High-statistics study of neutral-pion pair production in two-photon collisions

The differential cross sections for the process γγ→π0π0 have been measured in the kinematic range 0.6 GeV<W<4.1 GeV, |cosθ*|<0.8 in energy and pion scattering angle, respectively, in the γγ center-of-mass system. The results are based on a 223 fb-1 data sample collected with the Belle detector at the KEKB e+e- collider. The differential cross sections are fitted in the energy region 1.7 GeV<W<2.5 GeV to confirm the two-photon production of two pions in the G wave. In the higher-energy region, we observe production of the χc0 charmonium state and obtain the product of its two-photon decay width and branching fraction to π0π0. We also compare the observed angular dependence and ratios of cross sections for neutral-pion and charged-pion pair production to QCD models. The energy and angular dependence above 3.1 GeV are compatible with those measured in the π+π- channel, and in addition we find that the cross section ratio, σ(π0π0)/σ(π+π-), is 0.32±0.03±0.05 on average in the 3.1-4.1 GeV region.