Most of the mass in the visible universe—98% of it—comes not from the Higgs mechanism but from the invisible glue binding quarks and gluons inside protons and neutrons. This project aims to finally explain how that glue works. Physicists know the equations that govern the strong nuclear force, but those equations are too complex to solve for ordinary matter. This leaves a fundamental gap in the Standard Model: we cannot explain why a proton has the mass it does, or why quarks stay permanently locked inside particles. The researchers will run experiments at two of the world’s most powerful particle accelerators—Jefferson Lab in Virginia and MAMI in Germany—to probe the internal structure of hadrons and search for exotic states like glueballs and multiquark particles. This is fundamental science with no immediate practical application. But understanding how mass and matter emerge from empty space is one of the deepest questions in physics. Past work on similar problems—such as the discovery of the Higgs boson—led to technologies like medical imaging and particle accelerators used in cancer therapy. A clearer picture of QCD could, over decades, open new ways to manipulate matter at its most basic level.
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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. We have an ambitious plan of work that encompasses the development of research programmes from existing themes, possibilities for new physics measurements that have opened up recently, and the completion of physics projects from data that has been previously obtained. Our programme will build on the work of our previous consolidated grant and we will exploit the investment in manpower and equipment to address the current issues in our field at the highest possible level, at the world's top facilities. The challenges for the science programme are encapsulated by several key questions, which we group together as they roughly match the themes we have defined: * 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? * What is the equation of state of nuclear matter? * What is the nature of dark matter? We will address these questions by leading experimental programmes at two of the world's leading facilities: * Jefferson Lab, Newport News, Virginia, USA (JLab) * MAMI, Mainz, Germany:
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