  We incorporate fine-structure corrections into the dynamical diquark model of
multiquark exotic hadrons. These improvements include effects due to finite
diquark size, spin-spin couplings within the diquarks, and most significantly,
isospin-dependent couplings in the form of pionlike exchanges assumed to occur
between the light quarks within the diquarks. Using a simplified two-parameter
interaction Hamiltonian, we obtain fits in which the isoscalar $J^{PC} =
1^{++}$ state---identified as the $X(3872)$---appears naturally as the lightest
exotic (including all states that are predicted by the model but have not yet
been observed), while the $Z_c(3900)$ and $Z_c(4020)$ decay predominantly to
$J/\psi$ and $\eta_c$, respectively, in accord with experiment. We explore
implications of this model for the excited tetraquark multiplets and the
pentaquarks.

High Energy Physics - Phenomenology