Measurement of form factors for the decay η′ å ηπ−π+

Form factors measured for the decay η′ å ηπ − π + in π − A å η′π − A* reactions at a beam momentum of p π = 28 GeV are presented. The distribution in Dalitz variables for 7000 decay events is described well within a linear and a nonlinear parametrization of the relavant matrix element. The slope parameter in the variable y differs from zero significantly. It is Reα = −0.072±0.012 (stat.)±0.006 (syst.) in the linear and a = −0.120±0.027 (stat.)±0.015 (syst.) in the nonlinear parametrization. In this decay, the C-violation parameter is compatible with zero, c = 0.021±0.024.