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

这项研究计划的目的是通过与德国美因茨核物理研究所的A2小组继续合作，在我们测量对核子极化率敏感的极化观测值的成功计划的基础上再接再厉。我们将利用1.6GeV的Mami-C连续波电子束产生真正的光子，包括极化和未极化的光子，并与极化和未极化的核靶相结合，以探测中能强子的结构。在这个能区中，结构和反应动力学的描述处于基于核子/介子的模型和基于夸克/胶子自由度的QCD激发的模型(或有效理论)之间的过渡区域。不仅在美因茨实验室，而且在美国的JLab和其他中能实验室，了解非微扰QCD过渡区中的强子结构仍然是研究工作的前沿。*这个项目将专门关注A2中利用水晶球和TAPS探测器的主题。在过去的九年里，在美因茨，霍尼奇、胡伯和萨蒂一直在研究核子的极化率实验，此外还研究了介子产生的阈值。Sarty和Huber于2008年在Mami与Hornidge一起加入，这项提议是加拿大对A2合作的第三次延续，这一合作将持续到本十年末。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过去和计划的JLab项目有非常接近的科学目标。*作为我们当前项目拨款的一部分，我们已经获得了四个不同康普顿散射极化观测值的数据，最终目标是以较小的统计、系统和模型相关误差提取质子的标量和自旋极化率。这种极化率是基本的结构量，可以通过各种QCD激发的模型和有效的理论来计算。除了获取关于这四个不对称性中的三个的更多数据外，在接下来的三年里，我们将开始获取关于中子的数据，那里的极化率远不为人所知，我们打算继续我们的可行性研究，利用双轻子产生的质子对质子进行类时间虚拟康普顿散射。这将使我们第一次能够了解质子在低动量转移时的类时电磁结构。*最后，将继续开发和测试用于极化率实验和临界π介子生产的有源极化目标。通过使用这样的靶，我们将能够改进我们的双极化康普顿测量，以及阈值介子产生中的靶和束-靶不对称性的测量，以测试由于上下夸克质量差异而在几个百分点的水平上的同位旋破缺。

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.