Measurement of the reaction γγ→π⁺π⁺π⁻π⁻ at PETRA

Abstract

We have determined the cross section for γγ→π⁺π⁺π⁻π⁻ in a way free of assumptions about the relative contributions fromρ ⁰ ρ ⁰,ρ ⁰2π and 4π (uncorrelated phase space). We find a sharp onset above threshold and a rather high cross section of about 200 nb aroundW _γγ=1.5 GeV which consists to about 40% ofρ ⁰ ρ ⁰ production with sizeable contributions fromρ ⁰2π and 4π (PS). The total cross section as well as theρ ⁰ ρ ⁰ content fall rather fast at higher c.m. energies. Attempts to explain this behaviour in terms of production of known resonances are not successful so far. The angular distributions do not show any significant structure pointing to resonance formation in the 4π-system. Only theρ ⁰-meson is observed in the moment analysis. The decay distributions of theρ ⁰ for forward produced rhos are fairly consistent with helicity conservation of the produced rhos in accordance with the VDM picture.