A New Dynamical Picture for the Production and Decay of the $XY \! Z$ Mesons

Abstract

I introduce an entirely new dynamical description for exotic charmoniumlike hadrons, based upon the competing effects of the strong attraction between quarks in a diquark, and the inability of the diquark to hadronize on its own due to being a color nonsinglet. This mechanism naturally explains, for example, the strong preference of the $Z(4475)$ to decay to $\psi(2S)$ rather than the $J/\psi$, the existence of a state $X(4630)$ that decays to $\Lambda_c$ baryon pairs, and why some but not all exotics lie near hadronic thresholds. Owing to high-energy constituent counting rules, the four-quark nature of the states produces major changes to both the high-$s$ scaling of cross sections for producing such states and to the potency of the cusp effect of attracting resonances to pair-production thresholds. The recently observed $P_c^+$ pentaquark candidates are seen to fit naturally into this scheme.