Conceptual Design of the Muonium-to-Antimuonium Conversion Experiment (MACE)

TL;DR

This concept paper analyzes the Muonium-to-Antimuonium Conversion Experiment (MACE) as a sensitive probe of charged lepton flavor violation and physics beyond the Standard Model. It integrates a high-intensity muon source, a vacuum muonium target (including single- and multi-layer silica aerogel designs), and a three-component detector system (Michel electron magnetic spectrometer, positron transport system, and positron detection system) to search for M- anti-M oscillations with a target conversion probability around $P\sim\mathcal{O}(10^{-13})$. The work presents a SMEFT-based theoretical framework, detailed detector concepts, and background studies, culminating in projected sensitivities (SES ≈ $1.3\times10^{-13}$ and upper limits near $\sim$few$\times10^{-13}$) and a Phase-I plan to validate detector performance and explore additional muon cLFV channels. If realized, MACE could reach sensitivity to new physics scales in the multi-10 TeV range, complementing high-energy searches and advancing our understanding of lepton flavor violation in the charged sector.

Abstract

The spontaneous conversion of muonium to antimuonium is one of the interesting charged lepton flavor violation phenomena offering a sensitive probe of potential new physics and serving as a tool to constrain the parameter space beyond the Standard Model. The Muonium-to-Antimuonium Conversion Experiment (MACE) is designed to utilize a high-intensity muon beam, a Michel electron magnetic spectrometer, a positron transport system, and a positron detection system, to either discover or constrain this rare process with a conversion probability of $\mathcal{O}(10^{-13})$. This article presents an overview of the theoretical framework as well as a detailed description of the experimental design for the search for muonium-to-antimuonium conversion.

Figures (51)

• Figure 1: The SMEFT tree-level diagram for muonium-to-antimuonium conversion with one $\Delta L_\mu=2$ four-fermion effective vertex. The conversion probability is proportional to $1/\Lambda^4$.
• Figure 2: Charged lepton flavor violating scattering $\mu^+e^-\to\mu^-e^+$ and $\mu^+\mu^+\to \ell^+\ell^+~(\ell=e,\tau)$.
• Figure 3: The new physics scale accessible to experiments searching for $\Delta L_\ell=2$ processes.
• Figure 4: Energy spectrum and the leading-order diagram of antimuonium decay $\overline{\text{M}}\to e^+e^-\bar{\nu}_e\nu_\mu$. The energy spectrum of the fast decay $e^-$ is accurate to next-to-leading-order, atomic shell $e^+$ spectrum assumes $1s$ muonium Zhao:2024qjb.
• Figure 5: Muonium conversion to antimuonium by the Standard Model weak interaction. This process is strongly suppressed by the tiny neutrino masses.