Glueballs at BESIII · Search for the scalar glueball in a coupled channel amplitude analysis of J/psi decays with the BESIII experiment

Search for the scalar glueball in a coupled channel amplitude analysis of J/psi decays with the BESIII experiment

The Standard Model of Particle Physics describes the interactions of subatomic particles through three fundamental forces, the electromagnetic, the strong and the weak force. Within the Standard Model, predictions for electromagnetic and weak interactions can be made to high precision, whereas the strong interaction is more difficult to understand. The fundamental degrees of freedom of the strong interaction are colour charged quarks (q) and gluons (g). However, in nature, neither quarks nor gluons can directly be observed in experiments, a phenomenon known as confinement. What we observe in experiments are colour-neutral hadrons, with mesons made up from a quark anti-quark pair and baryons made up from three quarks. To date, a large number of these mesons and baryons have been identified. Using group theory, these particles can successfully be classified within the quark-model first derived by Gell-Mann and Zweig in 1964. Already in their fundamental works, Gell-Mann and Zweig mentioned the possibility of multi-quark hadrons, including tetraquark and pentaquark states. Ten years after their work, Jaffe and Johnson explored the possibility of hadrons containing constituent gluons, studying in detail the glueball, a massive particle consisting of massless gluons. These multi-quark states and gluonic hadrons are what we collectively call exotic hadrons, indicating their nature beyond our expectations from the quark model. While the existence of the glueball has been predicted by the discretized quantum field theory of the strong interaction, Lattice Quantum Chromo-Dynamics (LQCD), there is no conclusive experimental evidence for its existence yet, although multiple candidates are discussed in the literature. Experimental evidence for the glueball is thus a key open issue in the field of hadron spectroscopy and the main objective of this action. An experimental observation of this new form of matter is not only an important confirmation of the concepts of QCD, it will also provide valuable insight to the question of how gluonic components contribute to the mass of hadrons and thus to the overarching quest to arrive at a quantitative description of matter on the basis of QCD. With the tools developed in this action, exotic hadron candidates in the bottomonium sector were first studied in detail. Furthermore, we could confirm the f0(1710) as a likely candidate to overlap with the ground-state glueball, both by observing its strong presence in radiative J/psi decays and by non-observation of a potential isovector partner a0(1710) in hadronic chi_cJ decays.

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