Studies of hadronic structure using electromagnetic probes

现代物理学研究的中心问题之一是我们对原子核的构建单元的理解-质子，中子和其他粒子（介子）将它们结合起来。 一些重要的发现表明，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（称为量子色动学，或QCD）能够准确地描述夸克和胶子在极高能量下（或者等价地，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）的现象非常困难。 我们提高理解的障碍之一是缺乏高质量的数据，特别是对于提供QCD内部工作的最清晰图像的深排他反应。 这些研究是困难的，因为它们需要连续的，高亮度的电子束，和探测器具有良好的粒子识别和可重复的系统。杰斐逊实验室（JLab）12 GeV升级旨在解决这些问题。本研究主要包括四个方面的研究：*。1.π弹性形状因子（Fpi）：Fpi提供了我们直接观察QCD从长距离尺度的强约束到短距离尺度的渐近自由的转变的最大希望。我们的第一个目标是建立一个广泛的运动学范围内的π +电生产数据的π形状因子的提取的可靠性。 我们的第二个目标是获得从Q2=0.3到6.0 GeV 2的高质量Fpi测量。* 2. K+弹性形状因子（FK）：我们将在W>2.5 GeV时对p（e，e 'K+）Lambda和p（e，e' K+）Sigma 0反应进行L/T分离数据，以寻找质子``kaon云散射的证据。“如果数据允许，我们将首次提取K+形状因子的Q2依赖性。 这些数据也将提高我们对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.深度排斥π ±和K+电产生中的QCD因子分解：如果达到硬散射状态，散射振幅必须分解为硬过程和软过程。 事实上，因式分解是从深排斥反应中提取广义部分子分布（GPD）信息的先决条件，但因式分解适用的运动学机制仅对介子产生通道知之甚少。我们将在很宽的运动学范围内（直到Q2=9 GeV 2）测量π +、π-和K+ L/T-分离截面，以澄清QCD因子化机制的边界。4.在深排他矢量介子电产生中的QCD因子分解：作为QCD因子分解研究的扩展，我们还将采用排他的ρ，ω和phi电产生数据，直到Q2=6 GeV 2，以提供共线因子分解适用性的更好测试。

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.***