Angular correlations inγγ→ρ ⁰ρ⁰ near threshold

Abstract

We present an analysis ofρ ⁰ ρ ⁰ production by two photons in theρ ⁰ ρ ⁰ invariant mass range from 1.2 to 2.0 GeV. From a study of the angular correlations in the process γγ→ρ ⁰ ρ ⁰→π⁻π⁺π⁻ we exclude a dominant contribution fromJ ^P=0⁻ or 2⁻ states. The data indicate sizeable contributions fromJ ^P=0⁺ for four pion massesM _4π<1.7 GeV and fromJ ^P=2⁺ forM _4π>1.7 GeV. The data are also well described by a model with isotropic production and uncorrelated isotropic decay of theρ ^0,s. The cross section stays high below the nominalρ ⁰ ρ ⁰ threshold, i.e.M _4π<1.5 GeV. The matrix element forρ ⁰ ρ ⁰ production is found to decrease steeply with increasingM _4π. Upper limits for the couplings of the ι(1440) and Θ(1640) to γγ andρ ⁰ ρ ⁰ are given:Γ(ι→γγ)·B(ι→ρ ⁰ ρ ⁰)<1.0 keV andΓ(Θ→γγ)