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Modern hadron-spectroscopy experiments such as COMPASS collect data samplesof unprecedented size, so that novel analysis techniques become possible andnecessary. One such technique is the freed-isobar partial-wave analysis (PWA).In this approach, fixed parametrizations for the amplitudes of intermediatestates commonly modeled using Breit-Wigner shapes are replaced by sets ofstep-like functions that are determined from the data. This approach not onlyreduces the model dependence of partial-wave analyses, but also allows us tostudy the amplitudes of the intermediate states and their dependence on theparent system.  We will also present results of a freed-isobar PWA performed on the largedata set on diffractive production of three charged pions collected by theCOMPASS experiment, which consists of $46\times10^6$ exclusive events. We willfocus on results for the wave with spin-exotic quantum numbers $J^{PC}=1^{-+}$,in particular on its decay into $\rho(770)\pi$. Here, the freed-isobar PWAmethod provides insight into the interplay of three- and two-particle dynamicsand confirms the decay of the spin-exotic $\pi_1(1600)$ resonance to$\rho(770)\pi$ in a model-independent way.

Freed-Isobar Analysis of Light Mesons at COMPASS