Ph.D. Candidate: Kelly Yu-Ju Chiu
Research Advisor: Stanley J. Brodsky
Date: Monday, July 2nd
Time: 1:30 pm
Location: B48/Madrone Conference Room
Title: Angular Momentum Conservation Law in Light-Front Quantum Field Theory
This thesis investigates the angular momentum conservation law in light-front quantum field theory. We prove the light-front Poincaré invariance of the angular momentum conservation law and the helicity sum rule for relativistic composite systems. We show that the light-front wavefunction (LFWF), which describes the internal structure of a bound state, is in fact frame independent, in contrast to instant form wavefunctions. In particular, we demonstrate that $j^3$, the intrinsic angular momentum projected onto the light-front direction, is independent of the bound state’s $4$-momentum and the observer’s Lorentz frame. The frame independence of $j^3$ is a feature unique to the front form.
The angular momentum conservation law leads directly to a nonperturbative proof of the constraint $A(0)=1$ and the vanishing of the anomalous gravitomagetic moment $B(0)=0$. Based on the conservation of angular momentum, we derive a selection rule for orbital angular momentum which can be used to eliminate certain interaction vertices in QED and QCD. We also generalize the selection rule to any renormalizable theory and show that there exists an upper bound on the change of orbital angular momentum in scattering processes at any fixed order in perturbation theory.