Determination of the quadratic slope parameter in η → π0

Abstract

We have determined the quadratic slope parameter α η → π0 to be α = − 0.031(4) from a 99% pure sample of 106 η → 3π0 decays produced in the reaction π− p → nη close to the η threshold using the Crystal Ball detector at the AGS. The result is four times more precise than the present world data and disagrees with current chiral perturbation theory calculations by about four standard deviations. © 2001 American Physical Society.