Measurement ofπ0π0production in two-photon collisions

Abstract

The reaction $e^{+}{e}^{-}\to e^{+}{e}^{-}{\pi }^0{\pi }^0$ has been analyzed using 97 ${\mathrm{pb}}^{-1\mathrm{}}$ of data taken with the Crystal Ball detector at the DESY $e^{-}{e}^{+}$ storage ring DORIS II at beam energies around 5.3 GeV. For the first time we have measured the cross section for $\gamma \gamma \to {\pi }^0{\pi }^0$ for ${\pi }^0{\pi }^0$ mvariant masses ranging from threshold to about 2 GeV. We measure an approximately flat cross section of about 10 nb for $W=m_{{\pi }^0{\pi }^0}<0.8\mathrm{}$ GeV, which is below 0.6 GeV, in good agreement with a theoretical prediction based on an unitarized Born-term model. At higher invariant masses we observe formation of the $f_2\mathrm{}\left(1270\right)\mathrm{}$ resonance and a hint of the $f_0\mathrm{}\left(975\right)\mathrm{}$. We deduce the following two-photon widths: ${\Gamma }_{\gamma \gamma }\left(f_2\mathrm{}\right(1270\left)\right)=3.19\mathrm{}\pm {0.16\pm }_{0.28}^{0.29}$ keV and ${\Gamma }_{\gamma \gamma }\left(f_0\mathrm{}\right(975\left)\right)<\mathrm{}0.53\mathrm{}$ keV at 90% C.L. The decay-angular distributions show the ${\pi }^0{\pi }^0$ system to be dominantly spin 0 for $W<\mathrm{}0.7\mathrm{}$ GeV and spin 2, helicity 2 in the $f_2\mathrm{}\left(1270\right)\mathrm{}$ region, with helicity 0 contributing at most 22% (90% C.L.).