Eur. Phys. J. C (2026) 86: 140
https://doi.org/10.1140/epjc/s10052-026-15307-w

Regular Article - Theoretical Physics

Probing millicharged particles with NA64$\mu$ and LDMX

¹ Institute for Nuclear Research, 117312, Moscow, Russia
² Bogoliubov Laboratory of Theoretical Physics, JINR, 141980, Dubna, Russia
³ Millennium Institute for Subatomic Physics at the High-Energy Frontier (SAPHIR) of ANID, Fernández Concha 700, Santiago, Chile
⁴ Center for Theoretical and Experimental Particle Physics, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha 700, Santiago, Chile
⁵ Institut für Theoretische Physik, Universität Tübingen, Kepler Center for Astro and Particle Physics, Auf der Morgenstelle 14, 72076, Tübingen, Germany
⁶ ETH Zürich, Institute for Particle Physics and Astrophysics, 8093, Zürich, Switzerland
⁷ CSIC-Universitat de València, Instituto de Física Corpuscular (IFIC), 46980, Paterna, Spain
⁸ Matrosov Institute for System Dynamics and Control Theory SB RAS, Lermontov Str., 134, 664033, Irkutsk, Russia
⁹ Tomsk Polytechnic University, 634050, Tomsk, Russia

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Received: 23 May 2025
Accepted: 11 January 2026
Published online: 11 February 2026

Abstract

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$\mu$ and LDMX, to the parameter space of millicharged particles. For the NA64$\mu$ experiment, with an anticipated muon flux of $\text{MOT}\lesssim {10}^{14}$, our analysis reveals a detectable mass window of $10\text{MeV}\lesssim m_{\chi }\lesssim 150\text{MeV}$ and charge parameter range ${10}^{-4}\lesssim ϵ\lesssim 7\times {10}^{-4}$. This sensitivity arises from the bremsstrahlung-like missing energy signature $\mu N\to \mu N\gamma _{}^{\ast }(\to \chi \overline{\chi })$. Furthermore, we evaluate the discovery potential of the LDMX facility, considering its projected electron beam statistics, $\text{EOT}\lesssim 2\times {10}^{16}$, and energy, $E_{\text{e}}\simeq 8\text{GeV}$. Our results demonstrate that LDMX can probe heavier MCPs in the mass range $250\text{MeV}\lesssim m_{\chi }\lesssim 400\text{MeV}$, with sensitivities reaching ${10}^{-3}\lesssim ϵ\lesssim 1.5\times {10}^{-3}$. This parametric window can be accessible through the distinctive invisible decay channel $\rho \to \chi \overline{\chi }$, where $\rho$-meson photo-production $\gamma N\to N\rho$ plays a pivotal role.