# The Dynamical Diquark Model: Fine Structure and Isospin

## TL;DR

This paper enhances the dynamical diquark model of multiquark hadrons by including fine-structure effects, successfully explaining observed exotic states and predicting new ones through a simplified interaction Hamiltonian.

## Contribution

It introduces isospin-dependent couplings and finite diquark size effects into the model, providing a better fit to experimental data and new insights into exotic hadron structures.

## Key findings

- The $X(3872)$ naturally emerges as the lightest exotic state.
- The model explains decay patterns of $Z_c(3900)$ and $Z_c(4020)$.
- Predictions for excited tetraquark and pentaquark states.

## Abstract

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.