An international team has initiated the MACE experiment to detect a rare transformation of muonium into its antimatter counterpart, antimuonium. This process, if observed, would challenge the Standard Model of particle physics by violating lepton flavor conservation. The project aims to vastly improve upon previous searches conducted over two decades ago.
Led by researchers from Sun Yat-sen University and the Institute of Modern Physics of the Chinese Academy of Sciences, the MACE experiment targets an elusive event where muonium—a fleeting system of a positive muon bound to an electron—spontaneously converts to antimuonium. Such a discovery would signal new physics beyond the Standard Model, potentially revealing unknown forces or particles at high energy scales.
The research team describes the conversion as 'a clean and unique probe of new physics in the leptonic sector.' They emphasize its sensitivity to specific models, noting, '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.'
The last attempt to observe this effect occurred in 1999 at the Paul Scherrer Institute in Switzerland. MACE seeks to enhance sensitivity by over a hundredfold, targeting conversion probabilities around 10^{-13}. This requires innovations like a high-intensity surface muon beam, a silica aerogel target for muonium production, and advanced detectors to distinguish signals from background noise.
'Our design integrates advanced beam, muonium production target, and detector technology to isolate the signal from formidable backgrounds,' the team states. 'This makes MACE one of the most sensitive low-energy experiments searching for lepton flavor violation.'
In its initial Phase I, MACE will also probe other rare decays, such as muonium to two photons and muon to electron plus two photons, with unprecedented precision. A positive result could uncover physics at energies of 10 to 100 TeV, comparable to those of planned colliders.
Beyond fundamental insights, the experiment's technologies— including low-energy positron systems and high-resolution detectors—hold promise for applications in materials science and medical research. Hosted within Huizhou's research ecosystem, alongside facilities like the High-intensity heavy-ion Accelerator Facility and the China initiative Accelerator Driven System, MACE bolsters China's role in global particle physics. As the team puts it, 'We are not just building an experiment; we are opening a new window into the laws of nature.'