Probing millicharged particles with NA64$\mu$ and LDMX

TL;DR

This paper evaluates the projected sensitivity of NA64μ and LDMX fixed-target experiments to detect millicharged particles, identifying specific mass and charge ranges where these experiments can effectively probe new physics beyond the Standard Model.

Contribution

It provides the first detailed sensitivity estimates for NA64μ and LDMX experiments targeting millicharged particles across specific mass and charge ranges, highlighting their discovery potential.

Findings

NA64μ can detect millicharged particles with masses 10-150 MeV and charges 10^{-4} to 7×10^{-4}.

LDMX can probe heavier millicharged particles with masses 250-400 MeV and charges 10^{-3} to 1.5×10^{-3}.

Both experiments can explore new parameter space for millicharged particles via missing energy and invisible decay signatures.

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