The exotic $\eta'\pi^{-}$ Wave in 190 GeV $\pi^{-}p \to \pi^{-}\eta'p$ at COMPASS

A sample of 35,000 events of the type $\pi^{-}p\rightarrow\eta'p_{slow} (\eta'\rightarrow\eta\pi^{-}\pi^{+},\eta\rightarrow\gamma\gamma)$ with $-t >$ 0.1 GeV$^{2}/c^{2}$ was selected from COMPASS 2008 data for a partial-wave analysis. We study the broad $P_{+}$ structure known from previous experiments at lower energies, in particular its phase motion relative to the $D_{+}$-wave near the $a_{2}$(1320) mass and relative to a broad $D_{+}$-wave structure at higher mass. We also find the $a_{4}$(2040). We compare kinematic plots for the $\eta'\pi^{-}$ and $\eta\pi^{-}$ final states.