High-statistics study of eta pi(0) production in two-photon collisions

Abstract

The differential cross section for the process gamma gamma -> eta pi(0) has been measured in the kinematic range 0.84 GeV < W < 4.0 GeV, vertical bar cos theta*vertical bar < 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 B-gamma gamma(eta pi(0)) are measured to be 982.3(-0.7-4.7)(+0.6+3.1) MeV/c(2), 75.6 +/- 1.6(-10.0)(+17.4) 128(-2-43)(+3+502) 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 vertical bar cos theta*vertical bar < 0.8 has a W-n dependence with n = 10.5 +/- 1.2 +/- 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 -> pi(0)pi(0). The measured cross section ratio, sigma(eta pi(0))/sigma(pi(0)pi(0)) = 0.48 +/- 0.05 +/- 0.04, is consistent with a QCD-based prediction.