New measurement of the η → π 0 γγ branching ratio and the slope parameter for η → 3π 0 decay with the crystal ball

Abstract

We have measured the quadratic slope parameter α for η → 3π 0 decay. Our final result is α=−0.031(4). We have also obtained the branching ratio (BR) for the double radiative decay η → π 0 γγ. Our preliminary result is BR(η → π 0 γγ=3.2(9)×10−4. These new measurements were performed using the recent (1998) high-statistics sample of 1.9 × 107 η produced at the AGS at BNL in π − p → nη close to threshold. The η decay into neutrals was detected with the Crystal Ball multiphoton spectrometer. Both measurements provide important tests of chiral perturbation theory (χPTh). Our result for α is four times more precise than the present world data and disagrees with current χPTh calculations by about four standard deviations. Our branching ratio for η → π 0 γγ is about half the current value and is in agreement with the χPTh predictions.