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

A sample of 35 000 events of the type π−p → ηπpslow (η ′ → ηπ−π+, η → γγ) with −t > 0.1GeV2/c2 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 a2(1320) mass and relative to a broad D+wave structure at higher mass. We also find the a4(2040). We compare kinematic plots for the η′π− and ηπ− final states.