Abstract
Many of the new structures observed since 2003 in experiments in the heavy quarkonium mass region, such as the X (3872) and Z c (3900), are rather close to certain thresholds, and thus can be good candidates of hadronic molecules, which are loose bound systems of hadrons. We will discuss the consequences of heavy quark symmetry for hadronic molecules with heavy quarks. We will also emphasize that the hadronic molecular component of a given structure can be directly probed in long-distance processes, while the short-distance processes are not sensitive to it.
Highlights
Hadronic molecules are analogues of the light nuclei
Hadronic molecules are interesting for several reasons: (1) They are one type of realizations of color-neutral objects and the investigation of how hadronic molecular spectrum is organized would lead to a better understanding of the strong interactions at low energies
(3) Since they are close to thresholds, one has a good separation of energy scales 2μEB Λhadron, where Λhadron is typically a hadronic scale of the order of a few hundreds of MeV, so that they can be studied using techniques of effective field theory (EFT)
Summary
Hadronic molecules are analogues of the light nuclei They can be treated to a good approximation as composed systems made of two or more hadrons which are bound together via the strong interactions. Their size, ∼ 1/ 2μEB with μ and EB the reduced mass of the constituent hadrons and the binding energy, respectively, should be large so that the constituent hadrons can be recognized separately. (4) There have been a lot of new structures observed since 2003 in the heavy quarkonium mass region which do not fit in the conventional quarkonium spectrum, see Fig. 1 They are generally dubbed as XYZ states. Note that the X(3915) is listed with 0++, it is difficult to understand its properties as the χc0(2P) [3, 4] and it might be just the same state as the χc2(2P) with 2++ [5]
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