From Scalar Resonances to the Lightest Glueball - Hadronic ψ(2S) Decays at BESIII

• Applicant: Professor Dr. Alfons Khoukaz
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 547123630

The strong interaction mediates one of the four fundamental forces in the universe and is responsible both for the majority of the mass of our visible matter in the macrocosm and for the binding of the smallest particles in the microcosm, which is why it plays a crucial role in modern particle physics in particular. It is described theoretically by quantum chromodynamics (QCD), which characterizes the interaction between quarks and gluons. One of the most fundamental predictions of QCD are the glueballs, exotic particles that consist only of bound gluons. While the properties of the lightest glueball have already been calculated with high precision by theory, it could not be confirmed beyond doubt experimentally even more than 50 years after its postulation. In order to solve this highly topical problem, the particles f0(1370), f0(1500) and f0(1710) will be investigated in this research project as the most promising candidates for the lightest glueball. Their internal structure is still subject of highly controversial debate. Not only is one of these states supernumerary in the nonet of scalar mesons, but they also feature the same quantum numbers and similar masses as predicted for the lightest glueball. It is therefore assumed that either one of these states is the sought-after glueball itself or that the f0 states contain admixtures of the glueball in addition to possible quark-antiquark content. However, a clear determination of their internal structure is not yet possible according to the current state of research due to their partly imprecise or even completely unknown physical properties. This project will therefore analyze the eight most relevant and for the majority still undetermined hadronic decay channels of the f0 states. Using the world's largest psi(2S) data set of the BESIII collaboration with 2.7 billion events, decays of the psi(2S) charmonium will be analyzed in which the f0 states have been produced in combination with a phi or omega vector meson. Here, the vector mesons serve as an elegant filter for the quark composition of the f0 states. A highlight of this project will be the simultaneous description of all considered f0 decay channels by a fully analytical and unitary partial wave model. This highly sophisticated approach makes it possible to determine the physical properties of the f0 states with unprecedented accuracy and completely independently of previous analyses. These properties can subsequently be used to unravel the internal structure of the f0 states using theoretical models and calculations. Consequently, the results of this project will provide an essential contribution to answering the fascinating question of the true nature of the f0 states and therefore contribute significantly to the identification of the lightest glueball.

• DFG Programme: Research Grants