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.