Measuring Light-Meson Resonances in the $\omega\pi^-\pi^0$ and $K_S^0 K^-$ Final States at COMPASS

Abstract

COMPASS is a multi-purpose fixed-target experiment at the CERN SPS. One of its main goals is to probe the strong interaction at low energies by studying the excitation spectrum of light mesons in diffractive scattering reactions of a $190\ \text{GeV}/c$ $\pi^-$ beam. The analysis is done by first decomposing the data into partial-wave amplitudes with well-defined quantum numbers, and second, extracting meson resonance parameters from these amplitudes. We have collected the world's largest datasets of various final states. In this talk, we will focus on two of them: $\omega\pi^-\pi^0$ and $K_S^0 K^-$. They allow us to study light isovector mesons with spin, parity, and $C$-parity $J^{PC} = J^{++}$ and $J^{-+}$, i.e.\ $a_J$ and $\pi_J$ mesons. We will discuss the analysis and present new measurements of resonance parameters of several light mesons. The main focus of the $\omega\pi^-\pi^0$ analysis lies in the investigation of the nature of the $\pi_1(1600)$. Being a good candidate for the lightest hybrid meson, it is expected to predominantly decay into $b_1(1235)\pi$. In addition, the $\omega\pi^-\pi^0$ final state also gives access to other decay modes of the $\pi_1(1600)$, further testing theory predictions, and to a range of other $a_J$ and $\pi_J$ mesons. In the $K_S^0 K^-$ final state, only $a_J$ mesons with even spin $J$ appear, due to the high beam energy of COMPASS. This allows for an exclusive study of these mesons, up to high invariant masses, verifying the existence of several states claimed by other experiments and measuring their parameters.