Exotic $QQ\bar{q}\bar{q}$, $QQ\bar{q}\bar{s}$ and $QQ\bar{s}\bar{s}$ states

After constructing the possible $J^P=0^{-},0^{+},1^{-}$ and $1^{+}$ $QQ\bar{q}\bar{q}$ tetraquark interpolating currents in a systematic way, we investigate the two-point correlation functions and extract the corresponding masses with the QCD sum rule approach. We study the $QQ\bar{q}\bar{q}$, $QQ\bar{q}\bar{s}$ and $QQ\bar{s}\bar{s}$ systems with various isospins $I=0, 1/2, 1$. Our numerical analysis indicates that the masses of doubly-bottomed tetraquark states are below the threshold of the two bottom mesons, two bottom baryons and one doubly bottomed baryon plus one anti-nucleon. Very probably these doubly-bottomed tetraquark states are stable.