  The COMPASS experiment at CERN SPS features charged particle tracking as well
as good coverage by electromagnetic calorimetry, and our data provide an
excellent opportunity for simultaneous observation of new states in different
decay modes by the same experiment. The existence of the spin-exotic
$\pi_1(1600)$ resonance in the $\rho\pi$ decay channel is studied for the first
time at COMPASS in both decay modes of the diffractively produced $(3\pi)^{-}$
system: $\pi^{-}p \rightarrow \pi^{-}\pi^{+}\pi^{-}p$ and $\pi^{-} p
\rightarrow \pi^{-}\pi^{0}\pi^{0} p$. A preliminary partial-wave analysis
performed on the 2008 proton target data allows for a first conclusive
comparison of both $(3\pi)^{-}$ decay modes not only for main waves but also
for small ones. We find the neutral versus charged mode results in excellent
agreement with expectations from isospin symmetry. Both, the intensities and
the relative phases to well-known resonances, are consistent for the neutral
and the charged decay modes of the $(3\pi)^{-}$ system. The status on the
search for the spin-exotic $\pi_1(1600)$ resonance produced on a proton target
is discussed.
