HEAVY QUARKONIA – A Review of the Experimental Status

Because strong interactions are flavourindependent, it would appear that they can be equally well studied in the spectroscopy of light or heavy quarks. However, the highly relativistic nature of quarks in light-quark (u, d, s) hadrons, the large value of the strong coupling constant, αs (& 0.6), and the near equality of the masses of the u, d, s quarks, makes the light quark states strongly overlapping, with small spacing ≈ 14 MeV, large widths, Γ ≈ 150 MeV, and mostly mixtures of all three flavours. In contrast, bb̄ and cc̄ hadrons are relatively free from these problems, with 〈