At the J-PARC Muon Science Facility (MUSE), the MuSEUM collaboration is now performing new precision measurements of the ground state hyperfine structure (HFS) of both muonium and muonic helium atoms. High-precision measurements of the muonium ground-state HFS are recognized as one of the most sensitive tools for testing bound-state quantum electrodynamics theory to precisely probe the standard model and determine fundamental constants of the positive muon magnetic moment and mass. The same technique can also be employed to measure muonic helium HFS, obtain the negative muon magnetic moment and mass, and test and improve the theory of the three-body atomic system. Measurements at zero magnetic field have already yielded more accurate results than previous experiments for both muonium and muonic helium atoms. High-field measurements are now ready to start collecting data using the world’s most intense pulsed muon beam at the MUSE H-line. We aim to improve the precision of previous measurements ten times for muonium and a hundred times or more for muonic helium. We review all the key developments for these new measurements, focusing on the high-field experiment, and report the latest results and prospects.
Muonic helium hyperfine structure measurements at J-PARC MUSE
Patrick
Strasser, Seiso
Fukumura, Yu
Goto, and
13 more authors
Precision microwave spectroscopy of the ground-state hyperfine structure in muonium provides a stringent test of the Standard Model in particle physics. The MuSEUM collaboration is preparing for such a measurement, aiming for precision down to approximately 1 ppb, utilizing the world’s most intense pulsed muon beam at J-PARC. The measurement of the Zeeman-split structure with an external magnetic field of 1.7T also precisely determines the muon’s magnetic moment (approximately 10 ppb). In the future, improved precision of the magnetic moment can be potentially obtained by measurements with different magnetic field strengths, however, it entails upgrading current cylindrical microwave cavities to rectangular ones. As the first step for the upgrade, we have developed a dual-mode rectangular cavity for the measurement with 2.9T field. The electromagnetic design and production have been established, and frequency sweeping with two desired modes has been successfully demonstrated. Moreover, the overall performance of the measurement at 2.9T field was evaluated with Monte Carlo simulations. These studies pave the way for a further extension of the MuSEUM experiment at various strengths of the magnetic fields.
Present Status of Spectroscopy of the Hyperfine Structure and Repolarization of Muonic Helium Atoms at J-PARC
Seiso
Fukumura, Patrick
Strasser, Mahiro
Fushihara, and
13 more authors
The MuSEUM collaboration is planning measurements of the ground-state hyperfine structure (HFS) of muonium at the Japan Proton Accelerator Research Complex (J-PARC), Materials and Life Science Experimental Facility. The high-intensity beam that will soon be available, the H-line, allows for more precise measurements by one order of magnitude. We plan to conduct two staged measurements. First, we will measure the Mu-HFS in a near-zero magnetic field, and thereafter we will measure it in a strong magnetic field. We have developed two microwave cavities for this purpose. Furthermore, we evaluated the systematic uncertainties from such a fluctuation of microwave fields and confirmed the requirements for the microwave system; we use a microwave field distribution calculated with the finite element method.
New precise spectroscopy of the hyperfine structure in muonium with a high-intensity pulsed muon beam
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.
2020
Performance evaluation of a silicon strip detector for positrons/electrons from a pulsed a muon beam
T.
Aoyagi, Y.
Honda, H.
Ikeda, and
26 more authors