A plasma photocathode for spin-polarized electron beams using excited hydrogen halides

Spin-polarized electron beams are essential tools for probing fundamental symmetries and for the search beyond the Standard Model. While plasma-based accelerators are a promising pathway towards higher-energy frontiers, they have so far failed to deliver a competitive polarized source: existing proposals are challenging to realize and achievable polarizations remain far below conventional sources. Here, we introduce a photocathode-like scheme, applied to a gas of pre-polarized hydrogen halides. A VUV and a visible laser pulse excite the halide atoms to create a two-component ionization medium, consisting of low-threshold excited halide and high-threshold polarized hydrogen. Particle-in-cell simulations show witness beams of approx. 10 pC retaining $97\%$ of the initial polarization $P_0$, reaching $P \approx 68\%$ for unaligned bonds ($P_0 = 70\%$) and $P=97\%$ for aligned bonds ($P_0 = 100\%$), rivaling state-of-the-art conventional sources.