Final Report Summary - LQCDMESONSPEC (Excited Charmonium Spectroscopy from Lattice QCD)
Executive Summary:
The standard model of particle physics explains how the fundamental particles of nature interact. The strong interaction, one of the forces in the standard model, is responsible for binding quarks together to form hadrons: mesons, containing a quark and an anti-quark, and baryons, like protons and neutrons, containing three quarks. In turn, it binds protons and neutrons together into nuclei. In fact most of the mass of everyday matter arises, through mass-energy equivalence, from the energy of particles interacting via the strong interaction. It is also responsible for the properties of matter in more extreme conditions such as in neutron stars. Studying the strong interaction is therefore essential in order to understand the make-up of matter from small to large scales. This project carried out a detailed theoretical investigation of mesons made from a charm quark and anti-quark, known as charmonium.
Over the past decade, a host of new experimental discoveries in high-energy physics colliders have prompted vigourous new investigations of the spectrum of charmonium. There were many more of these states discovered than had been expected. The majority of the newly seen states are sufficiently heavy to decay via the strong force but the pattern of their decays did not fit with expectations from the simple models of charmonium that had proved reliable in the past. A full understanding requires the complete treatment of these system starting from quantum chromodynamics (QCD), the theory that describes the strong force. Making predictions from QCD is notoriously difficult, since the strength of the interactions inside bound states preclude using the standard techniques of quantum field theory which treat interactions between fundamental particles as a relatively minor correction. A formulation of the theory on a four-dimensional space-time lattice enables large-scale numerical simulation of QCD to be carried out on powerful computers. This project successfully extended recently devised new techniques to explore the spectroscopy of mesons to the charmonium system and related new results to experimental data.
Of particular importance in this project was exploring the possibility that some of the new meson excitations that had been seen were so-called hybrid mesons. In QCD, the strong force is carried by the gluon. For simple models of mesons, the gluon plays the role of just binding the quark to the anti-quark, and so the observable external properties of the bound-state are carried solely by the quarks. In a hybrid meson, the gluon acts now as a constituent, with a similar status as that of the quark and the anti-quark. As a result, a simple picture of the hybrid is a bound-state of a quark, a gluon and an anti-quark. This project included the theoretical probes needed to simulate these states in large-scale numerical studies of QCD on the lattice. A compelling picture emerges that clearly places the masses of these hybrid mesons within the energy range of the recently seen new charmonium states. This calculation establishes that the hybrid mesons studied in this calculation could appear in the excitation spectrum of charmonium at similar collision energies to the recent experimental discoveries and the work sets the stage for future investigations as a priority.
To begin, the research fellow made use of work previously carried out at TCD to accurately represent the charm quark field on the lattice. The charm quark is heavy relative to a typical QCD energy scale but not so heavy that its relativisitic motion can be neglected. This poses a problem for any lattice study with charm quarks. They are sufficiently heavy that their mass relative to the lattice spacing is large enough for artefacts to be significant, meaning a more careful theoretical parameterisation of their dynamics is needed to reduce the impact of these effects. Once this initial parameterisation was completed, the fellow's expertise in meson spectroscopy enabled him to develop and extend the Hadron Spectrum Collaboration's new techniques for precision computations to address how the charmonium system can be excited into higher-lying quantum states. The techniques used enabled the possibility of excitations including hybrid mesons to be explored in detail and a rich spectrum of possible candidate states was revealed.
As a next step, the excitation spectrum of mesons made up of a single charm quark bound to a lighter up, down or strange quark was computed, and a second major paper published. The fellow developed techniques to study how hadrons scatter off other hadrons in collaboration with researchers at the Thomas Jefferson National Accelerator Laboratory (JLab) in the US. This work strengthened existing links between the TCD group and researchers at JLab. These ideas were first tested for light quark mesons and the fellow extended this work to carry out the first calculations from QCD of how mesons with charm quarks inside scatter off other hadrons, with promising preliminary results. These calculations are a crucial next step for the collaboration and will uncover information on the decay properties of mesons with charm quarks. This next stage will provide useful insight into the puzzling experimental data that emerged over the last decade.
The project has made a substantial impact on the field. Our paper published on the charmonium spectrum during the project has received considerable attention and the fellow and TCD group members have received numerous invitations to describe this work in international conferences. As the excitations of the charm quark inside bound-states are understood, it is hoped more will be learned about the confining mechanism that binds quarks inextricably inside hadrons. This will reveal new insight into a fundamental and unsolved question about how matter behaves at the subnuclear scale.
In bringing new expertise to TCD, the fellow contributed greatly to a long-lasting exchange of knowledge. Other TCD group members benefited substantially and a Ph.D. student completed his training with significant support from the research fellow. This student has now taken up a postdoctoral position in the EU. During the fellowship, there was opportunity for significant transfer of knowledge as the fellow engaged with local researchers. The fellow contributed significantly to a journal club organised by the graduate students, he presented an informal lecture course on "Hadron phenomenology and lattice QCD" which was attended by experimental physicists from another Dublin university and he organised invitations to the local seminar series for researchers across Europe. The fellow also elected to deliver a module of formal lectures to third-year undergraduate theoretical physics students, which broadened his teaching experience significantly.The fellow has now taken up a permanent academic position in the UK and the collaboration started by this project will continue.