Measurement of π−p→π−pπ0 reaction near threshold and breaking of chiral symmetry

Abstract A full-kinematics measurement of the π − p→ π − p π 0 reaction in the incident π − momentum region from 295 to 450 MeV/ c is presented. The measurement was performed with the OMICRON spectrometer at the CERN synchrocyclotron. An incident-momentum dependent number of π − p→ π − p π 0 events, ranging from 6 to 430 has been extracted from collected data. Differential cross sections are found to be consistent with phase-space distributions. Integrated cross-sections are used to determine the value of the chiral symmetry breaking parameter ξ. The measured value of ξ =0.1 −0.7 +0.5 is obtained in a model-independent way, strongly favouring Weinberg's choice of ξ =0.