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

Project Information

Glueballs at BESIII

Grant agreement ID: 894790

DOI

10.3030/894790

Project closed

EC signature date 16 March 2020

Start date 1 November 2020

End date 31 October 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 246 669,12

EU contribution

€ 246 669,12

Coordinated by

JOHANNES GUTENBERG-UNIVERSITAT MAINZ
Germany

Periodic Reporting for period 2 - Glueballs at BESIII (Search for the scalar glueball in a coupled channel amplitude analysis of J/psi decays with the BESIII experiment)

Reporting period: 2022-11-01 to 2023-10-31

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

It has long been hypothesized that radiative decays of the charmonium-state J/psi, a gluon-rich process, are an ideal hunting ground for the glueball. The Beijing Spectrometer (BESIII) experiment gathered data for a total of ten billion J/psi decays in electron-positron annihilation – an unprecedented dataset to search for glueballs. In the first reporting period, we have analysed the huge dataset and identified several radiative decays J/psi, separating them from a large number of background processes. In close collaboration with theoretical physicists in the field of hadron spectroscopy, we have extended a computational framework that allows us to fit the complex amplitudes that theoretically describe the various decay mechanisms directly to the data in a coupled channel amplitude analysis. Due to the immense size of the dataset, the code is highly parallelized and developed such that calculations can also be performed on fast, modern graphics processing units (GPUs).
As part of the work on this project, a first test of our code with a direct application to the search for exotic hadrons containing bottom anti-bottom quark pairs has been published in Physical Review D, demonstrating both the viability of the approach, its potential in searches for exotic hadrons, and the general importance of a sound theoretical framework including coupled channel effects. Many properties of highly excited bottomonium states, among them a potential hybrid meson candidate, were determined for the first time, significantly extending our knowledge of the bottomonium-system. In the second funding period, we have performed coupled channel analyses of the radiative J/psi decays and of hadronic chi_cJ decays, with the aim to identify a glueball candidate and to search for a potential isovector partner-state to the f0(1710), the a0(1710) recently claimed in multiple works. While we find strong f0(1710) production in the gluon-rich J/psi decays, the a0(1710) is not confirmed in the highly precise study of chi_cJ decays, rendering the f0(1710) a likely candidate to largely overlap with the ground-state glueball. Both results will be published in timely fashion.
In total, results have been disseminated in 16 published articles, and through presentations at ten workshops or scientific conferences.
Furthermore, in collaboration with a German high school we have prepared a project in which students work directly with experimental data from the BESIII experiment, learning concepts of particle physics and data analysis, and directly applying them to our data. This project is continued and expanded with high school interns at the JGU Mainz.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

In the past, searches for exotic hadrons have followed similar approaches, with experimental works focusing on a single decay process, relying on simplified models. Theoretical approaches on the other hand typically involve more realistic and thus more complicated models that, however, are only applied to the result of an experimental analysis typically in the form of binned data. With this action, we want to pioneer a different approach, working closely with theory to directly apply elaborate models to the unbinned, experimental data from the BESIII experiment to combine the best of both worlds and enhance our discovery potential for the glueball, a key prediction of the strong interaction.
Such a direct collaboration between experimental and theoretical physicists in the scope of this action led to the first coupled channel analysis of the vector bottomonium system, including a new way to include challenging three-body decays in the established K-Matrix formalism. The results of our study significantly extend our knowledge of the bottomonium states above the open-bottom threshold, with many properties being measured for the first time. These will now be a key input for model builders to differentiate between a conventional meson assignment and more exotic possibilities like tetraquarks, molecules or hybrid mesons.
Applying these methods to large datasets of radiative J/psi decays and hadronic chi_cJ decays, we provide highly precise measurements of important properties like pole positions and partial decay rates of scalar and tensor mesons – including those that are regularly discussed as potential glueball candidates. These highly precise measurements will have a significant impact on our understanding of glueball candidates and thus the theory of the strong interaction, with the positive identification of a glueball being one of the ultimate goals of hadron spectroscopy. Furthermore, demonstrating a successful collaboration between experimental and theoretical particle physicists in the analysis of experimental data, maximizing its scientific impact on the field, will play an important role in how similar topics are addressed within the field in the future.

A coupled channel fit of electron-positron annihilation to bottom anti-bottom quark pairs.