Observation of spin-parity 2⁺ dominance in the reactionγγ→ρ ⁰ ρ ⁰ near threshold

Abstract

The reactionγγ→π ⁺ π ⁻ π ⁺ π ⁻ has been studied with the ARGUS detector. The rate in the invariant mass region below 1.8 GeV/c² is found to be largely due toρ ⁰ ρ ⁰ production. A spin-parity analysis shows a dominance of the partial wave (J ^P,J _z )=(2⁺, 2) with a small admixture fromJ ^P=0⁺. The contribution of negative parity states is consistent with zero. The large ratio of cross sectionsσ(γγ→ρ ⁰ ρ ⁰)/σ(γγ→ρ ⁺ ρ ⁻)≃4, and the dominance of theJ ^P=2⁺ wave in the reactionγγ→ρ ⁰ ρ ⁰ is a signature consistent with the production of an exotic (I=2) resonance.