Presently, the Standard Model of particle physics precisely explains most of the experimental results of elementary particle physics. However, phenomena that cannot be explained by the Standard Model exist, such as the origin of neutrino mass and the existence of dark matter, and therefore the discovery of new physics is now crucial.

The anomalous magnetic dipole moment (g-2) of muons, the second-generation leptons, is a physics observable that can be predicted precisely by the Standard Model. However, there has been a discrepancy between its value measured by a previous experiment and its predicted value from the Standard Model. This discrepancy suggests the contribution of new physics. We are currently preparing for the Muon g-2/EDM experiment at the Japan Proton Accelerator Research Complex (J-PARC), a high-intensity proton accelerator facility in Tokai, Japan. This experiment will measure the g-2 with a precision of 0.1 parts per million, and search for new physics from the ultra-precision-measurement frontier. Additionally, the electric dipole moment (EDM) of muons will be precisely measured. Observation of EDM at the expected sensitivity of this experiment will indicate a CP violation in the leptonic sector, providing a clue to new physics.

These dipole moments can be measured by observing the spin movements (precession) of muons in a magnetic field. This experiment utilizes a high-intensity beam of surface muons produced at about a hundred million muons per second by the J-PARC Material and Life Science Experimental Facility (MLF). By first cooling muons to room temperature and then accelerating them, this facility produces a highly-collimated muon beam with uniform momenta. This beam is accumulated by injecting it into a highly-uniform 3 Tesla magnetic field, which is realized using a technology developed for medical MRI devices. Muons eventually decay into positrons and neutrinos, and positron tracks are detected by a silicon-strip detector placed in a magnetic field. Since positrons tend to appear in the direction of the spin of muons, muon g-2 and EDM can be measured by observing the temporal change of the detected positron. This experiment uses a completely different method from previous experiments, which include the generation of highly-collimated muon beams, accumulation in a highly precise magnetic field of 3 Tesla, and the measurement of positrons using a track detector. At N Lab, we are working on the development of silica aerogel targets, which are used for the muonium production for the cooling of surface muons, and the development of a beam monitor installed in the linear accelerator, to commit to the realization of the project.

Left: Conceptual design of J-PARC Muon g-2 / EDM precision measurement experiment. Right: A photo taken during the beam test of muon acceleration, conducted at the J-PARC MLF in November 2019.

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