In a bold and groundbreaking initiative, an international team of physicists led by researchers from Sun Yat-sen University, the Institute of Modern Physics of the Chinese Academy of Sciences, and several collaborating institutions across China have unveiled the conceptual design of an ambitious experiment known as the Muonium-to-Antimuonium Conversion Experiment (MACE). This experiment is poised to explore one of the most intriguing and consequential questions in the realm of particle physics: the potential violation of lepton flavor conservation through the spontaneous transformation of muonium into antimuonium. This phenomenon, if observed, would mark a revolutionary departure from the Standard Model, which has long held lepton flavor conservation as an unbroken symmetry, thus opening portals to hitherto unexplored physics.
Muonium, a rare exotic atom comprised of a positive muon (μ⁺) and an electron (e⁻), presents a unique testing ground for new theoretical physics beyond the Standard Model. The crux of the MACE project is to detect the conversion of ordinary muonium into its antimatter counterpart, antimuonium, wherein the constituents switch to a negative muon and a positron (the electron’s antiparticle). This hypothetical process directly contravenes the conservation of lepton flavor number, specifically implicating leptonic number changes (ΔL_ℓ = 2) that are incompatible with standard theory. Physicists have long sought evidence of such flavor violation as it offers unparalleled insights into symmetry breaking phenomena and could potentially link to mechanisms behind neutrino masses and the matter-antimatter asymmetry observed in the Universe.
What makes the MACE experiment particularly compelling is its methodological sophistication. The apparatus centers on a sophisticated magnetic spectrometer tasked with tracking the high-energy electrons emerging from decay events, a transport solenoid that meticulously filters and accelerates low-energy positrons, and an advanced detection system capable of pinpointing the positrons’ exact spatial coordinates along with the associated gamma rays produced during annihilation. This level of precision is pivotal for isolating the extremely rare conversion events from the overwhelming background noise inherent in such high-sensitivity searches.
The experimental goal is ambitiously stringent; where the most recent upper limit was set in 1999 by the Paul Scherrer Institute in Switzerland, MACE aims to improve sensitivity by over two magnitudes—targeting an exceptionally low conversion probability on the order of 10⁻¹³. To achieve this, researchers are integrating cutting-edge technology encompassing a high-intensity surface muon beam, newly developed silica aerogel targets optimized for muonium production, and ultra-precise detector modules. These synergistic innovations operationalize a testing framework far beyond anything currently existing, potentially setting new standards in low-energy precision experiments.
From a technical standpoint, the MACE experiment harnesses a high-intensity beam of surface muons—muons generated when pions decay near the surface of a production target, offering a stable and intense particle source essential for producing a significant number of muonium atoms. The novel silica aerogel target material catalyzes muonium formation while minimizing background interactions. The magnetic spectrometer, finely tuned via computational simulation and modeling, tracks charged particle trajectories with exquisite temporal and spatial resolution, enabling efficient discrimination of signal from noise. The positron transport system, utilizing a solenoid with carefully calibrated magnetic fields, ensures that only relevant low-energy positrons reach the detection array, preserving signal integrity.
Beyond the primary objective of detecting muonium-to-antimuonium conversion, the experiment plans a Phase-I stage that will broaden scientific horizons by searching for other rare muonium decay channels including M→γγ and μ→eγγ processes. These decay modes, highly suppressed within the Standard Model, are fertile grounds for signs of new physics. Sensitivity improvements promised by the novel setup are anticipated to deliver unprecedented constraints on these rare events, potentially reshaping theoretical models about flavor-changing neutral currents and charged lepton flavor violation.
The scientific implications of confirming muonium-to-antimuonium conversion extend far beyond the intricacies of particle interactions; they reach the very foundations of our understanding of matter, symmetry, and the forces that govern the Universe. The discovery would demonstrate lepton flavor violation at energy scales possibly as high as 10 to 100 TeV, rivaling or exceeding the probing power of future collider experiments. This would not only validate various proposed extensions to the Standard Model, such as supersymmetry, left-right symmetric models, or theories involving heavy Majorana neutrinos, but also provide tangible empirical clues about the origin of neutrino mass and the baryon asymmetry problem.
MACE is emblematic of a broader strategic vision within China to enhance the nation’s position at the frontier of precision nuclear and particle physics research. By leveraging large-scale facilities like the High-intensity Heavy-Ion Accelerator Facility (HIAF) and the China initiative Accelerator Driven System (CiADS), MACE exemplifies the synergy between fundamental science and technological innovation. These infrastructures enable the deployment of state-of-the-art particle beams and detection systems to achieve experimental sensitivities that were unthinkable merely decades ago.
Another fascinating aspect of MACE lies in the potential cross-disciplinary applications stemming from the technologies developed. For instance, the muonium production target concept, low-energy positron transport technology, and high-resolution detectors are broadly relevant to fields ranging from condensed matter physics to medical imaging. Improved positron sources and detection techniques could revolutionize positron emission tomography (PET) scanners, materials characterization, and other domains where understanding particle-matter interactions at micro and nanoscale are crucial.
Equally important is the international collaborative spirit driving MACE forward. The project harnesses a confluence of expertise in experimental design, beam physics, detector technology, and theoretical modeling from Chinese institutions allied with global scientific communities. This collaborative framework not only accelerates the pace of discovery but ensures that findings from MACE will be rigorously scrutinized and integrated into the larger corpus of high-energy physics knowledge.
The researchers emphasize that MACE is more than an experiment; it is a gateway to new physics. Every component, from the initial particle beamline to the data acquisition software, has been meticulously optimized to untangle signals that could redefine prevailing paradigms. As the project advances from conceptual design into construction and data collection phases, the scientific community watches keenly for evidence that may help unravel some of the Universe’s deepest mysteries.
The potential detection of muonium-to-antimuonium conversion, a process so exotic it challenges the very lexicon of particle physics, underscores humanity’s relentless quest to comprehend the fundamental forces and building blocks of reality. Should MACE succeed, it will mark a seminal milestone that not only affirms the bold theoretical visions postulating physics beyond the conventional but also paves the way toward new generations of experiments probing matter at unprecedented depths.
In sum, MACE represents a masterpiece of experimental ingenuity, scientific curiosity, and international cooperation. With its unprecedented sensitivity and innovative approach, it holds the promise of either confirming one of the most elusive phenomena in particle physics or setting new boundaries that will inspire yet more audacious theories. As the field edges toward an era defined by precision and discovery, MACE stands ready to illuminate the path forward.
Subject of Research: Not applicable
Article Title: Conceptual design of the Muonium-to-Antimuonium Conversion Experiment (MACE)
News Publication Date: 28-Jan-2026
Web References: https://doi.org/10.1007/s41365-025-01876-0
References:
- Jian Tang et al., “Conceptual design of the Muonium-to-Antimuonium Conversion Experiment (MACE),” Nuclear Science and Techniques, 28-Jan-2026.
Image Credits: Jian Tang
Keywords
Particle physics, Supersymmetry, Lepton flavor violation, Muonium, Antimuonium, High-precision detector, Magnetic spectrometer, Silica aerogel target, Low-energy positron transport, Computational modeling, Rare muonium decays, Beyond the Standard Model
