Search For New Meson and Baryon Resonances

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It is now well-known that protons and neutrons, collectively called nucleons, are made from more fundamental particles called quarks. The goal of this research is to study the forces between quarks inside the nucleon. The project looks at how the quarks form quantum states of the nucleon after being bombarded by an energetic beam of particles, and studying how these quantum states release the energy in the form of new particles. By comparing these measurements to theoretical calculations, we can learn more about the forces between quarks that are confined inside the nucleon. Two experiments are included in the effort. The first one at the Japan Proton Accelerator Research Complex (J-PARC) uses a beam of particles called pi-mesons that impinge on a proton, resulting in a nucleon resonance that decays into two pi-mesons. This new experiment will expand by a factor of 100 the data for that reaction process. The last data of this type was taken back in the 1970s, and now precise data for this final state are needed as input to theoretical calculations of nucleon resonances. The second experiment is at the Thomas Jefferson National Accelerator Facility, located in Newport News, Virginia. This experiment uses an electron beam impinging on a proton to create a nucleon resonance, with the final particles going into a large acceptance spectrometer, called CLAS12. The nucleon resonance spectrum tells us about how quarks interact with each other inside the proton. Early career scientists, graduate, and undergraduate students play an important role in this project, which provides them with unique opportunities to further develop their training and education in leading research facilities.The scientific goal of this project is to understand the proton and how its valence quarks can be excited into resonant states. This moment is an exciting time to pursue this goal because we now have the theoretical tool of lattice gauge theory to calculate nucleon resonances from basic principles (the non-perturbative solutions to the equations of Quantum Chromodynamics, or QCD). As an analogy, measurements of excited states of atomic hydrogen led first to the simplified Bohr model, then to calculations using Schrodinger's equation, and finally to QED. Similarly, measurements of the excited states of the nucleon (N*'s) led first to the simplified constituent quark model, and then to calculations directly from QCD using lattice gauge theory. The latter predicts many more N* resonances than have been extracted from experimental data. Known as the missing baryons problem, it continues to motivate research on N* spectroscopy. The two experiments described above, one at J-PARC and the other at CLAS12, will help to resolve the N* spectrum and ultimately provide a proving ground for theoretical models of quarks.