Heavy Quarkonium ∗

I review heavy quarkonium physics in view of recent experimental results. In particular, I discuss new results on spin singlet states, photon and hadronic transitions, D−states and discovery of yet unexplained narrow X(3872) state. 1 Quarkonia Quarkonium is a bound state of a quark and its anti-quark. Some properties of light and heavy quarkonia are compared to properties of positronium in Table 1. Unlike positronium, light quarkonia are highly rela-tivistic. They also contain mixtures of quarks of different flavor and fall apart easily into other mesons. Charmonium (c¯ c) was the first heavy quarkonium discovered and is less relativistic; the number of long-lived states below the dissociation energy (i.e. the threshold for decay to D ¯ D meson pairs) equals the number of long-lived positronium states. Bottomo-nium (b ¯ b) is even more non-relativistic and has a larger number of long-lived states. The toponium system would have been completely non-relativistic. However, weak decays of the top quark will dominate over the strong binding and long-lived states will not be formed. Therefore, charmonium and bot-tomonium play a special role in probing strong interactions. The states below open flavor threshold live long enough for electromagnetic transitions between various excitations to occur. The electromagnetic transitions compete with transitions mediated by the emission of soft gluons. The latter materialize as light hadrons. Eventually the heavy quarks must annihilate into two or three hard gluons. Properties of these bound states and their decays are good testing grounds for QCD in both the non-perturbative and perturbative regimes. The first heavy quarkonium bound state above the D ¯ D or B ¯ B threshold acts as a factory of heavy-light mesons. The heavy quarks trapped in these mesons ultimately decay via weak interactions. This is a good place to look for physics beyond the standard model as discussed by Y. Grossman at this conference. 1 Many measurements of electroweak parameters are obscured by strong interactions. This provides an important motivation for trying to understand details of strong interaction phenomena. P. Lepage pointed out that at least some of new interactions to be discovered are likely to be strongly coupled , further motivating detailed studies of QCD. 2 Heavy quarkonia offer two small parameters: velocities (v) of constituent quarks and strong coupling constant (α s) in annihilation and production processes. Expansion of the full theory in these parameters allows for effective theories of strong interactions: in the past-purely …