From hadrons at unphysical quark masses to coupled-channel reaction dynamics in the laboratory

Matthias Lutz (GSI, Darmstadt)

Abstract:

In this talk I discuss recent progress on how to study QCD in its regime where non-perturbative structures emerge. While current QCD lattice simulations start to be available for masses of the hadrons in their ground states, computations of the excitation spectrum as measured in the laboratory pose still a formidable challenge for the lattice community. Nevertheless, it is argued that current lattice data taken at unphysical quark masses are instrumental to access QCD's excitation spectrum. These data can be used to determine the low-energy constants of effective chiral Lagrangians. Based on the latter coupled-channel approaches can then provide the missing link to the empirical data set. Such a program is illustrated at the hand of two systems. First we consider the quark-mass dependence of the baryon masses in their flavour octet and decuplet ground states. Here, any significant progress requires an improved understanding of the role played by the strange quarks. We find that a decomposition of the baryon masses into chiral moments does converge up to surprisingly large strange quark masses provided that such an expansion is formulated in terms of physical meson and baryon masses rather than the bare masses as requested by conventional Chiral Perturbation Theory ($\chi$PT). Given such a scheme a quantitative description of the baryon masses on all available QCD lattice ensembles is possible. In turn an accurate set of low-energy constants is established, which, moreover, is found compatible with the hirachy of sum rules as predicted by QCD with a large number of colors ($N_c$). The implications of this set on the baryon sigma terms is eluded on. The second case we discuss are meson systems that involve a charm quark. A quantitative reproduction of the masses of the D mesons in the $J^P = 0^-$ and $J^P = 1^-$ flavour triplet ground states on all available QCD lattice ensembles is achieved. The low-energy constants are used in a coupled-channel approach and predictions for the s-wave scattering phase-shifts and inelasticity parameters are made. Such results are scrutinized against first QCD lattice computations of the $\pi D$ scattering phase shifts by HSC at unphysical quark masses. A striking dependence on the choice of quark masses is predicted. At physical quark masses a clear signal of a member of an exotic flavour sextet multiplet in the $\eta D$ invariant mass distribution is foreseen.

 

HISKP, Hörsaal (Raum 0.023)

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