Vector meson pair production in two-photon collisions near threshold

The cross sections for several γγ→VV processes exhibit strong enhancements near threshold, where V denotes a vector meson. The pattern of these enhancements is not well understood; for example, γγ→ρ0ρ0 shows an appreciable peak, while γγ→ρ+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 γγ→J/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