Light-Meson Spectroscopy at COMPASS

TL;DR

The COMPASS experiment at CERN studies light meson spectroscopy using high-energy hadron beams, providing detailed measurements of known resonances and discovering new states like the $a_1(1420)$ with unprecedented data quality.

Contribution

This work presents the largest dataset for light meson spectroscopy, enabling high-precision measurements and the discovery of the new $a_1(1420)$ resonance.

Findings

High-precision measurements of known resonances.

Discovery of the new $a_1(1420)$ axial-vector state.

Confirmation of results across different decay channels.

Abstract

The goal of the COMPASS experiment at CERN is to study the structure and spectroscopy of hadrons. The two-stage spectrometer has large acceptance and covers a wide kinematic range for charged as well as neutral particles allowing to access a wide range of reactions. Light mesons are studied with negative (mostly $\pi^-$) and positive ($p$, $\pi^+$) hadron beams with a momentum of $190\,\text{GeV}/c$. The light-meson spectrum is measured in different final states produced in diffractive dissociation reactions with squared four-momentum transfer $t$ to the target between $0.1$ and $1.0\,(\text{GeV}/c)^2$. The flagship channel is the $\pi^-\pi^+\pi^-$ final state, for which COMPASS has recorded the currently world's largest data sample. These data not only allow us to measure the properties of known resonances with high precision, but also to search for new states. Among these is a new axial-vector signal, the $a_1(1420)$, with unusual properties. The findings are confirmed by the analysis of the $\pi^-\pi^0\pi^0$ final state.