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Quark structure pion.svg
The quark structure of the pion.
Composition: π+: ud
π0: (uu - dd)/√2
π: du
Statistical behavior: Bosons
Group: Mesons
Interaction: Strong
Symbol(s): π+, π0, & π
Theorized: Hideki Yukawa
Types: 3
Mass: π±: 139.57018(35) MeV/c2
π0: 134.9766(6) MeV/c2
Electric charge: π±: ±e
π0: 0
Spin: π±: 1(±1), 0
π0: 1(0), 0

In particle physics, a pion (short for pi meson; denoted π) is any of 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.


Basic properties

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


Charged pion decays

Feynman diagram of the dominating leptonic pion decay.

The π± mesons have a mass of 139.6 MeV/c2 and a mean lifetime of 2.6×10−8 s. They decay due to weak force processes. The primary decay mode of a pion, with probability 0.999877, is a purely leptonic decay into a muon and its neutrino:

π+ μ+ + νμ
π μ + νμ

The second most common decay mode of a pion, with probability 0.000123, is also a leptonic one into an electron and the corresponding neutrino. This mode was discovered at CERN in 1958:

π+ e+ + νe
π e + νe

The suppression of the electronic mode, with respect to the muonic one, by a factor of approximately (to within radiative corrections)

R_\pi = (m_e/m_\mu)^2 \left(\frac{M_\pi^2-M_e^2}{M_\pi^2-M_\mu^2}\right)^2

is a spin effect known as the helicity suppression. Measurements of the above ratio have for decades been considered to be tests of the so-called "V-A structure" of the charged "weak current" and of lepton universality.

Besides the purely leptonic decays of pions, there have also been observed some structure-dependent radiative leptonic decays, and also the very rare beta decay of pions (with a probability of about 1 x 10−8) with a neutral pion as the final state.

Neutral pion decays

The π0 meson has a slightly smaller mass of 135.0 MeV/c2 and a much shorter mean lifetime of 8.4×10−17 s. This pion decays in a electromagnetic force process. The main decay mode, with probability 0.98798, is into two photons:

π0 2 γ

Its second most common decay mode, with probability 0.01198, is the so-called "Dalitz" decay into a photon and an electron-positron pair:

π0 γ + e + e+

The rate at which pions decay is a prominent quantity in many sub-fields of particle physics, such as chiral perturbation theory. This rate is parametrized by the pion decay constant (ƒπ), which is about 90 MeV.

Particle name Particle
Rest mass (MeV/c2) IG JPC S C B' Mean lifetime (s) Commonly decays to

(>5% of decays)

Pion[2] π+ π ud 139.570 18(35) 1 0 0 0 0 2.6033 ± 0.0005 × 10−8 μ+ + νμ
Pion[3] π0 Self \tfrac{\mathrm{u\bar{u}} - \mathrm{d\bar{d}}}{\sqrt 2}[a] 134.976 6 ± 0.000 6 1 0−+ 0 0 0 8.4 ± 0.6 × 10−17 γ + γ

[a] ^ Make-up inexact due to non-zero quark masses.[4]


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 strong nuclear force (inferred from the radius of the atomic nucleus), Yukawa predicted the existence of a particle having a mass of about 100 MeV. Initially after its discovery in 1936, the muon (initially called the "mu meson") was thought to be this particle, since it has a mass of 106 MeV. However, later particle physics experiments showed that the muon did not participate in the strong nuclear interaction. In modern terminology, this makes the muon a lepton, and not a true meson.

In 1947, the first true mesons, the charged pions, were found by the collaboration of Cecil Powell, César Lattes, and Giuseppe Occhialini, et. al., at the University of Bristol, in England. Since the advent of particle accelerators had not yet come, high-energy subatomic particles were only obtainable from atmospheric cosmic rays. Photographic emulsions, which used the gelatin-silver process, were placed for long periods of 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 Mountains, where they were impacted by cosmic rays.

After the development of the photographic plates, microscopic inspection of the emulsions revealed the tracks of charged subatomic particles. Pions were first identified by their unusual "double meson" tracks, whice were left by their decay into another "meson". (It was actually the "muon". Note that the muon is not classified as a meson in modern particle physics.) In 1948, Lattes, Eugene Gardner, and their team first artificially produced pions at the University of California's cyclotron in Berkeley, California, by bombarding carbon atoms with high-speed alpha particles.

The Nobel Prize in Physics was awarded to Yukawa in 1949, for his theoretical prediction of the existence of mesons, and to Cecil Powell in 1950, for developing and applying the technique of particle detection using photographic emulsions.

Since the neutral pion is not electrically charged, it is more difficult to detect and observe than the charged pions are. Neutral pions do not leave tracks in photographic emulsions, and neither do they in Wilson cloud chambers. The existence of the neutral pion was inferred from observing its decay products in cosmic rays, a so-called "soft component" of slow electrons with photons. The π0 was identified definitively at the University of California's cyclotron in 1950 by observing its decay into two photons, and the same year, in cosmic-ray observing balloon experiments at the above-mentioned Bristol University.

The pion also plays a role in cosmology by imposing an upper limit on the energies of cosmic rays through the Greisen-Zatsepin-Kuzmin limit.

In the modern understanding of the strong force interaction, called "quantum chromodynamics", pions are considered to be the pseudo Nambu-Goldstone bosons of spontaneously broken chiral symmetry. This explains why the three kinds of pion's masses are considerably less than the masses of the other true mesons, such as the η′ meson (958 MeV). If their constituent quarks were massless particles, hypothetically, making the chiral symmetry exact, then calculations with the Goldstone theorem would give all pions zero masses. In reality, since all quarks actually have small masses, the pions also have nonzero rest masses.

The use of pions in medical radiation therapy, such as for cancer, was explored at a number of research institutions, including at the Los Alamos National Laboratory's Meson Physics Facility, which treated 228 patients between 1974 and 1981 in New Mexico,[5] and the TRIUMF laboratory in Vancouver, British Columbia.[6]

Theoretical overview

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 spinless, 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 fundamental representation 2 of SU(2), whereas the anti-quarks transform according to the conjugate representation 2*.

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 meson.

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


  1. ^ C. Amsler et al.. (2008): Quark Model
  2. ^ C. Amsler et al.. (2008): Particle listings – π±
  3. ^ C. Amsler et al.. (2008): Particle listings – π0
  4. ^ Griffiths, David J. (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4. 
  5. ^ von Essen CF, Bagshaw MA, Bush SE, Smith AR, Kligerman MM (September 1987). "Long-term results of pion therapy at Los Alamos". Int. J. Radiat. Oncol. Biol. Phys. 13 (9): 1389–98. PMID 3114189. 
  6. ^ "TRIUMF: Cancer Therapy with Pions". 

Further reading

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

External links


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also pion, and pión



Pion, n. (plural: Pionen)

  1. pion


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