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

Abstract

The differential cross section for the process $γγ\to π^0 π^0$ has been measured in the kinematic range 0.6 GeV $< W < 4.1$ GeV, $|\cos θ^*|<0.8$ 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. Using the data with $W > 1.4$ GeV, we obtain results on light-quark resonances and charmonia. We also compare the observed angular dependence and ratios of cross sections for neutral-pair and charged-pair production to QCD models. Differential cross sections are fitted in the energy region, $1.4 \GeV < W < 2.2 \GeV$, with a simple model where partial waves consist of resonances such as $f_2'(1525)$, $f_2(1950)$ and $f_4(2050)$ and smooth backgrounds. In the higher energy region, we observe production of the $χ_{c0}$ charmonium state and obtain the product of its two-photon decay width and the branching fraction to $π^0π^0$. The energy and angular dependences above 3.1 GeV are compatible with those measured in the $π^+π^-$ channel, and in addition we find that the cross section ratio, $σ(π^0π^0)/σ(π^+π^-)$, is $0.32 \pm 0.03 \pm 0.05$ on average in the 3.1-4.1 GeV region.

19 pages 8 figures, ICHEP08 conference paper