Electromagnetic decays of vector and pseudoscalar mesons in a dispersion-theoretic approach

A good understanding of the decays $V\to e^{-}{e}^{+}$, $V\to P\gamma$, and $P\to 2\gamma$, including the suppression of $\psi \to {\eta }_c\gamma$, has been obtained in a dispersion-theoretic approach, by assuming that the unitarity relation for the absorptive part in the dispersion integral is dominated by the quark-antiquark state near the threshold of quark-antiquark production. Moreover, it is assumed that the scaling mass in the dispersion integral as well as the coupling constants of vector and pseudoscalar mesons with quarks satisfy some simple scaling relations.