Quantum Chromodynamics (QCD) is the theory of the strong interaction, the fundamental force of nature responsible for the microscopic properties of matter. QCD has left us a signature in form of a rich resonance spectrum, in an energy region where the theory becomes genuinely non-perturbative ---a unique feature that attracts great theoretical interest. QCD manifests itself in these excited states through a complex phenomenology that is currently under intense experimental investigation at JLab and other facilities around the world. From the theory side, non-perturbative approaches have been developed in terms of quarks and gluons, but also relying on hadrons as the relevant degrees of freedom. To connect such approaches among each other, and also to experimental data, is one of the most challenging goals of hadron physics.
In this talk, I will address how modern tools of reaction theory help understand the wealth of current and future experimental data. Thanks to supercomputers, precise analyses of entangled pion- and photon-induced reactions are now within our grasp through dynamical coupled-channel approaches. Chiral unitary models complement the picture. Both ansätze, besides their phenomenological facets, are currently extended to analyze and interpret fundamental quark-based calculations, as I will discuss for the rapidly evolving field of lattice QCD. In particular, such analyses will play a key role once quark masses in lattice simulations drop towards the physical limit, obscuring the resonance spectrum through finite volume effects.
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