First measurement of the low-momentum negative-muon beam for the muon-to-electron conversion experiment at J-PARC
The COMET Phase- experiment successfully demonstrated the transport of low-momentum negative muons through the Transport Solenoid to the experimental area in 2023, providing the first experimental benchmark of beam characteristics and validating simulation models despite a systematic 30% yield discrepancy.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the subatomic world, particles usually obey a strict rulebook: a muon, a heavy cousin of the electron, should eventually decay into an electron and some invisible particles, but it should never simply turn into an electron while sitting still inside an atom. Physicists have long searched for a rare exception to this rule, a process where a muon stops in a metal target and transforms directly into an electron without any other particles appearing. If this transformation were found, it would shatter the Standard Model of particle physics, the current best theory of how the universe works, and hint at entirely new forces or particles. To catch this fleeting event, scientists need a machine that can deliver a very specific kind of beam: a stream of slow-moving, negatively charged muons, carefully filtered and guided to a precise stopping point. The challenge is that these particles are difficult to produce and even harder to steer, especially when they are moving slowly.
At the Japan Proton Accelerator Research Complex, a team of researchers recently took a crucial step toward building the machine needed for this search. They conducted the first test run of a new facility designed to capture and transport these slow muons. The experiment, known as Phase-α, was not the final search for the transformation itself, but rather a vital engineering test to prove that the facility could actually deliver the beam it was designed to carry. The researchers fired a powerful beam of protons at a target to create a shower of new particles, then used a massive, curved magnetic tunnel to sort through the debris. Their goal was to see if they could successfully guide the slow, negative muons through this tunnel and into the experimental area, where they would stop and wait for the rare transformation to occur.
The team used a specialized detector called a Range Counter to count the muons that made it through the journey. This device works like a trap: it lets charged particles enter, slows them down with layers of graphite, and stops them in a block of copper. When a muon stops in the copper, it eventually decays, emitting an electron a short time later. By waiting for this delayed electron, the detector can confirm that a muon had stopped there, distinguishing it from the chaotic background of other particles. The researchers found that they had indeed succeeded in transporting a measurable beam of slow negative muons to the experimental area. The beam was not a wild spray of particles; it was concentrated in the low-momentum region, meaning the muons were moving at the slow speeds required for the future experiment.
The team also mapped out the shape of the beam as it traveled through the curved magnetic tunnel. They discovered that the beam did not travel in a perfectly straight line or a uniform circle. Instead, it drifted in a specific direction, with the beam yield being more strongly suppressed when moving the detector upward compared to moving it downward. This pattern matched the predictions of physics for how charged particles behave in a curved magnetic field, confirming that the magnetic forces were guiding the beam exactly as the designers intended. The researchers compared their real-world measurements with a detailed computer simulation of the entire process. While the simulation correctly predicted the general shape and behavior of the beam, the actual number of muons they counted was about thirty percent higher than the computer had estimated.
This difference is not a failure but a valuable discovery. It tells the scientists that their current models of how these particles are produced and how they interact with the materials in the beamline are slightly incomplete. The fact that the beam arrived at all, and that its behavior matched the expected physics of the magnetic field, proves that the facility is ready for the next stage. The experiment successfully demonstrated that the complex system of magnets and tunnels can deliver the slow, negative muons needed for the main search. With this first benchmark established, the team now has real data to refine their computer models, ensuring that when the full-scale experiment runs, they will know exactly how many muons to expect and where they will be. This successful test run marks the transition from building the machine to understanding how it works, laying the necessary foundation for the future search for new physics.
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