; its ground state J/psi sits near 3.1 GeV. Bottomonium = bottom + anti-bottom; its ground state, the upsilon, sits near 9.5 GeV.
A heavy quark bound to its own antiquark forms a hydrogen-like ladder of states — an ideal proving ground for the strong force.
The quark and antiquark in quarkonium do eventually annihilate, but the strong binding makes that take a measurable, relatively long time. So quarkonium is a genuine, observable particle, not an instant flash of annihilation.