The hbar Expansion in Quantum Field Theory

TL;DR

The paper introduces an hbar expansion method in quantum field theory that maintains fundamental symmetries and offers new insights into perturbative and nonperturbative phenomena, especially in bound states.

Contribution

It develops a scheme for hbar expansion that preserves Lorentz and gauge invariance and applies it to bound states in QED and QCD, revealing new structural insights.

Findings

Each loop in perturbation theory corresponds to a factor of hbar.

Classical charge and fine structure constant are linear in hbar.

Bound state energies and momenta scale with hbar, affecting the expansion.

Abstract

We show how expansions in powers of Planck's constant hbar = h/2\pi can give new insights into perturbative and nonperturbative properties of quantum field theories. Since hbar is a fundamental parameter, exact Lorentz invariance and gauge invariance are maintained at each order of the expansion. The physics of the hbar expansion depends on the scheme; i.e., different expansions are obtained depending on which quantities (momenta, couplings and masses) are assumed to be independent of hbar. We show that if the coupling and mass parameters appearing in the Lagrangian density are taken to be independent of hbar, then each loop in perturbation theory brings a factor of hbar. In the case of quantum electrodynamics, this scheme implies that the classical charge e, as well as the fine structure constant are linear in hbar. The connection between the number of loops and factors of hbar is more subtle for bound states since the binding energies and bound-state momenta themselves scale with hbar. The hbar expansion allows one to identify equal-time relativistic bound states in QED and QCD which are of lowest order in hbar and transform dynamically under Lorentz boosts. The possibility to use retarded propagators at the Born level gives valence-like wave-functions which implicitly describe the sea constituents of the bound states normally present in its Fock state representation.