The OZI rule is a consequence of quantum chromodynamics that explains why certain decay modes appear less frequently than otherwisemight be expected. It was independently proposed by Susumu Okubo, George Zweig and Jugoro Iizuka in the 1960s. It states that any strongly occurring process will be suppressed if, through only the removal of internal gluonlines, its Feynman diagram can be separated into two disconnected diagrams: one containing all of the initial-state particles and one containing all of the final-state particles. An example of such a suppressed decay is the Phi meson into pions: It would be expected that this decay mode would dominate over other decay modes such as which have much lower values. In actuality, it is seen that φ decays to kaons 84% of the time, suggesting the decay path to pions is suppressed. An explanation of the OZI rule can be seen from the decrease of the coupling constant in QCD with increasing energy. For the OZI suppressed channels, the gluons must have high 2 and so the couplingconstant will appear small to these gluons. Another explanation of the OZI rule comes from the large- limit. The OZI suppressed processes have larger number of independent fermion loops compared with the non-suppressed processes, and those Feynman diagrams are suppressed by the number of independent fermion loops. A further example is given by the decays of excited states of charmonium. For states lighter than the charged D mesons, the decay must proceed just like the above example into three pions, with three virtual gluons mediating the interaction, each of which must have enough energy to produce a quark-antiquark pair. But above the D mesonthreshold, the original valence quarks need not annihilate; they can propagate into the final states. In this case, only two gluons are required, which share the energy of the light quark-antiquark pair that is spontaneously nucleated. They are thus lower in energy than the three gluons of the OZI-suppressed annihilation. The suppression arises from both the smaller values of the QCD coupling constant at high energies, as well as the greater number of interaction vertices.
