pion

American  

[pahy-on] / ˈpaɪ ɒn /

noun

Physics.

1. 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. π

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pion British  

/ ˈpaɪɒn /

noun

1. 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

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pion Scientific  

/ pī′ŏn′ /

1. 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.

2. Also called pi-meson

3. See Table at subatomic particle

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Etymology

Origin of pion

First recorded in 1950–55; pi (meson) + -on ¹

Example Sentences

In this case we study how the B meson decays into four other subatomic particles – a kaon, a pion and two muons.

From Science Daily • May 26, 2026

They had already found the pion for which Prof Powell would be awarded the Nobel Prize in 1950.

From BBC • Jul. 23, 2024

The negative pion can interfere with other negative pions.

From Scientific American • Jan. 11, 2023

The positive pion can interfere with other positive pions caused by other atomic flybys.

From Scientific American • Jan. 11, 2023

The pawn at chess is Fr. pion, a pawn, formerly also a foot-soldier, used contemptuously in modern French for a junior assistant master.

From The Romance of Words (4th ed.) by Weekley, Ernest