Study of spin structure of hadrons using intermediate energy photons

自从发现基本粒子有自旋以来，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取向。然而，到目前为止，使用更高能量的电磁探针（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使美国天主教大学的实验核物理小组能够继续开发和利用新的系统，这将允许光子和目标核的独立取向。这项研究将解决强子结构中的一个主要未解决的问题，质子和中子激发态的性质和数量。组成核子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，但只有少数迄今已明确确定。问题仍然是，这是否是由于一些额外的方面的理论尚未理解或由于实验问题，在检测这些激发态。实验上的困难主要在于这些激发态共振的广泛性和重叠性。只有通过测量，每次只选择性地激发这些共振中的一小部分，才能清楚地研究各个态的性质。在拟议的研究中，我们将使用一个新的委托偏振光子束线和一个新的偏振目标，沿着与高分辨率，高能量光子标签，以确定光子能量，开始这些共振的详细研究。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本质。除了基本夸克相互作用的研究之外，这项研究还有两个领域将产生更广泛的影响。该研究将为博士后、攻读高级学位的研究生、物理和工程专业的本科生提供培训。这些培训活动将涵盖大范围的实验活动，包括低温学、高速电子学、粒子探测器、真空系统和数据处理方面的经验。这种培训将扩大我国现有的科技人才库。大多数的研究，将进行在这个拟议的计划将涉及发展的instrumentation和经验，使用能量标记的偏振光子束。这些活动将导致改进，这将是必不可少的未来的实验，包括那些在更高的能量，现在正在计划。

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.