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

TL;DR

This study measures the differential cross sections for neutral-pion pair production in two-photon collisions across a broad energy range, confirming theoretical predictions and observing charmonium states with high statistical precision.

Contribution

It provides high-statistics measurements of $\gamma\gamma o \pi^0 ext{ extendash}\pi^0$ cross sections, confirms two-photon production mechanisms, and compares results with QCD models in a wide energy range.

Findings

Observation of the $\chi_{c0}\)$ charmonium state production.

Cross section ratio $\sigma(\pi^0\pi^0)/\sigma(\pi^+\pi^-)$ is approximately 0.32.

Energy and angular dependence consistent with QCD predictions above 3.1 GeV.

Abstract

The differential cross sections for the process $\gamma \gamma \to \pi^0 \pi^0$ have been measured in the kinematic range 0.6 GeV $< W < 4.1$ GeV, $|\cos \theta^*|<0.8$ in energy and pion scattering angle, respectively, in the $\gamma\gamma$ 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 $\chi_{c0}$ charmonium state and obtain the product of its two-photon decay width and branching fraction to $\pi^0\pi^0$. 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 $\pi^+\pi^-$ channel, and in addition we find that the cross section ratio, $\sigma(\pi^0\pi^0)/\sigma(\pi^+\pi^-)$, is $0.32 \pm 0.03 \pm 0.05$ on average in the 3.1-4.1 GeV region.