Existence and Non-Existence of Doubly Heavy Tetraquark Bound States

In this work we investigate the existence of bound states for doubly heavy tetraquark systems Q^-Q^{- 'qq'} in a full lattice-QCD computation, where heavy bottom quarks are treated in the framework of non-relativistic QCD. We focus on three systems with quark content b^-b^-ud, b^-b^-us and b^-c^-ud. We show evidence for the existence of b^-b^-ud and b^-b^-ud bound states, while no binding appears to be present for b^-c^-ud. For the bound four-quark states we also discuss the importance of various creation operators and give an estimate of the meson-meson and diquark-antidiquark percentages.