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Quark structure pion.svg
The quark structure of the pion.
Composition π+: ud
π0: uu or dd
π: du
Statistics Bosonic
Interactions Strong
Symbol π+, π0, and π
Theorized Hideki Yukawa (1935)
Discovered César Lattes, Giuseppe Occhialini (1947) and Cecil Powell
Types 3
Mass π±: 139.57018(35) MeV/c2
π0: 134.9766(6) MeV/c2
Electric charge π+: +1 e
π0: 0 e
π: −1 e
Spin 0
Parity −1

In particle physics, a pion (short for pi meson, denoted with π) is any of three subatomic particles: π0, π+, and π. Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force.

Pions are unstable, with the charged pions π+ and π decaying with a mean life time of 26 nanoseconds and the neutral pion π0 decaying with an even shorter lifetime. Charged pions tend to decay into muons and muon neutrinos, and neutral pions into gamma rays.

Pions are not produced in radioactive decay, but are produced commonly in high energy accelerators in collisions between hadrons. All types of pions are also produced in natural processes when high energy cosmic ray protons and other hadronic cosmic ray components interact with matter in the Earth's atmosphere. Recently, detection of characteristic gamma rays originating from decay of neutral pions in two supernova remnant stars has shown that pions are produced copiously in supernovas, most probably in conjunction with production of high energy protons that are detected on Earth as cosmic rays.[1]

Basic properties[edit]


Charged-pion decays[edit]

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 the weak interaction. The primary decay mode of a pion, with probability 0.999877, is a purely leptonic decay into an anti-muon and a muon neutrino:

π+ μ+ + ν
π μ + ν

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

π+ e+ + ν
π e + ν


 R_\pi = (m_e/m_\mu)^2 \left(\frac{m_\pi^2-m_e^2}{m_\pi^2-m_\mu^2}\right)^2 = 1.233 \times 10^{-4}



Also observed, for charged pions only, is the very rare "pion beta decay" (with probability of about 10−8) into a neutral pion plus an electron and electron antineutrino (or for positive pions, a neutral pion, positron, and electron neutrino).

π π0 + e + ν
π+ π0 + e+ + ν

Neutral pion decays[edit]

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 an electromagnetic force process. The main decay mode, with probability 0.98798, is into two photons (two gamma ray photons in this case):

π0 2 γ

Its second most common decay mode, with probability 0.01198, is the Dalitz decay into a photon and an electronpositron pair:

π0 γ + e + e+


Particle name Particle

(>5% of decays)

Pion[4] 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.[5]



In 1947, the first true mesons, the charged pions, were found by the collaboration of Cecil Powell, César Lattes, 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, which were left by their decay into another "meson". (It was actually the muon, which 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. Further advanced theoretical work was carried out by Riazuddin, who in 1959, used the dispersion relation for Compton scattering of virtual photons on pions to analyze their charge radius.[6]




In the standard understanding of the strong force interaction (called QCD, "quantum chromodynamics"), pions are understood to be the pseudo-Nambu-Goldstone bosons of spontaneously broken chiral symmetry. This explains why the three kinds of pions' masses are considerably less than the masses of the other mesons, such as the scalar or vector mesons. If their current quarks were massless particles, hypothetically, making the chiral symmetry exact, then the Goldstone theorem would dictate that all pions have zero masses. In reality, since the light quarks actually have minuscule nonzero masses, the pions also have nonzero rest masses, albeit almost an order of magnitude smaller than that of the nucleons, roughly[8] mπ ≈ √v mq / fπ ≈ √mq 45 MeV, where m are the relevant current quark masses in MeV, 5−10 MeVs.

The use of pions in medical radiation therapy, such as for cancer, was explored at a number of research institutions, including the Los Alamos National Laboratory's Meson Physics Facility, which treated 228 patients between 1974 and 1981 in New Mexico,[9] and the TRIUMF laboratory in Vancouver, British Columbia.[10]

Theoretical overview[edit]





See also[edit]


  3. ^ C. Amsler et al.. (2008): Quark Model
  9. ^ C. F. von Essen; M. A. Bagshaw; S. E. Bush; A. R. Smith; M. M. Kligerman (1987). "Long-term results of pion therapy at Los Alamos". International Journal of Radiation Oncology, Biology, Physics 13 (9): 1389–98. doi:10.1016/0360-3016(87)90235-5. PMID 3114189. 
  10. ^ "TRIUMF: Cancer Therapy with Pions". 

Further reading[edit]

External links[edit]