Exotic Hadrons

Strong interactions are described by quantum chromodynamics (QCD), a relativistic quantum gauge field theory with quarks and gluons as fundamental degrees of freedom. However, because of colour confinement, only hadrons colourless bound states of quarks and gluons are observed in nature. Remarkably, for decades only ordinary hadrons with the quantum numbers of quark-antiquark or three-quark systems have been established experimentally, although a priori all possible colourless bound states may exist in nature. Only recently experimental evidence for the existence of non-ordinary hadrons, so-called exotic states, has been gathered. Particularly convincing is the observation of a charged state minimally composed of a charmed quark, a charmed antiquark, an up-quark, and a down-antiquark, opening an entirely new chapter in QCD. The dynamics of both formation and structure of exotic states exhibit specific distinguishing features compared to the ordinary hadrons. Numerous models for the description of exotic hadrons have been proposed in the literature, but so far no QCD-based dynamical formalism for predicting the properties of exotic states has been formulated. The phenomenon of confinement, which provides the mechanism of hadron formation, is definitely of nonperturbative origin and therefore beyond reach of perturbation theory, the standard tool for analytic investigations in quantum field theory. Accordingly, hadronic features may be properly analysed only by nonperturbative approaches to QCD. This project will develop a formalism for the description of exotic states in QCD and the prediction of their characteristics, such as decay constants, form factors and decay rates, by exploiting the technique of QCD sum rules one of the main analytic nonperturbative theoretical tools in QCD. This analysis constitutes a first systematic dynamical exploration of the sector of exotic states the terra incognita of QCD. Upon completion, this project will result in reliable theoretical statements about properties of exotic states and will open the possibility of a better identification of such states in future experiments.

Elementary particle physics subsumes under the notion "hadrons" all those elementary particles that are subject to the strong interactions and can be observed as free particles. All hadrons prove to be composed of subcomponents, such as the quarks and their antiparticles. However, this class of particles encompasses not only bound states of quark and antiquark (like the pions) or of three quarks (e.g., proton and neutron) but also more exotic representatives, such as the "multiquarks", consisting of more than three quarks; examples of the latter are "tetraquarks" formed by two quarks and two antiquarks, or "pentaquarks" consisting of four quarks and one antiquark. Establishing, for such exotic states, their actual existence in nature by experiment has been achieved only recently. Quite generally, attempts of theoretical investigations of multiquarks face a conceptual difficulty: the components of any multiquark can likewise form two or more traditional non-exotic hadrons. A rather popular approach to bound states of quarks relies on the derivation of relations between observable hadron features and basic parameters of the theory governing the strong interactions. Hence, in order to separate the wheat from the chaff and to pave the way for trustable analyses of such exotic hadrons, the first objective of this project was the construction of rigorous criteria for the unambiguous identification of all contributions not related to multiquarks. Application of this procedure offers a means to discard the ballast of multiquark-irrelevant contamination. Thus, one of the main outcomes of this project is the definition and systematic formulation of a framework enabling, in the future, genuinely multiquark-adequate quantitative discussions of exotic hadrons which are not based on exclusively numerical techniques. In view of the fact that many previous multiquark studies retained the undesirable contributions, the insights gained in the project might induce a kind of shift of paradigm.