A Search for New Meson and Baryon Resonances

本文提出的研究主要针对两个项目。第一种是寻找一种被称为标量介子的粒子，这种粒子的量子数与理论上称为“胶球”的粒子具有相同的量子数。后者被认为是一个只有胶子的束缚系统，胶子是QCD(量子色动力学)理论中的载力粒子。在实践中，真实粒子是标量介子和假想胶球的量子力学混合物，因此研究标量介子的性质与QCD的预言有直接的联系。在一些强子束实验中已经观察到了标量介子，但在像杰斐逊实验室提出的那样的光子束实验中，几乎没有证据表明存在这些介子。这项研究项目将提供杰斐逊实验室CLAS12探测器产生的标量介子率的最终结果。第二个项目是将在日本的J-PARC设施进行的强子束实验。这个实验将探索在探测到两个pi介子的情况下终态的产生率。这种类型的最后一次数据是在20世纪70年代获得的，现在需要这种最终状态的准确数据作为核子共振的理论计算的输入。核子共振是质子的一种激发状态，它可以衰变成一个质子和一个或多个π介子(或其他粒子)。核子共振谱很重要，因为它告诉我们夸克如何在质子内部相互作用。核子共振谱现在可以用一种被称为晶格QCD的理论技术来计算，它是基于QCD理论的。理解QCD(量子色动力学)理论是当今核物理最重要的方面之一。QCD负责质子内部夸克的结合，也负责原子核内质子和中子的结合。因此，QCD本质上是一种导致更好地理解核能和其他核应用的理论。虽然今天核过程的应用，如核医学，可以从现象学的角度来理解，但未来的应用很可能取决于对QCD的理解。基于QCD理论的计算很困难，需要大型计算机，而且这种复杂的计算需要进行实验测试。这里提出的研究就是这样一种实验测试，可以导致对核力的更好理解。这项研究产生的其他更广泛的影响是对研究生进行构建尖端仪器和计算分析的培训。

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.