Heavy-Quark Symmetry Implies Stable Heavy Tetraquark Mesons $Q_i{Q}_j{\overline{q}}_k{\overline{q}}_l$

For very heavy quarks $Q$, relations derived from heavy-quark symmetry predict the existence of novel narrow doubly heavy tetraquark states of the form $Q_i{Q}_j{\overline{q}}_k{\overline{q}}_l$ (subscripts label flavors), where $q$ designates a light quark. By evaluating finite-mass corrections, we predict that double-beauty states composed of $bb\overline{u}\overline{d}$, $bb\overline{u}\overline{s}$, and $bb\overline{d}\overline{s}$ will be stable against strong decays, whereas the double-charm states $cc{\overline{q}}_k{\overline{q}}_l$, mixed $\mathrm{beauty}+\mathrm{charm}$ states $bc{\overline{q}}_k{\overline{q}}_l$, and heavier $bb{\overline{q}}_k{\overline{q}}_l$ states will dissociate into pairs of heavy-light mesons. Observation of a new double-beauty state through its weak decays would establish the existence of tetraquarks and illuminate the role of heavy color-antitriplet diquarks as hadron constituents.