Quantum Kinetic Theory for Quantum Chromodynamics

TL;DR

This paper develops a quantum kinetic theory for QCD that captures spin polarization effects in quark-gluon plasma, revealing different behaviors in vortical versus non-vortical gradients and proposing a mechanism for spin-orbital angular momentum conversion.

Contribution

It introduces a comprehensive quantum kinetic framework for QCD including all leading order collisions and analyzes spin polarization phenomena at different gradient orders.

Findings

Spin polarization arises from quantum kinetic equations in QCD.

Vortical gradients lead to spin polarization without collisional contributions.

Inelastic collisions may enable spin and orbital angular momentum conversion.

Abstract

We develop a quantum kinetic theory for QCD, which incorporates all leading order collision terms. At lowest order in gradient expansion, it reproduces the spin-averaged Boltzmann equation with both elastic and inelastic collisions. At next order in gradient expansion, the solution to the quantum kinetic equations give spin polarization of on-shell quarks and gluons in quark-gluon plasma when the gradients are of hydrodynamic ones. A power counting in the coupling shows the spin polarization behaves differently in vortical and non-vortical gradients: the former is free of collisional contribution to leading order, while the latter contains a collisional contribution at parametrically the same order as the free theory counterpart. We also find the inelastic collision in a spin basis provides a possible mechanism for conversion between spin and orbital angular momentum.