Probing millicharged particles with NA64μ and LDMX

Millicharged particles emerge as compelling candidates in numerous theoretically well-motivated extensions of the Standard Model. These hypothetical particles, characterized by an electric charge that is a small fraction of the elementary charge, have attracted significant attention in contemporary experimental physics. Their potential existence motivates dedicated search strategies across multiple experimental platforms, leveraging their distinctive electromagnetic interactions while evading conventional detection methods. In the present paper we estimated the projected sensitivity of fixed-target experiments, specifically NA64 μ μ and LDMX, to the parameter space of millicharged particles. For the NA64 μ μ experiment, with an anticipated muon flux of MOT 10¹⁴ MOT ≲ 10 14 , our analysis reveals a detectable mass window of 10~ MeV m_χ 150~ MeV 10 MeV ≲ m χ ≲ 150 MeV and charge parameter range 10⁻⁴ ε 7× 10⁻⁴ 10 - 4 ≲ ϵ ≲ 7 × 10 - 4 . This sensitivity arises from the bremsstrahlung-like missing energy signature μ N → μ N γ *( → χ χ ̄) μ N → μ N γ ∗ ( → χ χ ¯ ) . Furthermore, we evaluate the discovery potential of the LDMX facility, considering its projected electron beam statistics, EOT 2× 10¹⁶ EOT ≲ 2 × 10 16 , and energy, Eₑ 8~ GeV E e ≃ 8 GeV . Our results demonstrate that LDMX can probe heavier MCPs in the mass range 250~ MeV m_χ 400 ~ MeV 250 MeV ≲ m χ ≲ 400 <mml:mspace/