Speaker
Description
Our current understanding of hadrons is through QCD, and confinement in QCD leads to a rich spectrum of hadrons. Experimentally, hadronic resonances can appear as peaks in the invariant mass distributions. However, universal parameters of hadronic resonances are encoded theoretically in the poles of the $S$-matrix. Still, not all observed peaks necessarily correspond to hadronic resonances.
For example, the kinematical singularities, that correspond to intermediate particles going on shell and producing a peak in the invariant mass distributions, without any correspondence to a pole in the $S$-matrix. This was originally identified by Landau, and has been recently used to explain the $a_1(1420)$. We re-examine this scenario in the full three-body unitary formalism, utilising a recent approach, and extending it to the relevant coupled-channel system. We show the pattern and mechanism leading to the triangle singularity to all orders in exchange diagrams.
