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Muonium (/mjuː.ˈoʊ.ni.əm/,mew-OH-nee-əm) is anexotic atom made up of anantimuon and anelectron,[1] which was discovered in 1960 byVernon W. Hughes[2] and is given the chemical symbol Mu. During the muon's2.2 µs lifetime, muonium can undergo chemical reactions.[3]
Because, like a proton, the antimuon's mass is much larger than that of the electron, muonium (μ+
e−
) is more similar toatomic hydrogen (p+
e−
) thanpositronium (e+
e−
). ItsBohr radius and ionization energy are within 0.5% ofhydrogen,deuterium, andtritium, and thus it can usefully be considered as an exotic light isotope of hydrogen.[4]
Although muonium is short-lived, physical chemists study it usingmuon spin spectroscopy (μSR),[5] a magnetic resonance technique analogous tonuclear magnetic resonance (NMR) orelectron 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 bymuon spin rotation, in which the muonium atom's spin precesses in amagnetic field applied transverse to the muon spin direction (since muons are typically produced in aspin-polarized state from the decay ofpions), and byavoided level crossing (ALC), which is also calledlevel crossing resonance (LCR).[5] 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 alepton, the atomic energy levels of muonium can be calculated with great precision fromquantum electrodynamics (QED), unlike in the case of hydrogen, where the precision is limited by uncertainties related to the internal structure of theproton. For this reason, muonium is an ideal system for studying bound-state QED and also for searching for physics beyond theStandard Model.[6][7]
Normally in the nomenclature of particle physics, an atom composed of a positively charged particle bound to an electron is named after the positive particle with "-ium" replacing an "-on" suffix, in this case "muium". Replacing "-on" with (or otherwise appending) "-onium" is mostly used forbound states of a particle with its own antiparticle. The exotic atom consisting of a muon and an antimuon (which is yet to be observed) is known astrue muonium.[citation needed]
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