The Spectrum of $P$-Wave Hidden-Charm Exotic Mesons in the Diquark Model

We study the fine structure in the spectrum of known and predicted negative-parity hidden-charm exotic meson states, which comprise the lowest $P$-wave multiplet in the dynamical diquark model. Starting with a form previously shown to successfully describe the $S$-wave states, we develop a 5-parameter Hamiltonian that includes spin-orbit and tensor terms. After discussing the experimental status of the observed $J^{PC} = 1^{--}$ states $Y$ with respect to masses and decay modes (classified by eigenvalues of heavy-quark spin), we note a number of inconsistencies between measurements from different experiments that complicate a unique determination of the spectrum. Outlining a variety of scenarios for interpreting the $Y$ data, we perform fits to each one, obtaining results that demonstrate differing possibilities for the $P$-wave spectra. Choosing one of these fits for illustration, we predict masses for all 28 isomultiplets in this $1P$ multiplet, compare the results to tantalizing hints in the data, and discuss the rich discovery potential for new states.