An a0 resonance in strongly coupled πη, K(K)over-bar scattering from lattice QCD

We present the first calculation of coupled-channel meson-meson scattering in the isospin = 1, G-parity negative sector, with channels pi eta, K (K) over bar and pi eta', in a first-principles approach to QCD. From the discrete spectrum of eigenstates in three volumes extracted from lattice QCD correlation functions we determine the energy dependence of the S-matrix, and find that the S-wave features a prominent cusplike structure in pi eta -> pi eta close to the K (K) over bar threshold coupled with a rapid turn-on of amplitudes leading to the K (K) over bar final state. This behavior is traced to an a(0)(980)-like resonance, strongly coupled to both pi eta and K (K) over bar, which is identified with a pole in the complex energy plane, appearing on only a single unphysical Riemann sheet. Consideration of D-wave scattering suggests a narrow tensor resonance at higher energy.