Exploring Lepton Flavor Violation
Led by researchers from Sun Yat-sen University, the Institute of Modern Physics of the Chinese Academy of Sciences, and multiple collaborating institutions across China, MACE is designed to search for the spontaneous conversion of muonium—a bound state of a positive muon and an electron—into its antimatter counterpart, antimuonium. Such a transition would violate lepton flavor conservation, a symmetry upheld by the Standard Model of particle physics, and point directly to new physics beyond current theory.
"The conversion of muonium to antimuonium represents a clean and unique probe of new physics in the leptonic sector," explains the research team. "Unlike other charged lepton flavor violation processes, this conversion is sensitive to ∆Lℓ = 2 models that are fundamentally distinct and could reveal physics inaccessible to other experiments."
From Concept to Cutting-Edge Detector Systems
The last experimental limit on muonium-to-antimuonium conversion was set in 1999 at the Paul Scherrer Institute in Switzerland. MACE aims to improve upon that limit by more than two orders of magnitude, targeting a conversion probability as low as O(10-13). Achieving this requires innovations across the entire experimental setup, including a high-intensity surface muon beam, a novel silica aerogel target, and a high-precision detector system.
"Our design integrates advanced beam, muonium production target, and detector technology to isolate the signal from formidable backgrounds," says the team. "This makes MACE one of the most sensitive low-energy experiments searching for lepton flavor violation."
Implications for Fundamental Science and Beyond
If successful, MACE could probe new physics at energy scales up to 10–100 TeV—comparable to or even exceeding the reach of proposed future colliders. The experiment also features a planned Phase-I stage, which will search for other rare muonium decays and lepton flavor violating processes, such as M→γγ and μ→eγγ, with unprecedented sensitivity.
Beyond particle physics, the technologies developed for MACE—including muonium production target, low-energy positron transport system, and high-resolution detectors—could benefit applications in materials science, medical applications, and beyond.
Advancing Global Particle Physics Research
MACE is part of a broader effort at Huizhou's large-scale scientific facilities, including the High-intensity heavy-ion Accelerator Facility (HIAF) and the China initiative Accelerator Driven System (CiADS), to position China at the forefront of high-precision nuclear and particle physics. By leveraging these world-class infrastructures, MACE exemplifies how fundamental research can drive technological innovation and international collaboration.
"We are not just building an experiment; we are opening a new window into the laws of nature," the team notes. "Each component of MACE—from the beamline to the software—has been optimized to explore physics that could redefine our understanding of matter, symmetry, and the universe itself."
The complete study is via by DOI: https://doi.org/10.1007/s41365-025-01876-0
