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Odd and Even Partial Waves of $ηπ^-$ and $η'π^-$ in $π^-p\toη^{(\prime)}π^-p$ at $191\,\textrm{GeV}/c$

Exclusive production of $ηπ^-$ and $η'π^-$ has been studied with a $191\,\textrm{GeV}/c$ $π^-$ beam impinging on a hydrogen target at COMPASS (CERN). Partial-wave analyses reveal different odd/even angular momentum ($L$) characteristics in the inspected invariant mass range up to $3\,\textrm{GeV}/c^2$. A striking similarity between the two systems is observed for the $L=2,4,6$ intensities (scaled by kinematical factors) and the relative phases. The known resonances $a_2(1320)$ and $a_4(2040)$ are in line with this similarity. In contrast, a strong enhancement of $η'π^-$ over $ηπ^-$ is found for the $L=1,3,5$ waves, which carry non-$q\bar q$ quantum numbers. The $L=1$ intensity peaks at $1.7\,\textrm{GeV}/c^2$ in $η'π^-$ and at $1.4\,\textrm{GeV}/c^2$ in $ηπ^-$, the corresponding phase motions with respect to $L=2$ are different.