Search For New Meson and Baryon Resonances

现在大家都知道，质子和中子，统称核子，是由更基本的粒子——夸克构成的。这项研究的目的是研究核子内部夸克之间的力。该项目着眼于夸克在被高能粒子束轰击后如何形成核子的量子态，并研究这些量子态如何以新粒子的形式释放能量。通过将这些测量结果与理论计算进行比较，我们可以更多地了解被限制在核子内部的夸克之间的力。这项工作包括两个实验。第一个是在日本质子加速器研究中心（J-PARC）使用一束被称为π介子的粒子撞击质子，产生衰变为两个π介子的核子共振。这个新实验将把那个反应过程的数据扩充到原来的一百倍。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精确数据作为核子共振理论计算的输入。第二个实验在位于弗吉尼亚州纽波特纽斯的托马斯·杰斐逊国家加速器设施进行。这个实验使用电子束撞击质子来产生核子共振，最后的粒子进入一个叫做CLAS12的大型接受谱仪。核子共振谱告诉我们夸克是如何在质子内部相互作用的。早期职业科学家、研究生和本科生在这个项目中扮演着重要的角色，这为他们提供了在领先的研究机构进一步发展他们的培训和教育的独特机会。这个项目的科学目标是了解质子以及它的价夸克如何被激发成共振态。这一刻是追求这一目标的一个激动人心的时刻，因为我们现在有了晶格规范理论的理论工具，可以从基本原理（量子色动力学方程的非摄动解，或QCD）计算核子共振。作为一个类比，对氢原子激发态的测量首先导致了简化的玻尔模型，然后是使用薛定谔方程的计算，最后是QED。同样，对核子激发态（N* s）的测量首先导致了简化的组成夸克模型，然后使用晶格规范理论直接从QCD进行计算。后者预测的N*共振比从实验数据中提取的要多得多。这个问题被称为失踪重子问题，它继续激励着对N*光谱的研究。上面描述的两个实验，一个在J-PARC，另一个在CLAS12，将有助于解决N*谱，并最终为夸克的理论模型提供一个试验场。

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.