Simple spectrum of c(c)over-barc(c)over-bar states in the dynamical diquark model

We develop the spectroscopy of c (c) over barc (c) over bar and other all-heavy tetraquark states in the dynamical diquark model. In the most minimal form of the model (e.g., each diquark appears only in the color-triplet combination; the nonorbital spin couplings connect only quarks within each diquark), the spectroscopy is extremely simple. Namely, the S-wave multiplets contain precisely three degenerate states (0(++), 1(+-), 2(++)) and the seven P-wave states satisfy an equal-spacing rule when the tensor coupling is negligible. When comparing numerically to the recent LHCb results, we find the best interpretation is assigning X(6900) to the 2S multiplet, while a lower state suggested at about 6740 MeV fits well with the members of the 1P multiplet. We also predict the location of other multiplets (1S, 1D, etc.) and discuss the significance of the cc open-flavor threshold.