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Excited and exotic hadron resonances from Quantum Chromodynamics

Periodic Reporting for period 1 - XXQCD (Excited and exotic hadron resonances from Quantum Chromodynamics)

Reporting period: 2017-06-30 to 2019-06-29

"The project addresses the problem of understanding the fundamental structure of matter at the sub-nuclear scale. The atomic nucleus is made up of protons and neutrons, which in turn can be described as composites of constituent particles called quarks. Quarks are however never observed experimentally in isolation and this phenomenon, called ""quark confinement"" must arise from any successful theoretical description of the strong nuclear force. The behaviour of quarks is described by a quantum field theory called quantum chromodynamics (QCD). In QCD, quarks interact with force-carrying particles called gluons and these interactions should lead to confinement but a robust mathematical understanding of this emergent behaviour remains elusive. This project investigates new experimental data on the interactions between heavier charm quarks that might provide new insight into details of the dynamics of the quark and gluon fields. The investigation uses a framework for computing properties of QCD from large-scale numerical simulations called lattice QCD. This method allows the theory to be investigated from first principles on high-performance computing systems. These objectives are important for society as they attempt to answer fundamental scientific questions regarding the nature of matter at the smallest scale we can probe in current experiments. This deeper understanding of how matter behaves will drive fresh scientific discovery and basic research. As the project makes use of large-scale parallel computing systems, the work will also drive development of new methods for large-scale numerical computation and techniques for analysing large data-sets that may find uses in other scientific and technical research directions, giving new societal benefits.

The new ideas developed by this project will improve calculations in lattice QCD that investigate how particles interact as they collide in experiments and form short-lived excitations. The overall objective is to investigate recently discovered short-lived excitations seen in experiments creating a pairing of a charm quark and anti-quark known as charmonium. These new excitations, labelled the X, Y and Zs do not fit into the simplest models which described how a charm quark and anti-quark should interact. This inability to accommodate these new discoveries in a simplified approach suggests the quark and gluon fields inside these states exhibit more complex, exotic behaviour. This behaviour should however be predicted by a full treatment of the strong interactions using QCD and the project objective is to develop the necessary techniques to perform these calculations using lattice QCD.

The action terminated early as the research fellow takes up a new role. During the project, significant progress towards the goals and objectives were made and the research and collaboration developed during the span of the project will continue beyond the lifetime of the action."
The fellow started work on extending calculations he had previously been involved with to study how particles made of the light up- and down-quarks interact. This work was completed early in the project and has been published in Physical Review D (see publications section). The work is attracting significant interest in the research community. This computation studied coupled-channel interactions that enable transitions between quantum states of pions made of light up- and down-quarks and kaons which include the strange quark. These transitions occur through the formation of short-lived resonant structures, seen as peaks in scattering probabilities in experiments. These structures are labelled the sigma, f0 and f2 resonances and have been the subject of both experimental and theoretical scrutiny for decades.

The project then moved on to investigating how mesons made of both a heavy charm quark and light anti-quark interact. The relevant lattice QCD calculation probed how the D and D_s meson interact with the pions and kaons studied in the earlier phase. This work is on-going and experience gained over the duration of this project is proving invaluable to future progress. New ideas for investigating the spectrum of states formed by bottom quarks are being developed, again with the work carried out in this project being exploited to guide future research directions.
Within the limited time-span of the project, significant progress beyond the state-of-the-art has been made. The techniques to compute predictions directly from QCD, the theory of the strong nuclear force, on how charm quarks can form more exotic configurations have been extended and tested to higher precision. To investigate how particles interact via the strong interaction, the scattering parameters of mesons is being predicted from first-principles calculations using lattice QCD. Achieving this goal requires using a theoretical framework called the Luescher formalism. This formalism relates scattering properties of particles to the energies of quantum states constrained to live inside a small, finite three-dimensional volume. The technical challenges faced by these calculations meant the framework could not be used until recently. The project has made progress developing the pathways linking lattice QCD Monte Carlo calculations to the experimental observations of how particles interact and form short-lived quantum excitations called resonances. The publication of the research fellow has pushed beyond the state-of-the-art, including the first coupled-channel isoscalar scattering calculation and has already had deep impact on the discipline.

The knowledge exchange between the incoming fellow and the host institution has been substantial given the shortened duration of the project. The fellow has made significant contribution in training postgraduate early-stage researchers as they develop their projects and this transfer of knowledge directly illustrates one wider societal benefit of basic research of this type.
Scattering amplitudes for light mesons from a first-principles lattice QCD calculation