Muonic atoms and muonium spectroscopy

3 problems

• Fine-structure anomaly of 2p levels in heavy muonic atoms

  Partially resolved

  In muonic atoms of heavy elements such as zirconium, tin and lead, the measured gap between two closely spaced muon levels disagreed with theory for decades. Distortion of the nucleus by the muon turned out too small to explain it, and a 2025 calculation blames an incomplete treatment of the recoil of the nucleus instead.

• Ground-state hyperfine splitting of muonic hydrogen at the ppm level

  Open

  In muonic hydrogen the muon orbits very close to the proton, so the small magnetic splitting of its lowest level depends on how charge and magnetism are spread inside the proton. This splitting has never been measured directly, and several experiments are now trying.

• Muonium hyperfine and 1S-2S intervals at improved precision

  Open

  Muonium, an antimuon holding an electron, contains no proton, so its energy levels test the theory of light and charges without nuclear-structure uncertainty and give the muon mass and magnetism. Its best measurements date from about 25 years ago.