MuSEUM Experiment

Welcome to the MuSEUM (Muonium Spectroscopy Experiment Using Microwave) project. Our collaboration is dedicated to the ultra-precise measurement of the hyperfine structure (HFS) of muonium, an exotic atom consisting of a positive muon and an electron. As a simple two-lepton system, muonium is an ideal laboratory for testing the predictions of Quantum Electrodynamics (QED) and the Standard Model of particle physics without the complications of nuclear structure.

By performing high-precision microwave spectroscopy of muonium’s energy levels, we aim to determine fundamental physical constants, such as the muon-to-electron mass ratio and the muon magnetic moment, with unprecedented precision. These measurements provide a comprehensive test of the Standard Model and offer a window to search for new physics beyond it, including explorations of lepton universality and Lorentz invariance.

Our experiment is located at the J-PARC Materials and Life Science Experimental Facility (MLF), home to the world’s most intense pulsed muon beam. Leveraging this powerful beam and innovative techniques like Rabi oscillation spectroscopy, we are pushing the frontiers of precision measurement. The goal is to improve upon the last major measurement from 1999 by an order of magnitude, opening new avenues for discovery.

news

Jul 01, 2025	Website Renewal

selected publications

Improved Measurements of Muonic Helium Ground-State Hyperfine Structure at a Near-Zero Magnetic Field

P. Strasser, S. Fukumura, R. Iwai, and 9 more authors

Phys. Rev. Lett., Dec 2023

DOI
Rabi-oscillation spectroscopy of the hyperfine structure of muonium atoms

S. Nishimura, H. A. Torii, Y. Fukao, and 26 more authors

Phys. Rev. A, Aug 2021

DOI
New precise spectroscopy of the hyperfine structure in muonium with a high-intensity pulsed muon beam

S. Kanda, Y. Fukao, Y. Ikedo, and 20 more authors

Physics Letters B, Feb 2021

Abs DOI

A hydrogen-like atom consisting of a positive muon and an electron is known as muonium. It is a near-ideal two-body system for a precision test of bound-state theory and fundamental symmetries. The MuSEUM collaboration performed a new precision measurement of the muonium ground-state hyperfine structure at J-PARC using a high-intensity pulsed muon beam and a high-rate capable positron counter. The resonance of hyperfine transition was successfully observed at a near-zero magnetic field, and the muonium hyperfine structure interval of νHFS=4.463302(4)GHz was obtained with a relative precision of 0.9 ppm. The result was consistent with the previous ones obtained at Los Alamos National Laboratory and the current theoretical calculation. We present a demonstration of the microwave spectroscopy of muonium for future experiments to achieve the highest precision.