Lattice QCD investigation of the structure of the a 0 (980) meson

We investigate the quark content of the low-lying states in the I(JP)=1(0+) sector, which are the quantum numbers of the a0(980) meson, using lattice QCD. To this end, we consider correlation functions of six different two- and four-quark interpolating fields. We evaluate all diagrams, including diagrams, where quarks propagate within a time slice, e.g. with closed quark loops. We demonstrate that diagrams containing such closed quark loops have a drastic effect on the final results and, thus, may not be neglected. Our analysis, which is carried out at unphysically heavy u and d quark mass corresponding to mπ=296(3) MeV and in a single spatial volume of extent 2.9 fm, shows that in addition to the expected spectrum of two-meson scattering states there is an additional energy level around the two-particle thresholds of K+K̄ and η+π. This additional state, which is a candidate for the a0(980) meson, couples to a quark-antiquark as well as to a diquark-antidiquark interpolating field, indicating that it is a superposition of an ordinary q̄q and a tetraquark structure. The analysis is performed using AMIAS, a novel statistical method based on the sampling of all possible spectral decompositions of the considered correlation functions, as well as solving standard generalized eigenvalue problems.