High-statistics study of \({\boldmath \eta \pi^0}\) production in two-photon collisions

Abstract

The differential cross section for the process \(\gamma \gamma \to \eta \pi^0\) has been measured in the kinematic range \(0.84 \GeV < W < 4.0 \GeV\), \(|\cos \theta^*|<0.8\), where \(W\) and \(\theta^*\) are the energy and \(\pi^0\) (or \(\eta\)) 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. Clear peaks due to the \(a_0(980)\) and \(a_2(1320)\) are visible. The differential cross sections are fitted in the energy region \(0.9 \GeV < W < 1.46 \GeV\) to obtain the parameters of the \(a_0(980)\). Its mass, width and \(\Gamma_{\gamma \gamma} \B (\eta \pi^0)\) are measured to be \(982.3 ^{+0.6}_{-0.7} ^{+3.1}_{-4.7} \MeV/c^2\), \(75.6 \pm 1.6 ^{+17.4}_{-10.0} \MeV\) and \(128 ^{+3}_{-2} ^{+502}_{-43} \eV\), respectively. The energy and angular dependences above 3.1 GeV are compared with those measured in the \(\pi^0 \pi^0\) channel. The integrated cross section over \(|\cos \theta^*|<0.8\) has a \(W^{-n}\) dependence with \(n = 10.5 \pm 1.2 \pm 0.5\), which is slightly larger than that for \(\pi^0 \pi^0\). The differential cross sections show a \(\sin^{-4} \theta^*\) dependence similar to \(\gamma \gamma \to \pi^0 \pi^0\). The measured cross section ratio, \(\sigma(\eta \pi^0)/\sigma(\pi^0 \pi^0) = 0.48 \pm 0.05 \pm 0.04\), is consistent with a QCD-based prediction.