Muonium is an exotic atom made up of an antimuon and an electron, which was discovered in 1960 and is given the chemical symbol Mu. During the muon's 2.2 µs lifetime, muonium can enter into compounds such as muonium chloride (MuCl) or sodium muonide (NaMu). Due to the mass difference between the antimuon and the electron, muonium (μ+e−) is more similar to atomic hydrogen (p+e−) than positronium (e+e−). Its Bohr radius and ionization energy are within 0.5% of hydrogen, deuterium, and tritium.
Although muonium is short-lived, physical chemists study it using Muon spin spectroscopy (μSR), a magnetic resonance technique analogous to nuclear magnetic resonance (NMR) or electron spin resonance (ESR) spectroscopy. Like ESR, μSR is useful for the analysis of chemical transformations and the structure of compounds with novel or potentially valuable electronic properties. Muonium is usually studied by muon spin rotation, in which the Mu atom's spin precesses in a magnetic field applied transverse to the muon spin direction (since muons are typically produced in a spin-polarized state from the decay of pions), and by avoided level crossing (ALC), which is also called level crossing resonance (LCR). The latter employs a magnetic field applied longitudinally to the polarization direction, and monitors the relaxation of muon spins caused by "flip/flop" transitions with other magnetic nuclei.
Because the muon is a lepton, the atomic energy levels of muonium can be calculated with great precision from quantum electrodynamics (QED), unlike in the case of hydrogen, where the precision is limited by uncertainties related to the internal structure of the proton. For this reason, muonium is an ideal system for studying bound-state QED and also for searching for physics beyond the standard model.
True muonium vs. "muium"
The name "muonium" is, strictly speaking, etymologically incorrect, as this terminology is supposed to be reserved for bound states of a particle with its own antiparticle—in this case "true muonium", an exotic atom composed of a muon and an antimuon, which has never yet been observed. It may have been generated in the collision of electron and positron beams but has not been searched for in the particle debris.
An atom composed of a positively charged particle bound to an electron is supposed to be named after the positive particle with "-ium" appended, in this case "muium", which is hard to say (though not as hard as "piium" or "kaium"). For better or worse, the present convention is now firmly entrenched.
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- S.J. Brodsky, R.F. Lebed (2009). "Production of the smallest QED atom: True muonium (µ⁺µ⁻)". Physical Review Letters 102 (21): 213401. arXiv:0904.2225. Bibcode:2009PhRvL.102u3401B. doi:10.1103/PhysRevLett.102.213401.
- H. Lamm, R.F. Lebed (2013). "True Muonium (µ⁺µ⁻) on the Light Front: A Toy Model". arXiv:1311.3245.
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