Exotic QQ$\overline{q}\overline{q}$ states in QCD

QCD contains stable four-quark $QQ \overline{qq}$ hadronic states in the limit where the heavy quark mass goes to infinity. (Here $Q$ denotes a heavy quark, $\overline{q}$ a light antiquark and the stability refers only to the strong interactions.) The long-range binding potential is due to one-pion exchange between ground state $Q \overline{q}$ mesons, and is computed using chiral perturbation theory. For the $Q = b$, this long-range potential may be sufficiently attractive to produce a weakly bound two-meson state.