World's first cooling and acceleration of muon
〜The first muon accelerator finally coming to a reality.

Executive summary●Question
If muons can be accelerated in an accelerator, it is expected to be useful in avariety of fields such as elementary particle physics, material and life sciences,and earth science. For example, such muons are useful for ultra-precise
measurement of anomallous magnetic moment (g-2) and electric dipole
moment (EDM) to study new theory beyond the standard model of
elementary particles. But accelerating them is technically difficult.

●Findings
Generally, muons created in an accelerator have large variations in direction
and speed, making them unsuitable for acceleration. However, if a muon has apositive charge, it can be decelerated until it almost stops, and the direction
and speed can be made uniform (cooled). For the first time in the world, theresearch group succeeded in accelerating a positive muon to approximately
4% of the speed of light.

●Meaning
The research group demonstrated cooling and acceleration of positive muon
for the first time after continuous development of cooling and acceleration
technologies in the past. This is a major step towards enabling ultra-precise
survey of physics beyond the standard model. In addition, this technology
offers wide range of applications such as muon microscopy and otherinterdisciplinary research areas.

Figure 1 Cooling and accelerating a positive muon beam1

Figure 2 Experimental set up for muon cooling and acceleration at the J-PARC Materials andLife Science Experimental Facility (MLF). A positive muon beam enters from the right side,
is cooled in the muon cooling chamber on the right and is accelerated in the radiofrequency acceleration cavity on the left. The accelerated muon beam is measured by adiagnostics system at the exit of the acceleration cavity.

Overview
A muon is an elementary particle like an electron. Muons were first discovered in 1936
as cosmic rays falling from the sky. Natural muons originated from cosmic rays havebeen used to see through the interior of large and/or thick objects, such as pyramids.
Presently, muons can be produced in much higher intensity using accelerators for theuse of various research and applications.
Creation of muons in an accelerator is in three steps. First, protons are accelerated tonear the speed of light. When the accelerated protons hit material such as graphite,they create particles called pions, which decay to form muons.
At J-PARC, the high intensity proton accelerator in Tokai village, Ibaraki prefecture,
approximately 100 million muons are produced per second. However, the direction andspeed of each muon in the beam are quite different. There are some experiments where
it can be used as is. But it is not suitable for a new experiment to measure of muon g2/EDM with ultra-precision.
There are two types of muons with a negative charge and a positive charge, and they
have a particle/antiparticle relationship. A muon with a positive charge is called apositive muon, and it can be decelerated until it almost stops to align its direction andspeed (cooling). After they come to a near stop, they are accelerated by an electric2

field. At the end of this process, they become a highly directional.
However, muons are difficult to accelerate because they do not have the same directionor speed. The acceleration cavity used for acceleration is like a narrow tube. If thedirections are different, it will not be possible to efficiently put the muons into the tube.
Also, if the speeds are widely distributed, the efficiency of acceleration will be poor.
At J-PARC, positive muons having a speed of about 30% the speed of light produced ina proton accelerator are shot into a material called silica aerogel. Positive muonscombine with electrons in silica aerogel to form neutral atoms called muoniums. By
irradiating the muonium with a laser to strip off an electron, we obtain positive muonsthat have been slowed down (cooled) to a nearly stopped velocity at around 0.002%
of the speed of light.
After that, a radio-frequency electric field is applied to accelerate the positive muons.
Since the positive muon is almost stationary, the more it accelerates, the more
directional it becomes, creating a much more directional muon beam that can be used
for a variety of applications.
A joint research group consisting of KEK, Okayama University, Nagoya University,
Kyushu University, Ibaraki University, Japan Atomic Energy Agency, and Niigata
University conducted an experiment at the muon experimental facility in the J-PARC
MLF. By combining muon cooling technology and radio-frequency acceleration
technology, the research group successfully demonstrated that positive muons
accelerate to approximately 4% of the speed of light for the first time in the world.
Muons have a lifetime of only about 2 microseconds (2 millionths of a second), and ifthey are not accelerated quickly, they will decay. Also, since it is 200 times heavier thanan electron, it will need to be accelerated in stages, the research group plans to continue
developing the remaining parts of acceleration technology and eventually accelerate it
to 94% of the speed of light.
With the initial part of the muon acceleration technology now demonstrated, the world's
first muon accelerator is now within sight. The year 2024 is the first year of muon
acceleration. New research opportunities using accelerated muons will follow.

Research group
J-PARC center, Particle & Nuclear Physics Division, Materials & Life Science Division
Muon and Neutron group, Institute of Particle and Nuclear Studies, High Energy Accelerator Research
Organization (KEK)
Muon Science Laboratory, Institute of Materials Structure Science, KEK
Accelerator Laboratory, KEK
Division of Quantum Universe, Research Institute for Interdisciplinary Science, Okayama University

3

Nagoya University, Graduate School of Science, Department of Physics, High Energy Physics Laboratory
Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University
Experimental Particle Physics Laboratory, Department of Physics, Faculty of Science, Kyushu University
Research Center for Advanced Particle Physics, Kyushu University
Ibaraki University Graduate School of Science and Engineering, Institute of Quantum Beam Science,
Elementary particle physics laboratory
Japan Atomic Energy Agency
High Energy Physics Laboratory, Faculty of Science, Niigata University
Quantum Research Center, Institute for Research Administration, Niigata UniversityContacts
High Energy Accelerator Research Organization(KEK, Japan)
Dr. Tsutomu Mibe
Email : mibe@post.kek.jp
About KEK
PR office, High Energy Accelerator Research Organization (KEK, Japan)
Email : press＠kek.jp
About J-PARC
PR section, J-PARC
Email : pr-section＠j-parc.jp

4