QCD

Quarkonium

1. OZI rule: φ decays
   
2. Discovery of charm: J/Ψ
   In November 1974 a strong cross section enhancement in electron-positron annihilation into hadrons (SPEAR, SLAC) and in the electron-positron invariant mass in the reaction p Be -> e⁺e^- X (BNL) around 3.1 GeV CMS energy have been observed. The width of this resonance was smaller than the experimental resolution.
   
   The new particle was named by the two groups J and Ψ, respectively, and in the following additional narrow resonances have been found. The determined leptonic decay widths have values typical for vector mesons (J/Ψ: 5.36 keV).
   However, the total decay width of the J/Ψ has a value of only 86 keV! This observation was interpreted as the discovery of a new quark flavour called charm.
   
   In 1977, a second series of narrow resonances, called uspilon's (Υ), with masses of 9.46 GeV and higer, have been discovered and interpreted as the discovery of a fifth quark flavour called 'beauty' or also 'bottom'.
   
   
   INTERPRETATION OF THE RESONANCE
   The J/Ψ has been interpreted as a bound charm-anticharm state with J^PC=1^--) that is with quark spins being anti-parallel and no relative angular momentum.
   The small decay width is explained as follows:
   The mass of the J/Ψ is smaller than the sum of the masses of the two lightest charmed mesons (D⁺, D⁰).
   As a consequence, the OZI-allowed decays are kinematically forbidden and hence only OZI-forbidden decays to light hadrons are allowed leading to the small decay width.
   
   
   The discovery of the charm quark was not a total surprise:
   The gauge theory of weak interactions was demanding for a fourth quark with a mass of order 1.5 GeV (see 'GIM mechanism').
3. OZI rule in QCD
   
4. α_s from quarkonia decays