Abstract
The two-pole structure refers to the fact that particular single states in the spectrum as listed in the PDG tables are often two states. The story began with the Λ ( 1405 ) , when in 2001, using unitarized chiral perturbation theory, it was observed that there are two poles in the complex plane, one close to the K ¯ p and the other close to the π Σ threshold. This was later understood combining the SU(3) limit and group-theoretical arguments. Different unitarization approaches that all lead to the two-pole structure have been considered in the mean time, showing some spread in the pole positions. This fact is now part of the PDG book, although it is not yet listed in the summary tables. Here, I discuss the open ends and critically review approaches that cannot deal with this issue. In the meson sector, some excited charm mesons are good candidates for such a two-pole structure. Next, I consider in detail the D 0 * ( 2300 ) , which is another candidate for this scenario. Combining lattice QCD with chiral unitary approaches in the finite volume, the precise data of the Hadron Spectrum Collaboration for coupled-channel D π , D η , D s K ¯ scattering in the isospin I = 1 / 2 channel indeed reveal its two-pole structure. Further states in the heavy meson sector with I = 1 / 2 exhibiting this phenomenon are predicted, especially in the beauty meson sector. I also discuss the relation of these two-pole structures and the possible molecular nature of the states under consideration.
Highlights
The hadron spectrum is arguably the least understood part of Quantum Chromodynamics (QCD), the theory of the strong interactions
Hadronic molecules and atomic nuclei, that is multiquark states composed of a certain number of conventional hadrons; Hybrid states, which are composed of quarks and gluons; and Glueballs, bound states solely made of gluons, arguably the most exotic form of matter, which has so far been elusive in all searches
The observed hadrons are listed with their properties in the tables of the Particle Data Group (PDG) ( called “Review of Particle Physics” (RPP)) [4] within a certain rating scheme, just telling us that some states are better understood as others
Summary
The hadron spectrum is arguably the least understood part of Quantum Chromodynamics (QCD), the theory of the strong interactions. The only model-independent way (There are very few exceptions of isolated resonances on an energy-independent background where other methods can be used, but even in such cases a unique determination of the mass and the width is not always possible, see, e.g., the discussion of the ρ(770) in the RPP.) to pin down these basic resonance properties is to look for poles in the complex plane, where resonances are usually located on the second Riemann sheet at z R = (
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