Measurement and analysis of the reactionγγ→π+π−π+π−

We have measured the cross section of four charged pion production in photon-photon interactions in the invariant mass range 1.0≦W_γγ≦3.2 GeV and up toQ²=16 GeV². For 1.2 GeV≦W_γγ≦1.7 GeV the process is dominated by ρ⁰ρ⁰ production with a rapid rise in cross section around 1.2 GeV, well below the nominal ρ⁰ρ⁰ threshold. The observed distributions in the two particle masses and in the production and decay angles are well described by an incoherent sum of the phase-space subprocesses γγ →ρ⁰ρ⁰, →ρ⁰π⁺π⁻, and →π⁺π⁻π⁺π⁻. A spin-parity analysis of the ρ⁰ρ⁰ system showsJ^P=2⁺ to dominate, although 0⁺ is also possible forW_γγ≦1.4 GeV. Negative partity states are excluded.