Vector meson pair production in two-photon collisions near threshold

The cross sections for several $\gamma \overrightarrow{\gamma }\mathrm{VV}$ processes exhibit strong enhancements near threshold, where V denotes a vector meson. The pattern of these enhancements is not well understood; for example, $\gamma \overrightarrow{\gamma }{\rho }^0{\rho }^0$ shows an appreciable peak, while $\gamma \overrightarrow{\gamma }{\rho }^{+}{\rho }^{-}$ does not. Some possible mechanisms for this behavior are discussed. Tests are proposed involving production of systems containing heavier quarks, e.g., through the reaction $\gamma \overrightarrow{\gamma }J/\psi {\rho }^0.$ The importance of modeling ${\mathcal{l}}^{+}{\mathcal{l}}^{-}{\pi }^{+}{\pi }^{-}$ angular distributions in decays of threshold $J/\psi {\rho }^0$ enhancements is illustrated by comparing the expected distributions for S-wave decays of scalar particles and P-wave decays of pseudoscalar particles.