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[pahy-on] /ˈpaɪ ɒn/
noun, Physics.
the first meson to be discovered: it has spin 0 and may be positively or negatively charged or neutral; charged pions decay into a muon and a neutrino or antineutrino. Symbol: π.
Also called pi meson.
1950-55; pi (meson) + -on1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for pion
  • Place subatomic particles-pion or muons-on one side of the light cylinder, and a particle detector on the other side.
  • And then the lambda decays into a negative pion and a proton.
  • The time-reversed movie shows a pion and a deuteron colliding and producing two protons.
  • When physicists first observed the pion in cosmic ray experiments, they were puzzled.
  • The proton itself is apparently built from fractionally charged quarks, as is the neutron and the pion.
British Dictionary definitions for pion


(physics) a meson having a positive or negative charge and a rest mass 273.13 times that of the electron, or no charge and a rest mass 264.14 times that of the electron
Word Origin
C20: from Greek letter pi1 + on
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for pion

1951, from Greek letter pi + -on.

Online Etymology Dictionary, © 2010 Douglas Harper
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pion in Science
A meson occurring either in a neutral form with a mass 264 times that of an electron and a mean lifetime of 8.4 × 10-17 seconds or in a positively or negatively charged form with a mass 273 times that of an electron and a mean lifetime of 2.6 × 10-8 seconds. The pion was once believed to be the particle that mediates the strong force, which holds nucleons together in the nucleus; it is now believed that the gluon is the mediator particle. Pions do interact with nucleons, however, and are able to transform neutrons into protons and vice versa. Also called pi-meson. See Table at subatomic particle.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Related Abbreviations for pion


The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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