Speaker
Description
COMPASS is a multi-purpose fixed-target experiment at CERN aimed at studying the structure and spectrum of hadrons. The two-stage spectrometer has a good acceptance over a wide kinematic range and is thus able to measure a wide range of reactions. Light mesons are studied with a negative hadron beam (mostly $\pi^-$) with a momentum of $190~\text{GeV}/c$. The light-meson spectrum is investigated in various final states produced in diffractive dissociation. The flagship channel is the $\pi^-\pi^+\pi^-$ final state, for which COMPASS has acquired the so far world's largest dataset of $46~\text{M}$ exclusive events. We report on new results for this final state, which allows us to investigate $a_J$ and $\pi_J$ mesons, employing partial-wave analysis (PWA). In this method, the decay into $\pi^-\pi^+\pi^-$ is modeled as subsequent two-body decays in order to disentangle the contributions of different partial waves. The large size of our dataset allows to perform this analysis in narrow bins of the squared four-momentum transfer $t'$. Thus, we can also extract the $t'$ dependence of the various components from the data. Finally, the resonance parameters of $a_J$ and $\pi_J$ mesons are measured by disentangling resonant and non-resonant parts of $14$ selected partial waves simultaneously in a resonance-model fit. Combining $14$ partial waves in a single resonance-model fit allows us to study also weaker signals, e.g. from excited $a_1$, $a_2$, or $\pi_2$ states, by making use of their interference pattern and their different couplings to the various decay modes.
