Extraction of the amplitude of the π⁺π^- subsystem in diffractively produced π^-π⁺π^- at COMPASS

The two-stage Compass spectrometer has collected a huge data set of tens of millions of events for the channel π^-p → π^-π⁺π^-p. Therefore, spectroscopy analyses in this channel are dominated by systematic effects. The established method to analyze these data is Partial-Wave Analysis (PWA). For multi-particle final states PWA usually assumes subsequent two-particle decays, i.e. in case of π^-π⁺π^- one additional intermediate state which decays into π⁺π^-. Fixed mass shapes for this intermediate state, the so-called isobar, have to be assumed. These shapes, which in the most simple case may be given by a Breit-Wigner amplitude, have to be put into the analysis beforehand and therefore may introduce a model dependence leading to increased systematic uncertainties. We present a first analysis of diffractively produced π^-π⁺π^- events using a new method, which instead allows to extract binned isobar amplitudes directly from the data in a more model-independent way.