Session K2: Invited Session: Quarkonium - XYZ States

Charging up the $XYZ$ spectrum: recent experimental developments in charmonium spectroscopy

Over the last decade many new states, often dubbed $X$, $Y$, or $Z$, have been discovered in the excited charmonium spectrum. It has long been noted that several of these states have masses, widths, or decay modes that seem to be inconsistent with a simple quark-antiquark ($c\bar{c}$) interpretation. In the past year, the spectrum of $XYZ$ states has become broader with the discovery of multiple structures in the charmonium system that have electric charge, and therefore cannot be $c\bar{c}$ mesons. In this talk, I will review the recent developments in excited charmonium spectroscopy using data collected with the BESIII, Belle, and CLEO-c detectors. The results will be discussed in the context of both the known charmonium and bottomonium spectra, and prospects for future study will be presented.   

Lattice Results for Heavy Quarkonia

I will discuss some recent lattice QCD studies of heavy quarkonia with a focus on excited charmonia where there has been significant progress in the last few years. A highlight is a calculation of an extensive charmonium spectrum, including states with exotic quantum numbers and non-exotic hybrid mesons, with high statistical precision and the interesting phenomenology suggested by these results will be discussed. I also comment on investigations of unstable hadrons on the lattice, future prospects and opportunities to understand some puzzling experimental observations.   

Theoretical Interpretations of the XYZ States

The $XYZ$ mesons are unexpected $c \bar c$ and $b \bar b$ mesons that have been discovered during the last decade. The models for the $XYZ$ mesons that have been proposed, none of which have revealed a compelling pattern, include conventional quarkonium, quarkonium hybrids, and quarkonium tetraquarks (whose four constituents can be clustered in various ways). The Born-Oppenheimer approximation provides a coherent QCD framework for describing the $XYZ$ mesons that can be informed by lattice QCD. The additional hints that will be provided by current and upcoming experiments guarantee the eventual solution of the $XYZ$ puzzle.