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A short-lived association of an electron and a positron bound together in a configuration resembling the hydrogen atom.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(Atomic Physics) physics a short-lived entity consisting of a positron and an electron bound together. It decays by annihilation to produce two or three photons
[C20: from positron + -ium]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014


(ˌpɒz ɪˈtroʊ ni əm)

a short-lived atomic system consisting of a positron and an electron bound together.
[1945; positron + -ium2]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
References in periodicals archive ?
The singlet state referred to as parapositronium, may only decay into an even number of photons due to charge parity conservation, and has a lifetime (in vacuum) of 0.125 ns.
It follows from the CP-invariance of electromagnetic interactions, while the annihilation modes (the even number of [[gamma].sub.a]-quanta for parapositronium [sup.S]Ps and the odd number for TPs) are determined by conservation of the charge parity (C) and the total angular momentum (the spin, as the ground states of Ps have no orbital angular momentum).
The o-Ps species localizes itself in free volume cavities of radius 0.2-0.6 nm, a range which correlates to the non-bonded interatomic distances in polymers and the molecular radii of diffusing substances [21, 42], Analysis of the PALS data was carried out in terms of three lifetime components: [[tau].sub.1] which is attributed to parapositronium (p-Ps) annihilation; [[tau].sub.2] which is associated with free positron and positron-molecular species annihilation; and [[tau].sub.3] which is attributed to o-Ps annihilation.
A small contribution coming to [[tau].sub.1] from parapositronium atoms of lifetime 125 ps is ignored as the intensity of this component, one-third that of the orthopositronium intensity 73, is negligibly small.
The production of parapositronium via the collision of photons is analogous to pion production in two-photon interactions and Higgs production via gluon-gluon fusion.
This explains why orthopositronium, where the particle (electron and positron) spins are parallel and hence the magnetic component of the electromagnetic interaction is repulsive, has a higher energy state than parapositronium where the particle spins are anti-parallel and the magnetic component of QED is attractive.