  We investigate the quark content of the scalar meson $a_0(980)$ 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 timeslice, 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 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 $K +
\bar{K}$ and $\eta + \pi$. This additional state, which is a candidate for the
$a_0(980)$ meson, couples to a quark-antiquark as well as to a
diquark-antidiquark interpolating field, indicating that it is a superposition
of an ordinary $\bar{q} q$ 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.
