Exploring the Dark Axion Portal in the LUXE-NPOD Experiment

The optical dump at the LUXE experiment has the potential to create a large flux of $\mathcal{O}({\rm GeV})$ photons that can be used to look for new physics when directed at a solid material dump. The LUXE-NPOD extension of LUXE, which focuses the hard photons onto a slab of tungsten, offers two interaction points (laser-electron and photon-tungsten), making it well-suited to test theories containing two or more new particles. We examine the dark axion portal, a scenario involving both dark photons (DPs) and axion-like particles (ALPs) and their mutual interactions, and its implications on the phenomenology at LUXE-NPOD. To simulate the spectra of particle populations generated at the electron-laser interaction point, we solve a set of extended 1D cascade equations. We recover a photon spectrum consistent with previous analyses and present previously unconsidered DP and ALP spectra. We derive the overall sensitivity of LUXE-NPOD to various parameters of the new particles, and show that it is capable of probing previously uncharted regions in the dark axion parameter space. For ALPs in the $10$-$500$ MeV mass range and DPs either heavier ($\sim{\rm GeV}$) or significantly lighter $\mathcal{O}(10^{-18}$-$10^{-13})$ eV, we obtain novel constraints on DP kinetic mixing parameters smaller than $10^{-2}$ and on $\mathcal{O}(10^{-4}$-$10^{-3})$ GeV$^{-1}$ ALP-photon couplings. We find that restrictions on $\mathcal{O}\left(10^{-3}\right)$ kinetic mixing can be extracted for arbitrarily small DP masses. Our discussion aims to be systematic and demonstrates a practical method of analyzing constraints on multi-dimensional parameter spaces.