Spin-Resolved Decay of Axion-Like Particles into Electron–Positron Pairs in Strong Electromagnetic Fields

This study investigates the decay of axion-like particles (ALPs) into electron-positron pairs within intense laser fields. While previous research has focused on spin-summed rates, this paper addresses the need for a spin-resolved description, which is essential for understanding the underlying quantum dynamics and for implementing accurate simulations in strong-field physics.

The authors employ the Baier-Katkov quasiclassical operator method combined with the locally constant field approximation (LCFA). By deriving a compact analytic expression for the spin-resolved differential decay rate, they account for both finite ALP-mass effects and the spin degrees of freedom of the produced leptons. The analysis covers both the vacuum-forbidden kinematic region (where pair creation is entirely field-induced) and the vacuum-allowed region, providing a unified Airy-function representation that bridges these regimes.

The study demonstrates that the pseudoscalar coupling of the ALP induces unique spin-resolved channels and correlations that differ significantly from photon-induced pair production. A key finding is that finite ALP mass reorganizes the energy spectrum across the vacuum threshold; above this threshold, the coherent interplay between vacuum and field-assisted contributions generates spin-dependent oscillatory modulations. Furthermore, the entanglement of the produced pair is shown to be highly sensitive to the production mechanism: the pair is nearly maximally entangled and singlet-like near the vacuum threshold in weak fields, but transitions to a triplet-like state or becomes separable depending on whether strong-field or vacuum decay dominates.