OZI Rule

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The OZI rule is part of QCD which explains why certain decay modes appear less frequently than otherwise might be expected. It was independently proposed by Okubo, Zweig and Iizuka in the 1960s. It states that any strongly occurring process with a Feynman Diagram that can be split in two by cutting only internal gluon lines will be suppressed.

An example of such a suppressed decay is $\phi \rightarrow \pi^+ + \pi^- + \pi^0$ . It would be expected that this decay mode would dominate over other decay modes such as $\phi \rightarrow K^+ + K^-$ which have much lower Q values. In reality 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 fact that the coupling constant in QCD decreases with increasing energy (or momentum transfer). For the OZI suppressed channels the gluons must have high q² (at least as much as the rest mass energies of the quarks into which they decay) and so the coupling constant will appear small to these gluons.

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