The exotic eta'pi- Wave in 190 GeV pi-p --> 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.