In high energy physics, a quarkonium (pl. quarkonia) is a flavorless meson constituted by the association of a quark an its own antiquark, such as the charmonium or the bottomonium.

Some charmonium and bottomonium states

n 2S+1LJ	JPC	state	state
11S0	0−+	ηc(2980)	—
13S1	1−−	J/ψ(3097)	Υ(9460)
21S0	0−+	—	—
23S1	1−−	ψ(3686)	Υ(10023)
23P0	0++	χc0(3415)	χb0(9860)
23P1	1++	χc1(3511)	χb1(9892)
23P2	2++	χc2(3556)	χb2(9913)
21P1	1+−	—	—

Some quarkonium states are:

• states
  • meson
• (charmonium) states
  • η_c
  • J/ψ and ψ mesons
  • χ_c mesons
• (bottomonium) states:
  • Υ
  • χ_b mesons

Light quarks "u", "d" and "s", have binding energies that are comparable to their rest masses, which makes them have relativistic velocities inside the systems to which they are bound to. Relativistic mechanics are thuys needed to distinguish properties of particles and properties of the colour-field. Also, isospin related flavor-mixing must be considered.

The higher the rest mass of the quarks in any meson the less mixing there is and the less velocity the particles have relative to each other. Quarks "c" and "b" in their quarkonia have far more mass than binding energy. This makes them non-relativistic systems with little mixing involved.