The Exotic XYZ Charmonium-Like Mesons

Abstract

Charmonium, the spectroscopy of cc mesons, has recently enjoyed a renaissance with the discovery of several missing states and a number of unexpected charmonium-like resonances. The discovery of these new states has been made possible by the extremely large data samples made available by the B-factories at the Stanford Linear Accelerator Center and at KEK in Japan, and at the CESR e + e − collider at Cornell. Conventional cc states are well described by quark potential models. However, many of these newly discovered charmonium-like mesons do not seem to fit into the conventional cc spectrum. There is growing evidence that at least some of these new states are exotic, i.e. new forms of hadronic matter such as mesonic-molecules, tetraquarks, and/or hybrid mesons. In this review we describe expectations for the properties of conventional charmonium states and the predictions for molecules, tetraquarks and hybrids and the various processes that can be used to produce them. We examine the evidence for the new candidate exotic mesons, possible explanations, and experimental measurements that might shed further light on the nature these states.

Y(3940),Z(3930), and theXMolina1, Oset2 2009Phys. Rev. D

“…Indeed, for a charmonium state with J P C = 0 −+ (η c ) the mass is a little low and for J P C = 0 ++ (χ ′ c0 ) the mass is too high [20], also if it had a simple cc structure one would expect that the open charm decay modes would be dominant, and the other, ωJ/ψ, negligible. Also, the large partial width Y (3940) → ωJ/ψ, estimated above 1 MeV [21], is quite larger than the measured partial widths for any of the observed hadronic transitions between charmonium states.…”

“…In the production process the two photons can only produce DD in a 0 ++ or 2 ++ states. The Belle measurements favors the 2 ++ hypothesis, making the assignment of the Z(3930) to the 2 3 P 2 (χ ′ c2 ) charmonium state possible [21], following the arguments given in [27]. In this paper we will propose a theoretical explanation on the nature of some of these XYZ states, providing structure and quantum numbers for them.…”

The hidden-charm pentaquark and tetraquark states

“…Interested readers may also consult reviews in Refs. [40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,39].…”

Overview of $XYZ$ states and tetraquarks

“…Most of them cannot be described well by quarkonium potential models and may be the good candidates of exotic states [1,2,3]. Their unusual properties have stimulated considerable theoretical interest and various interpretations have been proposed, including tetraquarks, molecules, hybrids, or hadrocharmonia [1,3,4].…”