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 $\bar{Q}\bar{Q}'qq'$ 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 $\bar{b}\bar{b}ud$, $\bar{b}\bar{b}us$ and $\bar{b}\bar{c}ud$. We show evidence for the existence of $\bar{b}\bar{b}ud$ and $\bar{b}\bar{b}us$ bound states, while no binding appears to be present for $\bar{b}\bar{c}ud$. 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.