核子自旋、动量结构及其规范不变性研究

项目名称： 核子自旋、动量结构及其规范不变性研究

项目编号： No.10875082

项目类型： 面上项目

立项/批准年度： 2009

项目学科： 金属学与金属工艺

项目作者： 陈相松

作者单位： 四川大学

项目金额： 34万元

中文摘要： 规范不变性问题是困扰核子自旋结构研究长达20年的关键难题。在前一项目（10475057）的资助下，我们近期找到了既满足角动量对易关系、又规范不变的QCD总角动量分解，从而使夸克、胶子的自旋和轨道角动量具有了真正物理的含义，为核子自旋结构研究开辟了全新的、更明朗的道路。本项目将用具有正确物理意义的夸克与胶子角动量算子重新研究核子自旋组成，给出其真实图象，检查并澄清以前用非物理算子所得结果，包括：1）目前实验上正在测量、并用各种唯像模型计算的，与规范有关的算子相联系的所谓"胶子自旋"；2)被广泛讨论、并用格点QCD计算的，与不满足角动量对易关系的算子相联系的所谓"夸克轨道角动量"； 3）渐进极限下胶子携带约一半核子自旋的图象。由于QCD总动量的分解存在与总角动量分解完全类似的问题，本项目还将对核子动量结构进行重新研究，并检查以前基于非物理分解所得出的渐进极限下胶子携带约一半核子动量的结论。

中文关键词： 核子结构；规范不变性；自旋；动量；轨道角动量

英文摘要： Following our proposal of the gauge-field-decomposition method in[PRL100:232002], which gives a complete gauge-invariant decomposition of the nucleon spin, this project systematically developed this method and its application to the nucleon structure: 1. We pointed out that the quark and gluon distribution functions can have more reasonable gauge-invariant "canonical" constructions; perturbative calculation shew that the gauge-invariant canonical gluon momentum contributes only about 1/5 of the nucleon momentum in the asymptotic limit, instead of about 1/2 as in the conventional definition [PRL103:062001]. 2. Concerning the gluon spin which is receiving great attention in experiments, we did perturbative calculation to compare the conventional "gluon polarization" Δdefined in the light-cone gauge and the "gluon spin" Sg as we define to contain only physical degrees of freedom. We found that Sg is 5/9 of Δ and 4/5 of Sg is a "static-field" contribution involving no time-derivative, which cancels exactly a corresponding term in the gluon orbital angular momentum, and thus can be subtracted altogether[PLB700:21]. These results reveal that if properly defined, the nucleon can still be quark-dominated, and the real dynamical gluon degrees of freedom contribute little. 3. To address the controversy in nucleon spin decomposition, we made an analysis from a most basic perspective of constructing quantum eigenstates, and thus clarified what is a most practical decomposition [PRD83:071901]. 4. We successfully extended the physical decomposition of vector gauge field to tensor gauge field, which can be applied to the construction of gauge-invariant quantities in gravitation [PRD83:084006；061501].

英文关键词： nucleon structure; gauge invariance; spin; momentum; orbital angular momentum