Glueball Search with Holography and Phenomenology

The theory of the strong interaction Quantum Chromodynamics (QCD) emerged from the need to explain the vast amount of particles that were discovered throughout the twentieth century. It describes the dynamics of colour charges that are carried by quarks (which form, e.g., protons and neutrons) and gluons (that act as carriers of the strong interaction between quarks). QCD is asymptotically free: its coupling decreases with increasing energy. Therefore, at large energies, perturbative first-principles calculations regarding strongly interacting matter are possible. At low energies, however, the strength of QCD coupling is such that any perturbative treatment of strongly interacting matter fails; indeed, the relevant degrees of freedom in this, non-perturbative region of QCD are no longer quarks and gluons but rather hadrons into which quarks and gluons are confined. Gluons are analogous to photons present in electrodynamic processes (e.g., those involving electrons). However, there is an important difference: photons do not interact among themselves whereas gluons do possess this so- called self-interaction. That leads to the formation of glueballs, particles containing only gluons. Glueballs are very important for our understanding of physics as they are a rare kind of particles that do not obtain their mass from the Higgs boson. The scalar glueball (the one possessing the same quantum numbers as vacuum) is of particular interest because it is difficult to find in experiments. In this project we are developing two theoretical frameworks for a glueball search in the physical spectrum. One approach is built upon the renowned equivalence between the supergravity limit of a string theory on a curved space and a certain limit of a supersymmetric and conformal gauge theory on the boundary of the mentioned space (AdS/CFT correspondence). The other framework is the so-called Linear Sigma Model and it is based on symmetries of QCD. Using these theoretical approaches we intend to study not only features of the scalar glueball but also those of quark-antiquark states with the same quantum numbers in order to gain insight into fundamental principles of QCD.

The theory of the strong interaction Quantum Chromodynamics (QCD) emerged from the need to explain the vast amount of particles that were discovered throughout the twentieth century. It describes the dynamics of colour charges that are carried by quarks (which form, e.g., protons and neutrons) and gluons (that act as carriers of the strong interaction between quarks). Gluons are analogous to photons present in electrodynamic processes (e.g., those involving electrons). However, there is an important difference: photons do not interact among themselves whereas gluons do possess this so-called self-interaction. That leads to the formation of glueballs, particles containing only gluons and thus representing particles that are made purely of nuclear force. Although numerical studies have approximately pinned down the mass spectrum of glueballs, their experimental verification is still outstanding, because so far characteristic properties such as decay patterns could not be predicted unambiguously. In this project we have been developing two theoretical frameworks for a glueball search in the physical spectrum. One approach is built upon the famous equivalence between the supergravity limit of a string theory on a curved space and a certain limit of a supersymmetric and conformal gauge theory on the boundary of the mentioned space (AdS/CFT correspondence). The other framework is the so-called Linear Sigma Model and it is based on symmetries of QCD. In this project we could identify a hadronic particle with designation f0(1710) as the best candidate for the lightest glueball, with both methods coming to the same conclusion. New predictions for hitherto unexplored decay channels have been made, which could experimentally corroborate this identification in the near future. Moreover, the decay patterns of the next three heavier glueballs have been worked out. For those no experimental candidates exist so far, but they are being searched for experimentally in particle collider experiments.