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New source of millicharged particles: Secondary showers in the LHC forward absorber

Jyotismita Adhikary1,*, Peiran Li2,†, Zhen Liu2,‡, Sebastian Trojanowski1,3,§, and Azam Zabihi3,∥

  • *Contact author: jyotismita.adhikary@ncbj.gov.pl
  • †Contact author: li001800@umn.edu
  • ‡Contact author: zliuphys@umn.edu
  • §Contact author: sebastian.trojanowski@ncbj.gov.pl
  • ∥Contact author: azabihi@camk.edu.pl
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Phys. Rev. D 114, 015011 – Published 6 July, 2026

DOI: https://doi.org/10.1103/5s3t-x8l1

Abstract

Millicharged particles 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 target neutral beam absorber. By combining Monte Carlo simulations with geant4-based modeling, we show that these secondary cascades yield a substantial millicharged particle 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≲0.1  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.

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Phys. Rev. D 114, 015011– Published 6 July, 2026
  • Received 27 May 2026
  • Accepted 17 June 2026
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