Diffractive Dissociation into Three-Pion Final States at COMPASS

In order to study diffractive dissociation reactions, COMPASS has taken data with a 190 GeV/$c$ pion beam impinging on a liquid hydrogen target in 2008 and 2009. At squared four-momentum transfers to the target t' between 0.1 GeV$^2/c^2$ and 1.0 GeV$^2/c^2$ the number of events with three pions in the final state is about an order of magnitude larger than that acquired by any previous experiment. In COMPASS, the three-pion final state can be studied in the two channels $\pi^-\pi^-\pi^+$ and $\pi^-\pi^0\pi^0$. The large data sample in particular for the $\pi^-\pi^-\pi^+$ channel allows to find even small signals at the sub-percent level. The progress of the partial-wave analysis will be shown. Compared to previous COMPASS results, the analysis is now performed in bins of t', and the set of partial waves has been extended now including waves up to spin 6. The information from the t' dependence of the individual partial waves is very helpful in separating resonant and non-resonant contributions. As a consistency check results from the $\pi^-\pi^-\pi^+$ channel will be compared to the $\pi^-\pi^0\pi^0$ channel.