Muons cooled and accelerated in Japan

In a world first, a research group working at the J-PARC laboratory in Tokai, Japan, has cooled and accelerated a beam of antimatter muons (µ⁺). Though muon cooling was first demonstrated by the Muon Ionisation Cooling Experiment in the UK in 2020 (CERN Courier March/April 2020 p7), this is the first time that the short-lived cousins of the electron have been accelerated after cooling – an essential step for applications in particle physics.

The cooling method is ingenious – and completely different to ionisation cooling, where muons are focused in absorbers to reduce their transverse momentum. Instead, µ⁺ are slowed to 0.002% of the speed of light in a thin silica-aerogel target, capturing atomic electrons to form muonium, an atom-like compound of an antimatter muon and an electron. Experimenters then ionise the muonium using a laser to create a near monochromatic beam that is reaccelerated in radiofrequency (RF) cavities. The work builds on the acceleration of negative muonium ions – an antimatter muon bonded to two electrons – which the team demonstrated in 2017 (CERN Courier July/August 2018 p8).

Though the analysis is still to be finalised, with results due to be published soon, the cooling and acceleration effect is unmistakable. In accelerator physics, cooling is traditionally quantified by a reduction in beam emittance – an otherwise conserved quantity that reflects the volume occupied by the beam in the abstract space of orthogonal displacements and momenta. Estimates indicate a beam cooling effect of more than an order of magnitude, with the beam then accelerated from 25 meV to 100 keV. The main challenge is transmission. At present one antimatter muon emerges from the RF for every 10 million, which impact the aerogel. Muon decay is also a challenge given that the muonium is nearly stationary in the laboratory frame, with time dilation barely extending the muon’s 2.2 μs lifetime. Roughly a third of the µ⁺ decay before exiting the J-PARC apparatus.

The first application of this technology will be the muon g-2/EDM experiment at J-PARC, where data taking is due to start in 2028. The experiment will add valuable data points to measurements thought to have exceptional sensitivity to new physics (CERN Courier May/June 2021 p25). In the case of the anomalous magnetic moment (g-2) of the muon, theoretical showdowns later this year may either dissipate or reinforce intriguing hints of beyond-the-Standard-Model physics from the Muon g-2 experiment at Fermilab, potentially adding strong motivation to an independent test.

We are very impressed with the progress of our colleagues at J-PARC and congratulate them on their success

“Although our current focus is the muon g-2/EDM experiment, we are open to any possible applications of this technology in the future,” says spokesperson Tsutomu Mibe of KEK. “We are communicating with experts to understand if our technology is of any use in a muon collider, but note that our method cannot be adapted for negative muons.”

While proposals for a µ⁺µ⁺ or µ⁺e^– collider exist, a µ⁺µ^– collider remains the most strongly motivated machine. “Much of the physics interest in e⁺e^– and µ⁺µ^– colliders comes from the annihilations of the initial particles into a photon and/or a Z boson, or a Higgs boson in the case of µ⁺µ^–,” says John Ellis of CERN/KCL. “These possibilities are absent for a µ⁺e^– or µ⁺µ⁺ collider, making them less interesting in my opinion.” From an accelerator-physics perspective, it remains to be demonstrated that the technique can deliver the beam intensity needed for an energy-frontier collider – not least while keeping the emittance low.

“We are very impressed with the progress of our colleagues at J-PARC and congratulate them on their success, says International Muon Collider study leader Daniel Schulte of CERN. “This will profit the development of muon-beam technology and use. We are in contact to understand how we can collaborate.”