Helicity effects in the dynamically assisted Schwinger mechanism
We study vacuum electron-positron pair production in a spatially uniform bichromatic electric field within the quantum-kinetic framework for fermions. The external background models the superposition of two counterpropagating circularly polarized laser pulses and combines a strong slowly varying component with a weak rapidly oscillating one. We analyze the weak-field multiphoton regime, the strong-field tunneling regime, and their combination corresponding to the dynamically assisted Schwinger effect. Our main focus is on helicity-resolved observables. We show that dynamical assistance enhances not only the total yield but also the helicity asymmetry: right- and left-handed electrons preferentially populate opposite momentum half-spaces. Most importantly, within the parameter range considered here, the ratio of the momentum distributions for opposite helicities is governed predominantly by the polar angle with respect to the propagation axis and depends only weakly on the momentum magnitude and azimuthal angle. The corresponding asymmetry becomes more pronounced as the weak-field frequency is increased. These results identify a clear helicity signature of the dynamically assisted Schwinger effect in rotating strong-field backgrounds and provide a compact characterization of the associated helicity-resolved spectra.