Title: Gluon Spin, Canonical Momentum, and Gauge Symmetry

Speaker: Dr. Yong Zhao（University of Maryland）　　

Time: 3:00PM, July 13rd (Friday)

Place: Theory Division，319

Abstract：The first part of this talk will present some general remarks on gauge symmetry. The speaker will discuss the gauge symmetry in nonrelativistic systems such as the atom, and explain why it is natural to choose the Coulomb gauge to describe such systems. Then the talk will recall the quantization of QED, and show that a sufficient condition to have gauge-invariant physical matrix elements is to have manifestly gauge-invariant operators. It will also be shown that the boosting of the fermion wavefunction in non-covariant gauges like the Coulomb gauge is highly nontrivial and includes the contributions from the interactions. The second part of the talk will focus on the gauge-invariant decomposition of conserved quantities---the total energy-momentum and angular momentum. The speaker will discuss the two decompositions proposed by Chen et al. and Wakamatsu, and especially, the frame-dependence of the Chen decomposition. Finally, the talk will explain how the photon spin and orbital angular momentum are defined and measured in atomic physics.