Freed-isobar Partial-Wave Analysis of the Spin-Exotic J^PC = 1^-+ Wave at COMPASS

COMPASS is a multi-purpose fixed-target experiment at the CERN Super Proton Synchrotron aimed at studying the structure and spectrum of hadrons. The spectrum of isovector light-meson resonances is studied in diffractive-dissociation reactions using a 190 GeV/c π^- beam. The flagship channel is the π^-π^-π⁺ final state, for which COMPASS has recorded a large data sample of 46 million events. From these data, 3π resonances are extracted by performing a partial-wave analysis (PWA) using the isobar model. The model dependence of the PWA result is studied using a novel method, the freed-isobar PWA, which extracts the amplitudes of the π^-π⁺ subsytems from the data. Using this method, we studied the spin-exotic partial wave with J^PC = 1^-+ quantum numbers, which decays via an isovector π^-π⁺ subsystem with J^PC = 1^-. This wave is of particular interest because it has quantum numbers that are forbidden for q anti q systems in the non-relativistic limit. Continuous mathematical ambiguities may arise in the freed-isobar approach. We will discuss methods to resolve these ambiguities and present results of this analysis.