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、中子和其他结合它们的粒子（介子）的理解。一些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如何在极高的能量下相互作用（或者，当夸克非常接近时），但将QCD应用于较低能量（较长的距离）现象是非常困难的。阻碍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内部工作最清晰图像的深度独占反应。这些研究是困难的，因为它们需要连续的、高亮度的电子束，以及具有良好粒子识别和可重复系统的探测器。杰佛逊实验室（JLab） 12gev升级旨在解决这些问题。

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.