Abstract

In recent years, the spectroscopy of hadrons containing heavy quarks has brought forward a plethora of new and unexpected resonance-like signals. Many of these so-called X,Y,Z states are candidates for states beyond the quark-antiquark or three-quark configurations of mesons and baryons, respectively, which have been sought after since the introduction of the quark model. Similar studies in the light-quark sector are more challenging due to the wide and overlapping nature of known resonances, but are also expected to give easier access to hadrons with explicit gluonic degrees of freedom, so-called hybrids. Recently, lattice QCD started to make predictions on the multiplet structure of exotic hadrons, which may be used as a guideline in the experimental searches.

The COMPASS experiment at CERN uses a high-energy pion beam scattered off protons and heavier nuclei to produce mesonic excitations, which are observed in multi-particle final states subjected to partial-wave analyses. In addition to studying the properties of established mesons with unprecedented accuracy, the large existing data samples allow us to detect possibly exotic states. These either do not fit current theoretical expectations, like the recently observed a1(1420), or carry spin-exotic quantum numbers, like the pi1(1600). The talk will give an update on ongoing studies of these states involving novel analysis techniques and discuss possible interpretations.