title: Spin and orbital angular momentum in gauge theories: nucleon spin structure and multipole radiation revisited.
abstract: We address and solve the long-standing gauge-invariance problem of the nucleon spin structure. Explicitly gauge-invariant spin and orbital angular momentum operators of quarks and gluons are obtained. This was previously thought to be an impossible task and opens a more promising avenue towards the understanding of the nucleon spin. Our research also justifies the traditional use of the canonical, gauge-dependent angular momentum operators of photons and electrons in the multipole-radiation analysis and labeling of atomic states and sheds much light on the related energy-momentum problem in gauge theories, especially in connection with the nucleon momentum.
authors.affiliation: Department of Physics, Sichuan University, Chengdu, China. cxs@scu.edu.cn
authors.forename: Xiang-Song
authors.initials: XS
authors.lastname: Chen
authors.forename: Xiao-Fu
authors.initials: XF
authors.lastname: Lü
authors.forename: Wei-Min
authors.initials: WM
authors.lastname: Sun
authors.forename: Fan
authors.initials: F
authors.lastname: Wang
authors.forename: T
authors.initials: T
authors.lastname: Goldman