charmoniumlike

Charmoniumlike refers to hadronic states that contain a charm quark and an antiquark (hidden charm) but do not fit the predicted spectrum of conventional charmonium as described by potential models. These states are primarily observed in high-energy experiments and are often labeled with X, Y, or Z.

The term gained prominence after several discoveries in the 2000s and 2010s, including X(3872) found by Belle in 2003, and the Y(4260) seen in e+e- annihilation by BaBar. Some members are charged, such as Zc(3900) and Zc(4020), which cannot be conventional ccbar mesons.

Interpretations include tetraquarks (four-quark states), hadronic molecules composed of charmed mesons (for example D and D*), and hybrids with excited gluonic fields. Threshold effects and cusps at open-charm thresholds also play a role in some observations, making assignments challenging.

In experiments, charmoniumlike states are often produced in B decays or in e+e- collisions and studied through decays to J psi with light hadrons, or to open-charm final states. Their masses frequently lie near open-charm thresholds and their widths vary, complicating definitive classifications.

The usage of the label emphasizes that these states may not be conventional ccbar quarkonia, reflecting a spectrum that includes exotic configurations such as tetraquarks and hadronic molecules. Ongoing experiments continue to map their properties and refine theoretical descriptions within QCD.