A Program to Study Hadronic Matter using Electromagnetic Probes at Jefferson Lab

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。这笔拨款的重点是支持我们在托马斯杰斐逊国家加速器设施（JLab）的持续研究项目，使用电磁探针在非常基本和基本的水平上研究强子物质。我们的团队在三个独立的实验中发挥了关键作用，这些实验测试了粒子相互作用的本质，这些实验属于“标准模型”的范畴。一个名为Qweak的实验，利用弱相互作用的奇偶性来测量标准模型的一个基本参数，即温伯格（或混合）角，其相对误差在0.3%以内。这个角度与W和Z玻色子这两个粒子质量之比有关，并且作为传递到散射目标的动量的函数而变化。从这个实验中得到的精确测量，当与其他实验结合在一起时，将对我们目前版本的标准模型提出的扩展提出强有力的限制。在另一项实验中，我们小组的一名成员在杰斐逊实验室的B厅进行了介子寿命测量，旨在探索中性带电介子衰变为两个光子的机制。在本实验中，中性介子将通过Primakoff效应从原子核的库仑场产生，并将在高分段量热计中被检测到。该实验测量了一个可以在手性微扰理论中计算的基本量，代表了在约束尺度下可以进行的为数不多的严格的QCD实验测试之一。本课题组还开展了g8b（质子）和g13a/b（氘核）数据集中线性偏振光子的矢量介子光产生偏振观测数据提取实验，这是ISU三篇博士论文的主题。最近CLAS在光和电生产中提取单偏振和双偏振观测值的结果显示出它们对小产生振幅的高灵敏度，这是通过理解核子中夸克自由度的潜在对称性，入射光子和目标核子之间的奇偶交换的性质来提取激发态的关键。以及电磁反应中相关奇异性产生的机理。作为JLab重子共振研究部分的自然延伸，我们期望将研究扩展到北京正电子对撞机，在那里我们将在北京电子光谱仪（BES）上提取通过粲介子衰变的激发态重子。我们试图通过分析J/psi进入重子-反重子通道的衰变来协调这项研究与BES，我们期望在高达2 GeV的质量区域探测核子共振（N*）的补充手段。我们小组正在计划建造一套6个漂移室，用作杰斐逊实验室b厅的1区粒子跟踪系统。这个小组在JLab的研究项目，ISU的加速器设施，以及探测器科学实验室的连续探测器建设项目，结合在一个积极的项目中，在教育环境中提供广泛的经验，可以用来有效地培养下一代科学家。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The focus of this grant goes towards supporting our continuing research program at the Thomas Jefferson National Accelerator Facility (JLab) using electromagnetic probes to study hadronic matter at the very fundamental and basic levels. Our group has taken crucial roles in three separate experiments that test the nature of particle interactions which falls under the rubric of the "Standard Model". One experiment, Qweak, uses the parity-violating property of the weak interaction to measure a fundamental parameter of the Standard Model known as the Weinberg (or mixing) angle to within a relative error of 0.3%. This angle is related to the ratio of two particle masses, the W and Z bosons, and varies as a function of the momentum transfered to the scattering target. A precise measurement from this experiment, when combined with other experiments, will place strong constraints on proposed extensions to our present version of the Standard Model. In another experiment, a member of our group leads the pion lifetime measurements undertaken in Jefferson Lab's Hall B which seeks to probe the mechanism through which a neutrally charged pion can decay into two photons. In this experiment, neutral pions will be photoproduced from the Coulomb field of nuclei via the Primakoff effect, and will be detected in a highly segmented calorimeter. The experiment measures a fundamental quantity which can be calculated in the context of chiral perturbation theory and represents one of the few stringent experimental tests of QCD that can be made in the confinement scale regime. Our group also conducts experiments to extract polarization observables from vector meson photoproduction with linearly-polarized photons in the g8b (proton) and g13a/b (deuteron) datasets, which is the subject of three ISU PhD theses. Recent CLAS results on the extraction of single- and double-polarization observables in photo- and electroproduction show their high sensitivity to small production amplitudes, which is key in extracting excited baryon state by affording an understanding of the underlying symmetry of the quark degrees of freedom in the nucleon, the nature of the parity exchange between the incident photon and the target nucleon, and the mechanism of associated strangeness production in electromagnetic reactions. As a natural extension to the JLab baryon resonance studies component at JLab, we expect to expand our research to the Beijing Electron Positron Collider, where we will extract excited baryons states decaying through charmed mesons at the Beijing Electron Spectrometer (BES). We seek to coordinate this research with BES by analyzing the decay of the J/psi into baryon-antibaryon channels, where we expect a complementary means for probing nucleon resonances (N*) in the mass region up to 2 GeV. Our group is planning to construct a set of six drift chambers to be used as the region 1 particle tracking system in Jefferson Lab's Hall B. This group's research program at JLab, the accelerator facilities at ISU, and continuous detector construction projects in the groups Laboratory for Detector Science, combine in an active program to provide a breadth of experiences in an educational environment which can be used to effectively train the next generation of scientists.