In particle physics, pion (short for pi meson) is the collective name for three subatomic particles: π⁰, π⁺ and π⁻. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force.

Pion
Classification

Subatomic particle

Boson

Hadron

Meson

Pion

Properties

Symbol:	$\backslash pi^\backslash pm$	$\backslash pi^0$
Mass:	139.57018(35) MeV/c2	134.9766(6) MeV/c2
Electric Charge:	±e	0
I(Iz), JP:	1(±1), 0−	1(0), 0−
Quark Composition:	1 up, 1 anti-down ($\backslash pi^+$) 1 down, 1 anti-up ($\backslash pi^-$)	1 up, 1 anti-up or 1 down, 1 anti-down

Basic properties

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Pions have zero spin and are composed of first-generation quarks. In the quark model, an up and an anti-down quark compose a π⁺, while a down and an anti-up quark compose the π⁻, its antiparticle. The neutral combinations of up with anti-up and down with anti-down have identical quantum numbers, so they are only found in superpositions. The lowest-energy superposition is the π⁰, which is its own antiparticle. Together, the pions form a triplet of isospin; each pion has isospin-1 (I = 1) and third-component isospin equal to its charge (I_z = +1, 0 or −1).

The $\backslash pi^\backslash pm$ mesons have a mass of 139.6 MeV/c² and a mean life of 2.6×10⁻⁸ seconds. They decay due to weak processes. The main decay mode (99.9877%) is into a muon and its neutrino:

$\backslash pi^+\backslash to\backslash mu^++\backslash nu\_\backslash mu,~~~\backslash pi^-\backslash to\backslash mu^-+\backslash bar\{\backslash nu\}\_\backslash mu.$

The second largest decay mode (0.0123%) is into an electron and the corresponding neutrino:

$\backslash pi^+\backslash to\; e^++\backslash nu\_e,~~~\backslash pi^-\backslash to\; e^-+\backslash bar\{\backslash nu\}\_e.$

The $\backslash pi^0$ meson has as slightly smaller mass of 135.0 MeV/c² and a much shorter mean life of 8.4×10⁻¹⁷ seconds. It decays due to electromagnetic force. The main decay mode (98.798%) is into two photons:

$\backslash pi^0\backslash to2\backslash gamma$.

Its second largest decay mode (1.198%) is the so-called Dalitz decay into a photon and an electron-positron pair:

$\backslash pi^0\backslash to\backslash gamma\; e^+e^-$.

The rate at which pions decay features prominently in many subfields of particle physics such as chiral perturbation theory. This rate is parametrized by the pion decay constant (f_π), which is about 90 MeV.

Particle	Symbol	Anti- particle	Quark Makeup	Spin and parity	Rest mass MeV/c2	S	C	B	Mean lifetime s	Decays to	Notes
Charged Pion	$\backslash mathrm\{\backslash pi^+\}$	$\backslash mathrm\{\backslash pi^-\}$	$\backslash mathrm\{u\; \backslash bar\{d\}\}$	Pseudoscalar	139.6	0	0	0	2.60×10-8	μ+ + νμ
Neutral Pion	$\backslash mathrm\{\backslash pi^0\}$	Self	$\backslash mathrm\{\backslash frac\{u\backslash bar\{u\}\; -\; d\; \backslash bar\{d\}\}\{\backslash sqrt\{2\}\}\}$	Pseudoscalar	135.0	0	0	0	0.84×10-16	2γ	Makeup inexact due to non-zero quark masses

History

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Theoretical work by Hideki Yukawa in 1935 had predicted the existence of mesons as the carrier particles of the strong nuclear force. From the range of the nuclear force (inferred from the radius of the nucleus), Yukawa predicted the existence of a particle having a mass of about 100 MeV. Initially after its discovery in 1936, the muon was thought to be this particle, since it has a mass of 106 MeV. However, later experiments showed that the muon did not participate in strong interactions. In modern terminology, this makes it a lepton, not a meson.

In 1947 the first true mesons, the charged pions, were found by the collaboration of Cecil Powell, César Lattes and Giuseppe Occhialini at the University of Bristol. Since the age of particle accelerators had yet to arrive in those days, high energies were only accessible from atmospheric cosmic rays. Photographic emulsions using the gelatin-silver process were placed for a long time in sites located at high altitude mountains (first at Pic du Midi de Bigorre in the Pyrenees and later at Chacaltaya in the Andes), where they were exposed to cosmic rays. After recovery of the plates, microscopic inspection of the emulsions revealed the tracks of charged particles. Pions were first identified by their unusual "double meson" tracks, left by their decay into another "meson" (the muon). In 1948, Lattes and Eugene Gardner first achieved artificial production of pion particles at the University of California at Berkeley cyclotron by bombarding carbon atoms with alpha particles.

The Nobel Prize in Physics was awarded to Yukawa in 1949 (for predicting the existence of mesons) and to Powell in 1950 (for developing the technique of particle detection using photo-emulsions).

Since it is not electrically charged, the neutral pion is more difficult to observe than the charged pions; it doesn't leave a track in an emulsion. Its existence was inferred from its decay products in cosmic rays, a so-called "soft component" of electrons and photons. The π⁰ was identified at the Berkeley cyclotron in 1950 by its decay into two photons.

In the modern understanding of the strong interaction (quantum chromodynamics), pions are considered to be the pseudo Nambu-Goldstone bosons of spontaneously broken chiral symmetry. This explains why the pion masses are considerably lighter than the masses of other mesons like the $\backslash eta^\backslash prime$ meson (958 MeV). If their constituent quarks were massless (making chiral symmetry exact), the Goldstone theorem would predict that the pions should have zero mass. Since the quarks actually have small masses, the pions do as well.

Theoretical overview

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The pion can be thought of as the particle that mediates the interaction between a pair of nucleons. This interaction is attractive; it pulls the nucleons together. Written in a non-relativistic form, it is called the Yukawa potential. The pion, being a meson, has kinematics described by the Klein-Gordon equation. In the terms of quantum field theory, the effective field theory Lagrangian describing the pion-nucleon interaction is called the Yukawa interaction.

The nearly identical masses of π^± and π⁰ imply that there must be a symmetry at play; this symmetry is called the SU(2) flavour symmetry or isospin. The reason that there are three pions, π⁺, π^- and π⁰ is that these are understood to belong to the triplet representation or the adjoint representation 3 of SU(2). By contrast, the up and down quarks transform according to the irreducible, semi-simple fundamental representation 2 of SU(2), whereas the anti-quarks transform according to the conjugate representation 2*. Thus, one has

$SU(2)\; \backslash otimes\; \backslash overline\{SU(2)\}\; \backslash approx\; SO(3)\; \backslash oplus\; U(1)$

which is one of the many relationships which lends weight to the quark model of pions and nucleons.

With the addition of the strange quark, one can say that the pions participate in an SU(3) flavour symmetry, belonging to the adjoint representation 8 of SU(3). The other members of this octet are the four kaons and the eta.

Pions are pseudoscalars under a parity transformation. Pion currents thus couple to the axial vector current and pions participate in the chiral anomaly.

See also

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• Pionium
• List of particles
• Quark model

References

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• Gerald Edward Brown and A. D. Jackson, The Nucleon-Nucleon Interaction, (1976) North-Holland Publishing, Amsterdam ISBN 0-7204-0335-9

External links

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• Mesons at the Particle Data Group
• Mesons at Hyperphysics

Mesons

Пион | Pió | Pion | Pion | Pión | Pion (particule) | Pione | פאיון | Pion | Pion (natuurkunde) | Pion | パイ中間子 | Pion (cząstka) | Píon | Пион (частица)