Studies of hadronic structure using electromagnetic probes

现代物理学研究的中心问题之一涉及到我们对原子核的组成部分--质子、中子和其他束缚它们的粒子(介子)--的理解。值得注意的发现表明，这些粒子由更基本的成分--夸克和胶子组成。虽然一个非常好的理论框架(称为量子色动力学，或QCD)能够准确地描述夸克和胶子在极高能量下(或相当于当夸克非常接近时)如何相互作用，但将QCD应用于较低能量(较长距离)的现象一直是非常困难的。我们提高理解的障碍之一是缺乏高质量的数据，特别是提供QCD内部工作原理最清晰图景的深度排他性反应。这些研究是困难的，因为它们需要连续的、高光度的电子束，以及具有良好的粒子识别和可重现的系统学的探测器。Jefferson Lab(JLab)12 GeV升级旨在解决这些问题。 这项拟议的研究有四个关键调查。 1.介子弹性形变因子(FPI)：它为我们提供了直接观测QCD从长距离强禁闭到近距离渐近自由的最大希望。我们的第一个目标是确定在广泛的运动学范围内从pi+电子产生数据中提取π介子形状因子的可靠性。我们的第二个目标是获得从Q2=0.3到6.0GeV2的高质量的FPI测量。 2.K+弹性形状因子(FK)：我们将在W&gT；2.5GeV取p(e，e‘K+)Lambda和p(e，e’K+)Sigma0反应的L/T分离数据，以寻找质子‘Kaon云’散射的证据。如果数据保证，我们将首次提取K+形状因数的Q2依赖关系。这些数据还将提高我们对K+电产生的理解，包括核子-介子和夸克模型中所需的耦合常数。 3.深度独占pi±和K+电产生中的QCD分解：如果达到硬散射区域，则散射幅度必须分解为硬过程和软过程。事实上，因式分解是从深度排斥反应中提取广义部分子分布(GPD)信息的先决条件，但应用因式分解的运动学机制对于介子的产生渠道知之甚少。我们将在很大的运动学范围内(直到Q2=9GeV2)测量pi+，pi-和K+L/T分离的截面，以阐明QCD因式分解的界限。 4.深度独占矢量介子电生成中的QCD分解：作为QCD因式分解研究的扩展，我们还将取独家Rho、omega和Phi电生成数据，直到Q2=6GeV2，以更好地检验共线因式分解的适用性。

One of the central problems of modern physics research concerns our understanding of the building blocks of the atomic nucleus -- the protons, neutrons, and other particles (mesons) that bind them. Notable discoveries have indicated that these particles consist of yet more fundamental constituents, the quarks and gluons. While a very good theoretical framework (called Quantum Chromo-Dynamics, or QCD) is able to describe accurately how quarks and gluons interact at extremely high energies (or, equivalently, when the quarks are very close together), it has been very difficult to apply QCD to lower energy (longer distance) phenomena. One of the obstacles to our improved comprehension has been a lack of quality data, particularly for the Deep Exclusive Reactions which provide the clearest picture of the inner workings of QCD. These studies are difficult because they require continuous, high luminosity electron beams, and detectors with good particle identification and reproducible systematics. The Jefferson Lab (JLab) 12 GeV upgrade is intended to address these issues. The proposed research has four key investigations. 1. The Pion Elastic Form Factor (Fpi): Fpi offers our best hope of directly observing QCD's transition from strong-confinement at long-distance scales to asymptotic-freedom at short-distances. Our first objective is to establish the reliability of the extraction of the pion form factor from pi+ electroproduction data over a broad kinematic range. Our second objective is to obtain high quality Fpi measurements from Q2=0.3 to 6.0 GeV2. 2. The K+ Elastic Form Factor (FK): We will take L/T-separated data for the p(e,e'K+)Lambda and p(e,e'K+)Sigma0 reactions at W>2.5 GeV to search for evidence of scattering from the proton's ``kaon cloud.'' If warranted by the data, we will perform the first-ever extraction of the Q2-dependence of the K+ form factor. The data will also improve our understanding of K+ electroproduction, including the coupling constants needed in nucleon-meson and quark models. 3. QCD Factorization in Deep Exclusive pi± and K+ Electroproduction: If the hard scattering regime is reached, the scattering amplitude must factorize into hard and soft processes. In fact, factorization is a prerequisite for the extraction of Generalized Parton Distribution (GPD) information from Deep Exclusive Reactions, but the kinematic regime where factorization applies is only poorly known for meson production channels. We will measure pi+, pi- and K+ L/T-separated cross sections over a wide kinematic range (up to Q2=9 GeV2) to clarify the bounds of the QCD factorization regime. 4. QCD Factorization in Deep Exclusive Vector Meson Electroproduction: As an extension of the QCD factorization studies, we will also take exclusive rho, omega and phi electroproduction data up to Q2=6 GeV2, to provide a better test of the applicability of collinear factorization.