Investigations of Hadron Structure using CB-TAPS at the Mainz Microtron

这项研究计划的目的是建立在我们成功的计划，测量偏振可观的核子极化率敏感的继续与A2组在美因茨，德国核物理研究所的合作努力。 我们将利用1.6-GeV MAMI-C连续波电子束产生真实的光子，包括极化和非极化的，它们与极化和非极化的核靶相结合，以探测中能强子的结构。 在这个能量范围内，对结构和反应动力学的描述位于基于核子/介子的模型和基于夸克/胶子自由度的QCD启发模型（或有效理论）之间的过渡区域。 理解非微扰QCD过渡区的强子结构不仅在美因茨实验室，而且在美国JLab和其他中间能量实验室仍然处于研究工作的最前沿。该项目将特别关注A2中使用Crystal Ball和TAPS探测器的主题。 在过去的九年里，霍尼奇、胡贝尔和萨蒂在美因茨除了研究阈值π介子的产生外，还一直致力于核子极化率实验。 Sarty和Huber于2008年在MAMI与Hornidge合作，该提案是加拿大对A2合作的第三次继续，该合作将延续到本十年结束。 我们正在进行的实验不仅具有很高的科学影响力，而且它们与Huber和Sarty的过去和计划中的JLab项目有着非常接近的科学目标。作为我们目前的项目资助的一部分，我们已经采取了四种不同的康普顿散射极化观测数据，最终目标是提取质子的标量和自旋极化率，具有小的统计，系统和模型相关的误差。 这种极化率是基本的结构量，可以通过各种QCD启发的模型和有效的理论进行计算。 除了对这四种不对称性中的三种获取更多的数据外，在接下来的三年里，我们将开始获取中子的数据，因为中子的极化率还不太为人所知，我们打算继续利用双轻子产生对质子进行类时虚康普顿散射的可行性研究。 这将使我们第一次能够在低动量转移时获得质子的类时电磁结构。最后，将继续研制和测试一种用于极化率实验和阈π介子产生的主动极化靶。 通过使用这样的目标，我们将能够改善我们的双极化康普顿测量沿着与目标和束靶的不对称性测量在阈值π介子生产测试的百分之几的水平，由于上下夸克质量差的同位旋断裂。

The objective of this research proposal is to build upon our successful program of measuring polarization observables sensitive to the nucleon polarizabilities by continuing collaborative efforts with the A2 group at the Institute for Nuclear Physics in Mainz, Germany. We will exploit the 1.6-GeV MAMI-C continuous-wave electron beam to generate real photons, both polarized and unpolarized, which are combined with polarized and unpolarized nuclear targets in order to probe the structure of hadrons at intermediate energies. In this energy regime, description of structure and reaction dynamics lies in the transition region between nucleon/meson-based models and QCD-inspired models (or effective theories) based on quark/gluon degrees of freedom. Understanding hadronic structure in this transition region of non-perturbative QCD remains at the forefront of research efforts not only at the Mainz laboratory, but also at JLab in the USA, and other intermediate energy labs.******This project will focus specifically on topics within A2 that make use of the Crystal Ball and TAPS detectors. Over the last nine years in Mainz, Hornidge, Huber, and Sarty have been working on nucleon polarizability experiments in addition to studying threshold pion production. Sarty and Huber joined with Hornidge at MAMI in 2008, and this proposal is the third continuation of the Canadian contributions to the A2 Collaboration that will extend to the end of the decade. The experiments we are working on have not only high scientific impact, but they have very close scientific goals to the past, and planned, JLab projects of Huber and Sarty.******As part of our current project grant, we have taken data on four different Compton scattering polarization observables with the end goal of extracting both the scalar and spin polarizabilities of the proton with small statistical, systematic, and model-dependent errors. Such polarizabilities are fundamental structure quantities, amenable to calculation via various QCD-inspired models and effective theories. In addition to taking more data on three of these four asymmetries, over the next three years we will begin taking data on the neutron, where the polarizabilities are much less well known, and we intend to continue our feasibility studies of timelike virtual Compton scattering on the proton using dilepton production. This will allow us, for the first time, to access the timelike electromagnetic structure of the proton at low momentum transfer.******Finally, work will continue on the development and testing of an active, polarized target to use in both polarizability experiments and threshold pion production. By using such a target we will be able to improve on our double-polarization Compton measurements along with the target and beam-target asymmetry measurements in threshold pion production to test isospin breaking at the few percent level due to the up-down quark mass difference.