Muon Beams Manipulated
About every second, the average human has a rare messenger from the edge of space passing through their body. Muons―similar to electrons but with 200 times the mass―are created naturally when cosmic ions strike the upper atmosphere, producing a shower of particles. Muons can also be created artificially, but these muon beams are very sparse compared to more conventional electron, proton, and ion beams. Over the past few decades, researchers have developed a way to make much denser beams [1, 2], but the difficulty of working with such beams has kept scientists from fulfilling their potential as a research tool. Now a team at the Japan Proton Accelerator Research Complex (J-PARC) has successfully demonstrated the capability to manipulate a dense muon beam, accelerating the muons in a radio-frequency device for the first time [3]. Muon beams have previously been accelerated using electrostatic fields, but such methods cannot reach the >100 MeV particle energies that researchers would like to achieve. The J-PARC team’s use of radio-frequency acceleration is an important step toward extending the range of application of muon beams and could deliver new ways of probing the limits of the standard model of particle physics.
Despite the extremity of their natural origins, artificially produced muons are used daily in laboratories around the world, where they enable, for example, precise measurements of new materials. They might also offer clues to new physics. Because of the weird nature of quantum physics, every muon acts like a tiny bar magnet as it flies through space. The orientation of this dipole precesses as the muon travels through a magnetic field, with the frequency of the precession indicating the strength of the muon’s magnetic moment. Precise measurements of this magnetic moment were performed by the g – 2 team at the Fermi National Accelerator Laboratory (Fermilab), Illinois, who observed muons in a very-well-characterized magnetic field. Tiny deviations of the spin precession from the expected value were observed, forcing physicists to question their understanding of the way forces and particles interact on a fundamental level (see Special Feature: The Muon g–2 Anomaly Explained, Viewpoint: Muon’s Escalating Challenge to the Standard Model, and Research News: Muon Experiment Calls It a Wrap).
Resolving this discrepancy is a top physics priority, but to make progress, researchers need intense muon beams with high and uniform energies. Muon beams are usually produced artificially by firing high-energy protons onto a graphite or metal target. Like cosmic rays striking the upper atmosphere, the protons have enough energy to shatter nuclei in the target, creating showers of unstable particles called pions. These pions, which survive for only a few billionths of a second, decay predominantly into muons, which emerge from the target with a broad range of trajectories. Beam scientists have several tools for marshaling charged particles into energetic, coherent beams, but those techniques are not practical for muons. Magnetic lenses, for example, can exchange a spread in the particles’ propagation angles for a spread in position and vice versa. But the way muons are created from exploding nuclei means that they are too energetic to be easily controlled in this manner. Creating a useful muon beam also means accelerating the particles to the desired energy. For more easily manipulable particles such as protons and electrons, this can be achieved using radio-frequency cavities, which contain an oscillating electromagnetic field. In principle, this technique is also applicable to muons, but whereas proton and electron beams are typically created with a small energy spread, the nuclear origin of muons means that these particles encompass a much greater energy range. This huge range would cause the flux of the resultant beam to be too low to be useful.
One approach to overcoming these challenges splits the problem into two steps: concentrating the particles then accelerating them. The first step, which has been previously demonstrated [1, 2], increases the muon density by slowing the particles down dramatically. In this technique, known as laser ionization cooling, the muons released from the target are stopped in a very-low-density aerogel sponge. Here, the muons bind to electrons in the aerogel to form muonium, a hydrogen-like atom made up of a muon and an electron, which drifts through the material. When the muonium particles emerge from the aerogel, a powerful laser is used to knock off the electrons, leaving bare muons in a compact, extremely-low-energy beam.
The J-PARC team has now demonstrated the second step, in which the cold muon beam produced by laser ionization cooling is subsequently reaccelerated. The team did this by passing the beam through a special radio-frequency cavity known as a radio-frequency quadrupole (RFQ), a device normally used for accelerating low-energy proton beams. In an RFQ, an oscillating electric field tightly focuses the particles to stop them from escaping transversely while giving them a boost longitudinally, increasing their energy before they spill out of the end of the device. Crucially, the success of this reacceleration process depends on the prior development of the laser ionization stage: The aperture of the RFQ is too small to accept a conventional muon beam, necessitating the initial collimation step, and the RFQ’s 324-MHz oscillation frequency demands a short muon pulse that can fit within a single 3-ns radio-frequency period.
Although the beam produced by the J-PARC team is of good quality (in terms of having low emittance), its energy and intensity are not yet high enough for the experiments that researchers eventually hope to make. Nevertheless, the demonstration of the potential to reaccelerate cold muons is an exciting step forward. Other groups are investigating alternative techniques for producing such beams, for example, using a novel combination of crossed electric and magnetic fields to cool the muons [4, 5] or using higher-energy ionization cooling [6, 7], which can be applied to relativistic muons.
Whatever the technique behind their production, muon beams are an exciting and growing field, with application across a broad range of fundamental and applied sciences. When the team at J-PARC has achieved sufficiently high energies, one of the first uses of the beam will be to make an extremely precise measurement of the muon magnetic moment, using a far smaller device than that used by the Fermilab team. This will enable a complementary indirect probe of the standard model of particle physics. The J-PARC result also paves the way for acceleration of muons to record-breaking energies. Such a beam could be collided with a high-energy electron beam or another muon beam, allowing a direct test of the standard model of particle physics with a stringency comparable to that of the proposed Future Circular Collider at CERN but in a much smaller facility [8–10].
References
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- P. Bakule et al., “Pulsed source of ultra low energy positive muons for near-surface µSR studies,” Nucl. Instrum. Methods Phys. Res., Sect. B 266, 335 (2008).
- S. Aritome et al., “Acceleration of positive muons by a radio-frequency cavity,” Phys. Rev. Lett. 134, 245001 (2025).
- D. Taqqu, “Compression and extraction of stopped muons,” Phys. Rev. Lett. 97, 194801 (2006).
- A. Antognini et al. (muCool Collaboration), “Demonstration of muon-beam transverse phase-space compression,” Phys. Rev. Lett. 125, 164802 (2020).
- MICE Collaboration, “Demonstration of cooling by the Muon Ionization Cooling Experiment,” Nature 578, 53 (2020).
- MICE Collaboration, “Transverse emittance reduction in muon beams by ionization cooling,” Nat. Phys. 20, 1558 (2024).
- Y. Hamada et al., “µTRISTAN,” Prog. Theor. Exp. Phys. 2022 (2022).
- “US particle physicists want to build a muon collider — Europe should pitch in,” Nature 625, 423 (2024), editorial.
- K. R. Long et al., “Muon colliders to expand frontiers of particle physics,” Nat. Phys. 17, 289 (2021).