Precision measurements of muonium and muonic helium hyperfine structure at J-PARC Patrick Strasser, Mitsushi Abe, Kanta Asai, Seiso Fukumura, Mahiro Fushihara, Yu Goto, Takashi Ino, Ryoto Iwai, Sohtaro Kanda, Shiori Kawamura et al. (13 more) Eur. Phys. J. D, 79 3 (2025) 20 Published online: 14 March 2025 DOI: 10.1140/epjd/s10053-025-00959-2