An international team led by scientists at Sun Yat-sen University and the Institute of Modern Physics of the Chinese Academy of Sciences has launched an ambitious experiment known as MACE. The project is designed to look for an extremely rare event in which muonium, a short lived system made of a positive muon bound to an electron, spontaneously transforms into antimuonium, its antimatter counterpart. Observing such a process would break a fundamental rule of particle physics called lepton flavor conservation, which is a key part of the Standard Model, and would provide direct evidence for physics beyond existing theories.

"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."

Pushing Experimental Sensitivity to New Limits

The most recent experimental constraint on muonium converting into antimuonium was established in 1999 at the Paul Scherrer Institute in Switzerland. MACE aims to go far beyond that result by improving sensitivity by more than a hundred times, with the goal of detecting conversion probabilities as small as O(10-13). Reaching this level requires advances throughout the entire experimental system, including a powerful surface muon beam, a newly developed silica aerogel target, and detectors capable of extremely precise measurements.

"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."

What a Discovery Could Reveal

If the experiment succeeds, it could allow scientists to explore new physics at energy scales ranging from 10 to 100 TeV, a level that rivals or even surpasses what future particle colliders are expected to achieve. MACE is also planned to operate in an initial Phase I stage, during which it will investigate other exceptionally rare muonium decay processes and lepton flavor violating events, including M→γγ and μ→eγγ, with record breaking sensitivity.

The impact of MACE extends beyond fundamental physics. Technologies developed for the experiment, such as advanced muonium production targets, low energy positron transport systems, and high resolution detectors, may also find uses in fields like materials science and medical research.

Strengthening Global Particle Physics Efforts

MACE is part of a larger scientific push centered on Huizhou’s major research facilities, including the High-intensity heavy-ion Accelerator Facility (HIAF) and the China initiative Accelerator Driven System (CiADS). Together, these projects aim to establish China as a global leader in high precision nuclear and particle physics. By drawing on these advanced facilities, MACE demonstrates how basic research can fuel both technological progress 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."