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An exotic atom, also called the pi-mu atom, which is similar in structure to the hydrogen atom but with the proton replaced by a pion and the electron replaced by a muon. Pionium is unique among atoms that have been observed in the laboratory in that all of its constituents are unstable particles not found in ordinary matter. Pionium is formed during the decay of a certain heavier particle called the neutral kaon. A kaon has many modes of decay, one of which results in the formation of a pion, a muon, and a neutrino. See Elementary particle

The nomenclature of exotic atoms is not well established, and the name pionium may also refer to the pion-electron atom.

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


(particle physics)
An exotic atom consisting of a muon orbiting about an oppositely charged pion. Also known as pi-mu atom.
An exotic atom consisting of an electron in orbit about an oppositely charged pion.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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In order for the paper to be self-contained let's recall that the pionium is formed by a [[pi].sup.+] and [[pi].sup.-] mesons, the positronium is formed by an antielectron (positron) and an electron in a semi-stable arrangement, the protonium is formed by a proton and an antiproton also semi-stable, the antiprotonic helium is formed by an antiproton and electron together with the helium nucleus (semi-stable), and muonium is formed by a positive muon and an electron.
From the exotic atom, the pionium, positronium, protonium, antiprotonic helium, and muonium are unmatter.