An axion framework for Particle-in-Cell codes with Monte-Carlo sampling: emission, absorption, and detailed balance in plasmas
We present an extension of the OSIRIS particle-in-cell (PIC) code that introduces an axion macroparticle species and three axion-production channels commonly used in thermal-plasma axion phenomenology: screened Primakoff conversion $(γ+ Z \leftrightarrow a + Z)$, Compton-like photoproduction on electrons in a blackbody photon bath $(γ+ e \to a + e)$, and thermal axion bremsstrahlung from electron-ion and electron-electron scattering $(e + Z \to e + Z + a$ and $e + e \to e + e + a)$. The package is integrated into the existing OSIRIS quantum-electrodynamics (QED) Monte Carlo infrastructure and provides Poisson macro-event sampling with unbiased weight rescaling for variance control. Optional modules implement conservative cell-local energy and momentum feedback and temperature-field evolution, and each channel includes an inverse absorption operator constructed to satisfy detailed balance with a thermal bath. We benchmark forward spectral emissivities for uniform plasmas at $T_e = 1.3~\mathrm{keV}$, $3~\mathrm{keV}$, and $5~\mathrm{keV}$ against screened analytic results based on Raffelt-style calculations, finding percent-level agreement in integrated power for all channels and good reproduction of spectral peak positions. In addition, homogeneous relaxation tests with forward and inverse operators enabled show that, for all three implemented channels, the axion population and total axion energy evolve toward stable steady-state values, providing an initial validation of detailed-balance recovery in the inverse-process implementation. These results establish a foundation for kinetic simulations of axion production, absorption, and transport in high-energy-density plasmas, while more extensive validation of feedback physics and fully dynamic multidimensional coupled scenarios remains future work.