Gluon Spin, Canonical Momentum, and Gauge Symmetry

Abstract

It is well known that in gauge theories, the spin (and orbital angular momentum) of gauge particles is not gauge invariant, although the helicity is; neither are the canonical momentum and canonical angular momentum of charged particles. However, the simple appeal of these concepts has motivated repeated attempts to resurrect them as physical descriptions of gauge systems. In particular, measurability of the gluon-spin-contribution to the proton helicity in polarized proton scattering has generated many theoretical efforts in generalizing it and others as gauge-invariant quantities. In this work, we analyze the constraints of gauge symmetry, the significance of gluon spin in the light-cone gauge, and what is possible and natural in QCD parton physics, emphasizing experimental observability and physical interpretation in the structure of bound states. We also comment on the measurability of the orbital angular momentum of the Laguerre-Gaussian laser modes in optics.

17 pages, latex, 1 figure

References in corpus (8)

• Quantized Rotation of Atoms From Photons with Orbital Angular Momentum
• Reply to the Comment on "Spin and orbital angular momentum in gauge theories: Nucleon spin structure and multipole radiation revisited" [PRL 100:232002 (2008)]
• Spin and orbital angular momentum in gauge theories: Nucleon spin structure and multipole radiation revisited
• Do gluons carry half of the nucleon momentum?
• Gauge- and frame-independent decomposition of nucleon spin
• On the controversy concerning the definition of quark and gluon angular momentum
• Gauge Invariance and k_T-Factorization of Exclusive Processes
• Reply to the Comment of X. Ji on "Do gluons carry half of the nucleon momentum?" [PRL 103:062001 (2009)]

Cited by in corpus (16)

• The angular momentum controversy: What's it all about and does it matter?
• 50 Years of Quantum Chromodynamics
• The Spin Structure of the Nucleon
• Is gauge-invariant complete decomposition of the nucleon spin possible ?
• Twist analysis of the nucleon spin in QCD
• Geometrical approach to the proton spin decomposition
• Physics of Gluon Helicity Contribution to Proton Spin
• Probing Parton Orbital Angular Momentum in Longitudinally Polarized Nucleon
• Justifying the Naive Partonic Sum Rule for Proton Spin
• Unravelling High-Energy Hadron Structures with Lattice QCD
• Gauge symmetry and background independence: Should the proton spin decomposition be path independent?
• Gauge Invariant Descriptions of Gluon Polarizations
• On the two remaning issues in the gauge-invariant decomposition problem of the nucleon spin
• Are there infinitely many decompositions of the nucleon spin ?
• Proton spin after 30 years: what we know and what we don't?
• Gauge Invariant Descriptions of Gluon Polarizations Revisited