Japanese physicists have accelerated positive muons to 100 kiloelectronvolts. To do this, they created ultraslow muons through multiphoton ionization of muonium atoms and accelerated them in a high-frequency quadrupole. A report on the work is available on the preprint portal arXiv.org.
Muon accelerators could become important tools for both fundamental science (for example, for the precise measurement of the muon's anomalous magnetic moment) and applied applications. In particular, it will be possible to create a muon microscope with much higher penetrating power than an electron microscope for studying thick materials.
Building an efficient muon accelerator is a challenging task. An intense muon flux can be produced through the decay of pi mesons, which are formed by irradiating stationary targets with proton beams. The muons produced in this way occupy a large volume of phase space, so they must be cooled to form muon bunches, which then need to be accelerated. The problem lies in the short muon lifetime (on the order of two microseconds), making traditional particle cooling methods unsuitable. Scientists have already solved this problem by cooling the particles with cryogenic-temperature helium gas, but successful acceleration of muons after cooling has not yet been reported.
Physicists from the MUSE experiment at Japan's J-PARC proton accelerator solved both problems: they were able to cool muons after their generation and then accelerate them to 100 kiloelectronvolts. To do this, the scientists directed a stream of muons produced by the decay of pi mesons at an 8-millimeter-thick silicon aerogel (SiO2) target, both sides of which were irradiated with a pulsed laser. Some of the muons were slowed in the target, forming muonium atoms (𝜇+e-). The muonium atoms then decayed under the influence of laser photons, and the cooled muons were directed by an electrical field into the accelerator. The physicists used a high-frequency quadrupole approximately two meters long, with a peak power of 2.6 kilowatts and a frequency of approximately 324 megahertz. The scientists analyzed the beam characteristics using a horizontal bending magnet, a microchannel anode, and a beam profile monitor installed after the quadrupole in the diagnostic line.
As a result, the scientists accelerated positive muon beams to 100 kiloelectronvolts, which corresponds to approximately four percent of the speed of light in a vacuum. They estimated the muon cooling and extraction efficiency at 19 percent, and muon losses in the beams at three percent due to muon decay. The transverse normalized emittance of accelerated muons in the horizontal and vertical planes was 0.85𝝅 and 0.25𝝅 millimeters⋅milliradians, respectively, which, according to the scientists, corresponds to a phase space reduction of two orders of magnitude and demonstrates the accelerator's high efficiency.
According to physicists, the obtained results demonstrate the possibility of creating a muon accelerator for studying muons directly, as well as for other physics problems.
Read about how muons are already being studied in our article "Fallen from the Sky."
