Research team achieves first-ever acceleration of positive muons to 100 keV

A team of engineers and physicists affiliated with a host of institutions across Japan, working at the Japan Proton Accelerator Research Complex, has demonstrated acceleration of positive muons from thermal energy to 100 keV—the first time muons have been accelerated in a stable way. The group has published a paper describing their work on the arXiv preprint server.

Muons are sub-atomic particles similar to electrons. The main difference is their mass; a muon is 200 times heavier than an electron. They are also much shorter lived. Physicists have for many years wanted to build a muon collider to conduct new types of physics research, such as experiments that go beyond the standard model.

Unfortunately, such efforts have been held back by the extremely short muon lifespan—approximately 2 microseconds—after which they decay to electrons and neutrinos. Making things even more difficult is their tendency to zip around haphazardly, which makes forming them into a single beam extremely challenging. In this new effort, the research team has overcome such obstacles using a new technique.

The team started by shooting positively charged muons into a specially designed silica-based aerogel, similar to that used for thermal insulation applications. As the muons struck the electrons in the aerogel, muoniums μ⁺e⁻ (an exotic atom consisting of a positive muon and an electron) were formed. The research team then fired a laser at them to remove their electrons, which forced them to revert back to positive muons, but with greatly diminished speed.

The following step involved guiding the slowed muons into a radio-frequency cavity, where an electric field accelerated them to a final energy of 100 keV, achieving approximately 4% of the speed of light.

The research team acknowledges that despite their achievement, building a working muon collider is still a distant goal. And while their technique might play a role in such a development, there are still problems that must be worked out, such as how to scale an apparatus to a usable size.

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