A New Source of Millicharged Particles: Secondary Showers in the LHC Forward Absorber

TL;DR

This paper identifies secondary particle showers in the LHC as a significant new source of millicharged particles, which can enhance detection prospects for future experiments.

Contribution

It quantifies secondary production of millicharged particles in the LHC environment using simulations, showing a 50% increase in expected signals for certain masses.

Findings

Secondary showers produce a substantial flux of millicharged particles.

Secondary production can increase signal yield by about 50%.

Simulated spectra are publicly available for future research.

Abstract

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 $m_\chi \lesssim 0.1~\textrm{GeV}$. 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.