Breakthrough in Muon Beam Technology Paves Way for Future Colliders

Researchers at the Japan Proton Accelerator Research Complex (J-PARC) have achieved a significant milestone in particle physics by successfully accelerating muons into a high-intensity beam with energies reaching 100 keV. This groundbreaking advancement is expected to enable next-generation experiments, enhancing measurements related to the muon's anomalous magnetic moment, which could reveal new physics beyond the Standard Model. The findings, reported in *Physical Review Letters*, underscore the potential of muon technology in collider applications.

Muons, subatomic particles similar to electrons but approximately 200 times heavier, offer unique advantages in high-energy physics. Their increased mass allows them to radiate less energy while traveling in circular paths, theoretically enabling more energetic collisions than conventional electron accelerators for the same energy input. However, the practical application of muons poses significant challenges, primarily due to their short lifespan of only 2 milliseconds. Traditional techniques for cooling muon beams, such as synchrotron radiation and laser cooling, are ineffective due to this brief existence.

To address these challenges, a team led by particle physicist Shusei Kamioka at the MUon Science Facility (MUSE) developed an innovative cooling and acceleration technique. This method involves cooling positively charged muons, or antimuons, to thermal energies of 25 meV before accelerating them using radio-frequency (rf) cavities. The researchers utilized a silica aerogel target, which allows a muon that stops within it to be re-emitted as a muonium atom—an exotic atom composed of an antimuon and an electron—at significantly lower thermal energy. Following this, a laser beam was employed to remove the electrons from the low-energy muonium atoms, resulting in antimuons with more uniform velocities.

The final beam produced by this process exhibited an intensity of 2 × 10⁻³ μ+ per pulse, with a remarkable reduction in emittance by factors of 200 in the horizontal direction and 410 in the vertical direction. This achievement represents a two-orders-of-magnitude decrease in the spread of particle positions and momenta, which enhances the efficiency of muon acceleration. Kamioka noted, "These improvements are crucial steps towards developing a functional muon collider."

Looking forward, the MUSE team aims to further increase the beam's energy and intensity, with plans for a 4 MeV acceleration with 1000 muons per second by 2027, and a more ambitious 212 MeV acceleration with 100,000 muons per second targeted for 2029. These advancements could yield a 100,000 to 1,000,000 fold increase in muon rates, facilitating critical experiments such as the muon g−2/EDM experiment at J-PARC.

This research not only highlights the technical prowess of the team at J-PARC but also emphasizes the potential implications for future collider technologies. If successful, muon colliders could open new frontiers in particle physics, enabling scientists to probe deeper into the fundamental components of matter and the forces governing the universe.

The findings thus represent a promising leap in particle accelerator technology, with implications that could reshape the landscape of high-energy physics research for years to come.