The confined gluon: precision spectroscopy with charm quarks

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The charmonium system consists of a charm and anti-charm quark pair. Interest in charmonium underwent a revolution after a number of entirely unexpected narrow resonances, called the X, Y and Zs were discovered by the Belle and BaBar experiments at the start of the millenium. In spite of almost two decades of theory investigations, no clear picture of the internal mechanics of these resonances has emerged. This project aims to develop a robust framework using lattice QCD to calculate properties of charmonium including hadronic decays and to exploit this framework to progress our understanding of recent experimental discoveries.One of the major goal of the project is to get new insight into the charmonium spectrum, including the XYZ states by developing and optimising new techniques for high-precision hadron spectroscopy Monte Carlo calculations. These developments will focus on optimising the creation operators and the Monte Carlo evaluation of their correlation functions to determine the spectrum of low-lying states in finite volume to high precision. After first extending techniques for single-hadron correlation functions and developing links with other projects in the Research Unit, we will investigate the multi-hadron correlation functions needed to compute the widths of some of the charmonium states below the open-charm threshold including their OZI suppressed decays into glueballs and light hadrons. As efficient statistical sampling methods are combined with creation operators optimised for physics goals, hybrid charmonium states will be explored. They lie above the open-charm threshold and their decays into charmed mesons are OZI allowed. We will perform a first scattering analysis in an elastic setting realized for sufficiently large light quark masses.The other major goal of the project will be a study of light-quark and gluon degrees of freedom in orbital excitations to the potential between static quarks. New techniques for efficient computations will be developed, in close collaboration with the other projects of the Research Unit. The saturation of the ground-state potential in the presence of constituent gluons due to the formation of static-light-meson pairs will be examined in detail. This phenomenon is known as string-breaking. This flattening provides a connection to the strong decay physics of charmonium hybrids in the heavy-quark limit. In this limit, the Born-Oppenheimer approximation treats hybrid charmonium by solving the non-relativistic Schrödinger equation in the adiabatic potential generated by the relativistic light quarks and intrinsic gluons, which will be computed using lattice QCD. The technology developed in this project for hybrid potentials can be applied to tetraquark potentials which provide insight into the internal structure of exotic configurations of static sources with non-trivial spin and isospin. Tetraquark potentials are planned for a second funding period of the Research Unit.