Millicharged particles (mCPs) are a well-motivated target for far-forward searches at the Large Hadron Collider. We identify and quantify a significant new source of these particles: secondary production in hadronic and electromagnetic showers initiated by energetic neutral particles striking the TAXN absorber. By combining Monte Carlo simulations with \texttt{Geant4}-based modeling, we show that these secondary cascades yield a substantial mCP flux that complements the primary production from the interaction point. For the proposed FORMOSA detector, this contribution can enhance the expected signal yield by approximately 50% for mχ≲0.1GeV. Our results demonstrate that secondary production in downstream infrastructure is an essential ingredient for realistic sensitivity projections and new-physics searches at the High-Luminosity LHC. The simulated secondary spectra are made publicly available to facilitate future forward physics studies.
@article{arxiv.2605.13964,
title = {A New Source of Millicharged Particles: Secondary Showers in the LHC Forward Absorber},
author = {Jyotismita Adhikary and Peiran Li and Zhen Liu and Sebastian Trojanowski and Azam Zabihi},
journal= {arXiv preprint arXiv:2605.13964},
year = {2026}
}