Exotic tetraquark states with the qq (Q)over-bar(Q)over-bar configuration

In this work, we study systematically the mass splittings of the qq (Q) over bar(Q) over bar (q = u, d, s and Q = c, b) tetraquark states with the color-magnetic interaction by considering color mixing effects and estimate roughly their masses. We find that the color mixing effect is relatively important for the J(P) = 0(+) states and possible stable tetraquarks exist in the nn (Q) over bar(Q) over bar (n = u, d) and ns (Q) over bar(Q) over bar systems either with J = 0 or with J = 1. Possible decay patterns of the tetraquarks are briefly discussed.