Quarkonium
= Quarkonium
A <quarkonium> is a <bound state> of a heavy <quark> and its own <antiquark>. Examples are <charmonium> and the bottom-antibottom states called <Upsilon mesons>. Spectroscopic labels $n\,{}^{2S+1}L_J$ distinguish radial excitation, total quark <spin>, relative <orbital angular momentum> and total <angular momentum>.