Vector meson pair production in two-photon collisions near threshold
  The cross sections for several $\gamma \gamma \to VV$ processes exhibit strong enhancements near threshold, where $V$ denotes a vector meson. The pattern of these enhancements is not well understood; for example, $\gamma \gamma \to \rho^0 \rho^0$ shows an appreciable peak, while $\gamma \gamma \to \rho^+ \rho^-$ 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 $\gamma \gamma \to J/\psi \rho^0$. The importance of modeling $\ell^+ \ell^- \pi^+ \pi^-$ 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. 
Jonathan L. Rosner