Lattice Prediction for Deeply Bound Doubly Heavy Tetraquarks.

We investigate the possibility of qq^{'}b[over ¯]b[over ¯] tetraquark bound states using n_{f}=2+1 lattice QCD ensembles with pion masses ≃164, 299, and 415 MeV. Motivated by observations from heavy baryon phenomenology, we consider two lattice interpolating operators, both of which are expected to couple efficiently to tetraquark states: one with a diquark-antidiquark and one with a meson-meson structure. Using nonrelativistic QCD to simulate the bottom quarks, we study the udb[over ¯]b[over ¯], ℓsb[over ¯]b[over ¯] channels with ℓ=u, d, and find unambiguous signals for strong-interaction-stable J^{P}=1^{+} tetraquarks. These states are found to lie 189(10) and 98(7) MeV below the corresponding free two-meson thresholds.