Strange-Meson Spectroscopy with COMPASS

While the spectrum of non-strange light mesons is well known, many predicted strange mesons have not yet been observed, and many potentially observed states require further confirmation. Using the $K^{-}$ component of the hadron beam at the M2 beamline at CERN, we study the strange-meson spectrum with the COMPASS experiment. The flagship channel is the $K^{-}{\pi }^{-}{\pi }^{+}$ final state, for which COMPASS has obtained the world’s largest sample. Based on this sample, we have performed the most detailed and comprehensive partial-wave analysis of this final state to date. For example, we observe a clear signal from the well-known $K_2^{\ast }(1430)$, and for the first time we study the $K_2(1770)$, $K_2(1820)$, and $K_2(2250)$ in a single analysis. We also find evidence for a supernumerary signal called $K(1630)$, suggesting that this signal is a pseudoscalar exotic strange meson.