Study of spin structure of hadrons using intermediate energy photons

自从发现基本粒子自旋以来，很明显，这些粒子之间相互作用的强度取决于这些自旋的相对方向。然而，到目前为止，使用更高能量的电磁探测器(光子)，只能研究所有可能的自旋取向的平均性质。对这一提议的支持将允许美国天主教大学的实验核物理小组继续开发和开发新的系统，从而允许光子和靶核的独立定向。这项研究将解决强子结构中尚未解决的主要问题之一，即质子和中子的激发态的性质和数量。组成原子核的各个夸克的自旋决定了激发态的性质。有许多预测的激发态，但到目前为止只有几个被明确地识别出来。问题仍然是，这是由于理论的其他方面尚未被理解，还是由于在检测这些激发态方面的实验问题。实验的大部分困难在于这些激发态共振的广泛性和重叠性。只有通过一次只有选择性地激发其中几个共振的测量，才能清楚地研究各个状态的性质。在拟议的研究中，我们将使用新委托的偏振光子光束线和新的偏振目标，以及高分辨率、高能光子标记器来确定光子能量，以启动对这些共振的一些详细研究。这样的测量将清楚地区分相互竞争的理论模型，并导致更好地理解夸克之间基本力的性质。在基本夸克相互作用的研究之外，这项研究将在两个领域产生更广泛的影响。这项拟议的研究将为博士后助理、寻求高级学位的研究生以及本科生物理和工程专业学生提供培训。这些培训活动将涵盖广泛的实验活动，包括低温、高速电子学、粒子探测器、真空系统和数据处理方面的经验。这一培训将加强该国现有的受过科学和技术培训的人才库。在这一拟议计划内进行的大部分研究将涉及仪器的发展和使用能量标记偏振光子束的经验。这些活动将带来对未来实验至关重要的改进，包括目前正在规划的更高能量的实验。

Ever since the discovery that elementary particles have spin, it hasbeen clear that the strength of the interaction between these particlesdepends on the relative orientation of these spins. Until now, however,with higher energy electromagnetic probes (photons) it has only beenpossible to study properties averaged over all possible spinorientations. Support for this proposal will allow the experimentalnuclear physics group at The Catholic University of America to continueto develop and then exploit new systems which will permit theindependent orientation of both the photon and the target nucleus. Theresearch will address one of the major unsolved problems in hadronicstructure, the nature and number of excited states of the proton andneutron. The spins of the individual quarks that make up the nucleondetermine the properties of the excited states. There are manypredicted excited states but only a few have thus far been unambiguouslyidentified. The question remains as to whether this is due to someadditional aspect of the theory not yet understood or due toexperimental problems in detecting these excited states. Much of theexperimental difficulty lies in the broad and overlapping nature ofthese excited state resonances. It is only by performing measurementsthat selectively excite only a few of these resonances at a time thatthe properties of the individual states can be cleanly studied. In theproposed studies we will use a newly commissioned polarized photon beamline and a new polarized target, along with a high-resolution,high-energy photon tagger to determine the photon energy, to initiate anumber of detailed studies of these resonances. Such measurements willcleanly discriminate between competing theoretical models and lead to amuch better understanding of the nature of the fundamental forcesbetween the quarks. There are two areas where this research will have broader impact outsidethe study of fundamental quark interactions. The proposed research willprovide training for postdoctoral associates, graduate students pursingan advanced degree, and undergraduate physics and engineering students. These training activities will cover a large range of experimentalactivities including experience with cryogenics, high-speed electronics,particle detectors, vacuum systems, and data processing. This trainingwill enhance the pool of scientifically and technically trained talentavailable in this country. Most of the research that will be conductedwithin this proposed program will involve the development ofinstrumentation and experience with the use of energy-tagged polarizedphoton beams. These activities will lead to improvements which will beessential to the future experiments, including those at higher energy,now being planned.