Hadrons in covariant models of quantum chromodynamics

In modern particle physics, hadrons are understood to experience the strong interaction, which is one of the four fundamental forces of nature. The strong interaction is an important reason for why the world is as it is, and in particular for our very existence. The most well-known hadrons, the proton and the neutron, constitute atomic nuclei. Since protons are positively charged and neutrons are neutral, the nucleus would not be stable without the presence of the strong interaction. This interaction can be understood theoretically via the concept of quarks and gluons, the elementary particles in quantum chromodynamics, which is believed to be the underlying quantum field theory of the strong interaction. Neither quarks nor gluons can ever reach a particle detector alone, but they appear in certain groups and form bound states, which appear as hadrons. Hadrons with mainly three quarks inside - held together by gluon exchange - are called baryons. Hadrons consisting of mainly a quark and an antiquark again held together by gluon exchange - are called mesons. Many different mesons and baryons have been found experimentally, and it is the goal of theoretical hadron physics to explain their properties from the underlying theory of quantum chromodynamics. This project aims at doing this in a certain framework with the help of coupled integral equations. The equations and their solutions have very convenient properties such that hadrons can easily be studied. While it is not possible to use the exact input from quantum chromodynamics, sophisticated models can be constructed that retain important properties of the underlying theory, but are at the same time simple enough to be treatable by present- day numerical methods and computing facilities. More precisely, emphasis of this project lies on a comprehensive study of hadron (both meson and baryon) masses, in particular regarding their various excited states, for the various kinds of quarks that can be the constituents of the hadrons. Starting with the heaviest quark known to form bound states, the bottom quark, one goal of the project is to find a new effective interaction capable of providing a unified description of meson and baryon properties in the same setup. Other investigations in this project concern the various possible interactions of hadrons, again both of mesons and baryons, with photons (the electromagnetic interaction), and weak bosons (the weak interaction) as well as with each other (via the strong interaction). Such processes happen in particle accelerators and detectors and need to be understood in order to produce reliable experimental data in particle physics.

In modern particle physics, hadrons are understood to experience the strong interaction, one of the four fundamental forces of nature. The strong interaction is an important reason for why the world is as it is, and in particular for our very existence. The most well-known hadrons, the proton and the neutron, constitute atomic nuclei. Since protons are positively charged and neutrons are neutral, the nucleus would not be stable without the presence of the strong interaction.This interaction can be understood theoretically via the concept of quarks and gluons, the elementary particles in quantum chromodynamics, which is believed to be the underlying quantum field theory of the strong interaction. Neither quarks nor gluons can ever reach a particle detector alone, but they appear in certain groups and form bound states, which appear as hadrons. Hadrons with mainly three quarks inside - held together by gluon exchange - are called baryons. Hadrons consisting of mainly a quark and an antiquark - again held together by gluon exchange - are called mesons. Many different mesons and baryons have been found experimentally, and it is the goal of theoretical hadron physics to explain their properties from the underlying theory of quantum chromodynamics.This project used coupled integral equations for such an investigation. Sophisticated models, in which all important features of the underlying theory remain while they are simple enough for numerical treatment, were constructed from quantum chromodynamics.The results for mesons made from heavy quarks expectedly provided a somewhat better agreement with experimental data than for light quarks. In general the description of many meson masses and other properties was successful. For example, in the bottomonium system, mesons not experimentally seen so far were calculated and their properties of decay via the weak interaction predicted. For other mesons that contain two of the heaviest kinds or flavors of quarks, one meson which has not yet been seen experimentally was predicted in line with previous theoretical studies and a corresponding world theory average for this prediction was compiled from 40 individual numbers.Another focus of the results is on so-called exotic mesons, whose properties are not immediately explained by the traditional and well-established particle model. In particular, such hadrons often require a more complex structure in theoretical approaches. In the approach used here, however, they are relatively simple and comparable to conventional hadrons. After a study of the two exotic states seen with experimental confidence, new quasi-exotic mesons were predicted for the first time, which are connected to exotic mesons in very precise ways, and their properties were discussed in detail.