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

We investigate spin-resolved decay of an axion-like particle (ALP) into an electron--positron pair in an intense laser field. Using the Baier--Katkov quasiclassical operator formalism and the locally constant field approximation, we derive a compact analytic rate retaining finite ALP-mass effects and the lepton spin degrees of freedom. In the massless limit, the spin-summed rate has the same weak- and strong-field asymptotic scalings as the corresponding photon-induced pair-creation rate, while the pseudoscalar coupling induces distinct spin-resolved channels and spin correlations. A finite ALP mass reorganizes the spectrum across the vacuum threshold, producing purely field-induced pair creation below threshold and spin-dependent oscillatory modulations above threshold through the coherent interplay of vacuum and field-assisted contributions. The entanglement of the produced pair reflects the dominant production mechanism. Near the vacuum threshold in weak fields, the pair is nearly maximally entangled and singlet-like. Away from threshold, the reduced spin state becomes triplet-like, retaining a concurrence of 1/2 when strong-field production dominates but becoming separable when vacuum decay dominates. These results identify spin-resolved spectra and entanglement as signatures of finite-mass and threshold effects in strong-field ALP searches.