Martin Lüscher-CERN: The famous Witten-Veneziano formula for the mass of the eta' meson links the fluctuations of the topological charge density in QCD to an experimentally accessible quantity. As it stands, the formula is however affected by short-distance singularities, whose subtraction leads to a non-perturbative ambiguity on the QCD side of the equation. The issue proved to be a persistent one that could only be resolved after important theoretical advances in lattice and continuum QCD were made, many of which are interesting on their own. In this talk, the most relevant of these developments are reviewed with emphasis on the underlying concepts and key results.

DATA: 07-04-2016