Jefferson Laboratory Upgrade Project

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Approximately 98% of the mass of nucleons, and therefore of the visible universe, emerges from the interactions among their constituents, the quarks and gluons. The Higgs mechanism, which gives mass to the bare quarks, is responsible for only a small fraction of the nucleon mass. The confinement of quarks within mesons and baryons is a direct consequence of their fundamental interactions. The field theory of the strong nuclear force, Quantum Chromodynamics (QCD), is now well established, and yet the phenomena described above cannot be understood from the QCD Lagrangian; they are emergent properties that arise from the unique complexity of these interactions. QCD is as yet intractable at the mass scale of nucleons and nuclei, so a clear picture of the fundamental nature of matter at these energies does not yet exist. However, this situation is set to change, and both theoretical and experimental developments in the coming decade are expected to revolutionise our understanding of nuclear matter within the context of QCD and the Standard Model. There are several key questions that will be addressed by the work proposed here: * What is the mechanism for confining quarks and gluons in strongly interacting particles (hadrons)? * What is the structure of the proton and neutron and how do hadrons get their mass and spin? * Can we understand the excitation spectra of hadrons from the quark-quark interaction? * Do exotic hadrons (multiquark states, hybrid mesons and glueballs) exist? * How do nuclear forces arise from QCD?* Can nuclei be described in terms of our understanding of the underlying fundamental interactions?The scientific vision behind the upgrade to the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, is to address fundamental issues such as how constituent quarks acquire mass, and why they are confined. It is therefore ideally suited to tackling the pivotal questions laid out above. New insights into the fundamental structure of the nucleon will be obtained from measurements of nucleon form factors, Generalised Parton Distributions (GPDs) and Transverse Momentum-dependent parton Distributions (TMDs). The fundamental nature of QCD confinement will be studied through the investigation of the light hadron spectrum, and the search for the existence of exotic states, in which the confining gluonic field provides an additional degree of freedom to the quarks. These states are a direct prediction of the QCD theory but remain unobserved experimentally. The Jefferson Lab 12 GeV Upgrade offers an exciting opportunity for the UK Nuclear Physics community to lead and instigate world-leading hadron physics research. The central scientific motivation for undertaking this project is a continuing desire to understand QCD physics: to obtain new information on the structure of nucleons, and to investigate the mechanism of quark confinement. This proposal represents a bid by the Edinburgh and Glasgow nuclear physics groups to build on established leadership roles, and enhance their impact on JLab's future science programme for the coming decades.