COMPASS

One of the main goals of the COMPASS experiment is to study the spectrum of light mesons. COMPASS has collected large data sets for a diffractive production of various final states using a 190 GeV negative hadron beam on a liquid hydrogen target. The flagship channel of this data is the process $\pi^-p\to\pi^-\pi^+\pi^-p$, for which $46\times10^6$ exclusive events were recorded.

This dataset was subjected to a partial-wave analysis (PWA) using a large set of 88 partial waves and splitting the data into narrow bins of the reduced squared four-momentum transfer $t^\prime$ from the beam to the target. This partial-wave decomposition resolves the contributions to the process with an unprecedented level of detail. The PWA result was further analyzed by performing a resonance-model fit to extract the masses and widths of the contributing $3\pi$ resonances in a novel approach, using the information on the $t^\prime$ dependence of the partial wave amplitudes. One of the resonances that were included in the fit is the spin-exotic $\pi_1$ resonance, which was disputed in previous analyses.