Freed-isobar analysis of light mesons at COMPASS

Modern hadron-spectroscopy experiments such as COMPASS collect data samples of unprecedented size, so that novel analysis techniques become possible and necessary. One such technique is the freed-isobar partial-wave analysis (PWA). In this approach, fixed parametrizations for the amplitudes of intermediate states commonly modeled using Breit-Wigner shapes are replaced by sets of step-like functions that are determined from the data. This approach not only reduces the model dependence of partial-wave analyses, but also allows us to study the amplitudes of the intermediate states and their dependence on the parent system. We will also present results of a freed-isobar PWA performed on the large data set on diffractive production of three charged pions collected by the COMPASS experiment, which consists of $46\times10^6$ exclusive events. We will focus 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 PWA method provides insight into the interplay of three- and two-particle dynamics and confirms the decay of the spin-exotic $\pi_1(1600)$ resonance to $\rho(770)\pi$ in a model-independent way.