Multi-scattering processes and spectral properties of low-energy QCD

Abstract

We compute quark and meson spectral functions in low-energy QCD, including all-order scatterings and decays of pions and the scalar $σ$-mode. For low energies the gluons decouple and the dynamics of two-flavour QCD is well-described by a Quark-Meson model. The computations are performed directly in Minkowski space, using the spectral functional renormalisation group. The full mesonic realtime dynamics is captured with the emergent composite approach which is extended here to realtime processes. The inclusion of all-order scatterings and decays is achieved through the self-consistent treatment of the scattering tails and the momentum-dependent resummation of the four-meson vertex via its Bethe-Salpeter equation. We illustrate the importance of higher-order scatterings using the example of the $π\to 3π$ scattering threshold - the lowest kinematically accessible channel for the pion.