Highlights from the COMPASS experiment at CERN -- Hadron spectroscopy and excitations

TL;DR

The COMPASS experiment at CERN has made significant advances in hadron spectroscopy by precisely measuring pion polarisability, discovering a new axial-vector state, and investigating exotic mesons and charmonium-like states, enhancing understanding of hadron structure.

Contribution

This paper provides a comprehensive overview of recent experimental results from COMPASS, including new measurements and discoveries in hadron spectroscopy and exotic states.

Findings

Precise measurement of pion polarisability resolving a long-standing puzzle

Observation of a new narrow axial-vector state, $a_1(1420)$

Insights into the exotic $1^{-+}$-wave and search for $Z_c(3900)$

Abstract

The COMPASS experiment at the CERN-SPS studies the spectrum and the structure of hadrons by scattering high energy hadrons and polarised muons off various fixed targets. Recent results for the hadron programme comprise highlights from different topics. A selective overview is given and, among others, the following results are discussed. The precise determination of the pion polarisability, a long standing puzzle that has been solved now, is presented as well as measurements of radiative widths. The observation of a new narrow axial-vector state, the $a_1(1420)$, as well as deeper insights into the exotic $1^{-+}$-wave, which is under study since decades by several experiments, are discussed and further, the search for the charmonium-like exotic $Z_c(3900)$ state in the COMPASS data is covered.