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

Abstract

The differential cross sections for the process $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ have been measured in the kinematic range $0.6\text{ }\text{ }\mathrm{GeV}<W<4.1\text{ }\text{ }\mathrm{GeV}$, $|\mathrm{cos}{\ensuremath{\theta}}^{*}|<0.8$ in energy and pion scattering angle, respectively, in the $\ensuremath{\gamma}\ensuremath{\gamma}$ center-of-mass system. The results are based on a $223\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ data sample collected with the Belle detector at the KEKB ${e}^{+}{e}^{\ensuremath{-}}$ collider. The differential cross sections are fitted in the energy region $1.7\text{ }\text{ }\mathrm{GeV}<W<2.5\text{ }\text{ }\mathrm{GeV}$ to confirm the two-photon production of two pions in the $G$ wave. In the higher-energy region, we observe production of the ${\ensuremath{\chi}}_{c0}$ charmonium state and obtain the product of its two-photon decay width and branching fraction to ${\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$. 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 ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ channel, and in addition we find that the cross section ratio, $\ensuremath{\sigma}({\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0})/\ensuremath{\sigma}({\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})$, is $0.32\ifmmode\pm\else\textpm\fi{}0.03\ifmmode\pm\else\textpm\fi{}0.05$ on average in the 3.1--4.1 GeV region.