Vector meson pair production in two-photon collisions near threshold

Abstract

The cross sections for several $γγ\to VV$ processes exhibit strong enhancements near threshold, where $V$ denotes a vector meson. The pattern of these enhancements is not well understood; for example, $γγ\to ρ^0 ρ^0$ shows an appreciable peak, while $γγ\to ρ^+ ρ^-$ 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 $γγ\to J/ψρ^0$. The importance of modeling $\ell^+ \ell^- π^+ π^-$ angular distributions in decays of threshold $J/ψρ^0$ enhancements is illustrated by comparing the expected distributions for S-wave decays of scalar particles and P-wave decays of pseudoscalar particles.