Partial-wave decomposition of the diffractively produced ${\pi }^{-}{\pi }^{+}{\pi }^{-}\eta$ final state at COMPASS

One of the prime goals of the COMPASS experiment at CERN is the study of the light meson spectrum, with a particular emphasis on the search for exotic states. We present the first high-statistics partial-wave decomposition of the diffractive reaction ${\pi }^{-}+p\to {\pi }^{-}{\pi }^{+}{\pi }^{-}\eta +p$, which spans a wide range of decay channels, such as $f_1(1285){\pi }^{-}$, $a_2^{-}(1320)\eta$, ${\eta }^{\mathrm{\prime }}{\pi }^{-}$, and $\rho (770)a_0^{-}(980)$. This analysis also includes decay channels, i.e. $f_1(1285){\pi }^{-}$ and ${\eta }^{\mathrm{\prime }}{\pi }^{-}$, predicted by theoretical models for the lightest hybrid meson and providing the opportunity to verify the hybrid meson hypothesis of the ${\pi }_1(1600)$.