Vector meson pair production in two-photon collisions near threshold

The cross sections for several γγ⃗VV processes exhibit strong enhancements near threshold, where V denotes a vector meson. The pattern of these enhancements is not well understood; for example, γγ⃗ρ⁰ρ⁰ shows an appreciable peak, while γγ⃗ρ⁺ρ^- 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 γγ⃗J/ψρ⁰. The importance of modeling l⁺l^-π⁺π^- angular distributions in decays of threshold J/ψρ⁰ enhancements is illustrated by comparing the expected distributions for S-wave decays of scalar particles and P-wave decays of pseudoscalar particles.