Hadron physics at the COMPASS experiment

Quantum Chromodynamics (QCD), the theory of strong interactions, in principle describes the interaction of quark and gluon fields. However, due to the self-coupling of the gluons, quarks and gluons are confined into hadrons and cannot exist as free particles. The quantitative understanding of this confinement phenomenon, which is responsible for about 98% of the mass of the visible universe, is one of the major open questions in particle physics. The measurement of the excitation spectrum of hadrons and of their properties gives valuable input to theory and phenomenology. In the Constituent Quark Model (CQM) two types of hadrons exist: mesons, made out of a quark and an antiquark, and baryons, which consist of three quarks. But more advanced QCD-inspired models and Lattice QCD calculations predict the existence of hadrons with exotic properties interpreted as excited glue (hybrids) or even pure gluonic bound states (glueballs). The Compas