Beauty and Charm Hadrons and Their Decays

Hadrons containing the comparatively heavy bottom and charm quarks are in the focus of interest of theorists and experimentalists all over the world for several reasons. First, the experimental investigation of such hadrons offers both unique tests of the standard theory of elementary particle physics and probes of expected "new physics" beyond the standard theory. Second, these particles allow to study the complicated quantum chromodynamics (QCD) effects in systems containing both heavy and light quarks. The main difficulty in all theoretical descriptions of such systems is the necessity to treat the nonperturbative QCD phenomena responsible for the formation of hadrons and for the properties of these bound states: QCD is a relativistic quantum field theory formulated at the fundamental level of quark and gluons whereas - as summarized under the notion of confinement of all colour degrees of freedom - only hadrons (colourless bound states of these quarks and gluons) are observed in nature. A novel feature of this project is the combination of two, very promising approaches to nonperturbative QCD, namely, the relativistic quark model and the method of QCD sum rules. This move renders possible to deal with such cumbersome problems in a systematic way and to improve the predictions arising within each of these techniques. The main goals of this project are to establish a sound connection between the constituent quark model, on the one hand, and QCD, on the other hand; to increase our understanding of the parameters and the accuracy of QCD sum rules; to apply new versions of QCD sum rules to both heavy-quark systems and weak decays of heavy hadrons; and to extend the methods developed for the usual hadrons to the study of new exotic multiquark states. The outcome of this project should provide a better understanding of the nonperturbative QCD dynamics, by emphasizing the advantageous features of each of the nonperturbative approaches and diminishing all uncertainties, and by leading to more reliable predictions for experimentally measured observables.

In elementary particle physics, any strongly interacting elementary particle observed in nature is called a "hadron". To this class of particles belong the well-known protons, neutrons and pions, as well as objects containing also heavier quarks, discriminated by a quantum number called "flavour". The entire structure and all characteristics of the hadrons are governed by the strong interactions between their constituents: quarks and gluons. Within the standard theory of elementary particle physics, the strong interactions are described by a relativistic quantum field theory called quantum chromodynamics (abbr. QCD), involving quarks and gluons as its fundamental degrees of freedom. However, until now limitations induced by the inherent mathematical structure of QCD, determined by symmetry considerations, prevent to solve QCD exactly. Even the application of methods of perturbation theory (the only fully analytical tool for deriving approximate solutions) is feasible only under certain well-defined circumstances. Consequently, in order to study strongly interacting physical systems one has to rely on semi- analytical or numerical methods, or to construct appropriate simplified models. In view of this, the goal of the present research project was to investigate nonperturbative QCD effects within the framework of two frequently employed approaches to QCD: "QCD sum rules", a powerful method for analyzing properties of ground-state hadrons, and the constituent quark model, which constitutes the only framework for the analysis of the features of excited hadrons, in particular of processes mediated by weak interactions and involving hadrons that contain the comparatively heavy b- or c-quarks and carry thus nonvanishing values of the flavour quantum numbers "beauty" and/or "charm". In particular, weak interactions with participation of a b-quark provide, on the one hand, an ideal environment to perform precision tests of the standard theory of elementary particle physics, to investigate a phenomenon commonly known as "CP violation", and to measure so far unknown parameters of the standard theory that describe the mixing of heavy and light quarks, and open, on the other hand, windows to study new physics beyond the realm of the standard theory. In order to scrutinize this standard theory by b-physics experiments, the b-quark`s - comparatively well understood - weak interactions and its - definitely more complicated - strong interactions have to be disentangled; the latter exhibit the phenomenon of "confinement" of quark and gluon degrees of freedom to hadrons, a feature that can be accommodated properly only by nonperturbative approaches to QCD. This project resulted in a considerable improvement of the method of QCD sum rules, which constitutes the main semi-analytical tool for theoretical analyses of features of individual hadrons directly from QCD. The standard techniques of QCD sum rules applied so far in hadron physics proved not to yield reliable error estimates for predicted parameters of individual hadron states. Now, this grave shortcoming, as far as sum-rule applications to precision electroweak physics are concerned, has been overcome: As main outcome of this project, the relevant modifications of the QCD sum-rule formalism that yield a significant increase of its predictive power for various characteristics of hadrons have been identified and proposed. Taking advantage of these insights and improvements, it was rather straightforward to extract from QCD sum rules more accurate predictions of the properties of ground-state hadrons involving c- and b-quarks.