A Search for New Meson and Baryon Resonances

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The research proposed here is primarily for two projects. The first is a search for a type of particle known as a scalar meson, which has the same quantum numbers as a theoretical particle called the "glueball". The latter is conjectured to be a bound system of only gluons, which are the force-carrying particles in the theory of QCD (quantum chromodynamics). In practice, real particles are a quantum-mechanical mixture of scalar mesons and the hypothetical glueball, so studying the properties of the scalar mesons gives a direct connection with the predictions of QCD. Scalar mesons have been observed in some hadron-beam experiments, but there is scant evidence of these mesons in photon-beam experiments like the ones proposed at Jefferson Lab. This research project will provide definitive results on scalar meson rates produced at the CLAS12 detector at Jefferson Lab. The second project is a hadronic-beam experiment to be carried out at the J-PARC facility in Japan. This experiment will explore the production rate of final states where two pi-mesons are detected. The last data of this sort was taken back in the 1970s, and now precise data for this final state are needed as input to theoretical calculations of nucleon resonances. A nucleon resonance is an excited state of the proton, which can decay into a proton and one or more pi-mesons (or other particles). The nucleon resonance spectrum is important because it tells us about how quarks interact with each other inside the proton. The nucleon resonance spectrum can now be calculated using a theoretical technique called lattice QCD, which is based on the theory of QCD.Understanding the theory of QCD (quantum chromodynamics) is one of the most important aspects of nuclear physics today. QCD is responsible for the binding of quarks inside the proton, and also for the binding of protons and neutrons inside the nucleus. Hence, QCD is essentially the theory that leads to a better understanding of nuclear energy and other nuclear applications. While today's applications of nuclear processes, such as nuclear medicine, can be understood in terms of phenomenology, the applications of the future may well depend on an understanding of QCD. Calculations based on the theory of QCD are difficult, requiring large computers, and such complex calculations need to be tested experimentally. The research proposed here is one such experimental test that can lead to a better understanding of the nuclear force. Other broader impacts resulting from this research are the training of graduate students in building leading-edge instrumentation and in computational analysis.