Pion
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2008-07-13T16:31:56Z
PedroFonini
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/* Basic properties */ link error
{{Infobox Particle
| bgcolour =
| name = Pion
| image = [[Image:Quark structure pion.svg|250px]]
| caption = The quark structure of the pion.
| num_types = 3
| composition = {{SubatomicParticle|Pion+}}: {{SubatomicParticle|Up quark}}{{SubatomicParticle|Down antiquark}}<br />{{SubatomicParticle|Pion0}}: {{SubatomicParticle|Down quark}}{{SubatomicParticle|Down antiquark}} / {{SubatomicParticle|Up quark}}{{SubatomicParticle|Up antiquark}}<br />{{SubatomicParticle|Pion-}}: {{SubatomicParticle|Down quark}}{{SubatomicParticle|Up antiquark}}
| family = [[Boson]]s
| group = [[Meson]]s
| generation =
| interaction = [[Strong interaction|Strong]]
| antiparticle =
| theorized = [[Hideki Yukawa]]
| discovered =
| symbol = {{SubatomicParticle|Pion+}}, {{SubatomicParticle|Pion0}}, & {{SubatomicParticle|Pion-}}
| mass = {{SubatomicParticle|Pion+-}}: 139.57018(35) [[MeV]]/[[Speed of light|c]]<sup>2</sup><br />{{SubatomicParticle|Pion0}}: 134.9766(6) [[MeV]]/[[Speed of light|c]]<sup>2</sup>
| decay_time = {{SubatomicParticle|Pion+-}}: 2.6×10<sup>−8</sup>[[second|s]], {{SubatomicParticle|Pion0}}: 8.4×10<sup>−17</sup>
| decay_particle =
| electric_charge = {{SubatomicParticle|Pion+-}}: ±[[Elementary charge|e]]<br />{{SubatomicParticle|Pion0}}: 0
| color_charge =
| spin = {{SubatomicParticle|Pion+-}}: 1(±1), 0<sup>−</sup><br />{{SubatomicParticle|Pion0}}: 1(0), 0<sup>−</sup>
| num_spin_states =
}}
In [[particle physics]], '''pion''' (short for '''pi meson''') is the collective name for three [[subatomic particle]]s: {{SubatomicParticle|Pion0}}, {{SubatomicParticle|Pion+}} and {{SubatomicParticle|Pion-}}. Pions are the lightest [[meson]]s (excluding the misnamed "Mu Meson" or [[muon]]) and play an important role in explaining low-energy properties of the [[strong nuclear force]].
==Basic properties==
[[Image:PiPlus-muon-decay.png|left|thumb|300px|[[Feynman diagram]] of a common pion decay.]]
Pions have zero [[spin (physics)|spin]] and are composed of first-[[generation (particle physics)|generation]] [[quark]]s. In the [[quark model]], an up and an anti-down quark compose a {{SubatomicParticle|Pion+}}, while a down and an anti-up quark compose the {{SubatomicParticle|Pion-}}, its [[antiparticle]]. The neutral combinations of up with anti-up and down with anti-down have identical [[quantum number]]s, so they are only found in [[quantum superposition|superposition]]s. The lowest-energy superposition is the {{SubatomicParticle|Pion0}}, 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''<sub>z</sub> = +1, 0 or −1).
The π<sup> ±</sup> mesons have a [[mass]] of 139.6 MeV/''c''<sup>2</sup> and a [[mean life]] of 2.6×10<sup>−8</sup> seconds. They decay due to [[Weak force|weak]] processes. The main decay mode (99.9877%) is into a [[muon]] and its [[neutrino]]:
: {{SubatomicParticle|Pion+}} → {{SubatomicParticle|Antimuon}} + {{SubatomicParticle|Muon neutrino}}
: {{SubatomicParticle|Pion-}} → {{SubatomicParticle|Muon}} + {{SubatomicParticle|Muon antineutrino}}
The second largest decay mode (0.0123%) is into an [[electron]] and the corresponding neutrino:
: {{SubatomicParticle|Pion+}} → {{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron neutrino}}
: {{SubatomicParticle|Pion-}} → {{SubatomicParticle|Electron}} + {{SubatomicParticle|Electron antineutrino}}
The {{SubatomicParticle|Pion0}} meson has a slightly smaller mass of 135.0 MeV/''c''<sup>2</sup> and a much shorter mean life of 8.4×10<sup>−17</sup> seconds. It decays due to [[electromagnetic]] force. The main decay mode (98.798%) is into two [[photon]]s:
: {{SubatomicParticle|Pion0}} → 2 {{SubatomicParticle|gamma}}
Its second largest decay mode (1.198%) is the so-called [[Richard Dalitz|Dalitz]] decay into a photon and an [[electron]]-[[positron]] pair:
: {{SubatomicParticle|Pion0}} → {{SubatomicParticle|gamma}} + {{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron}}
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''<sub>π</sub>), which is about 90 MeV.
{| class="wikitable"
! Particle
! Symbol
! Anti-<br>particle
! [[Quark]]<br>Makeup
! [[Spin (physics)|Spin]] and [[Parity (physics)|parity]]
! [[Rest mass]]<br>[[electron volt|MeV]]/[[speed of light|c]]<sup>2</sup>
! [[Strangeness (particle physics)|S]]
! [[charm (quantum number)|C]]
! [[bottomness|B]]
! [[Mean lifetime]]<br>[[second|s]]
! Decays to
! Notes
|-
| Charged<br>Pion
| {{SubatomicParticle|Pion+}}
| {{SubatomicParticle|Pion-}}
| {{SubatomicParticle|Up quark}}{{SubatomicParticle|Down antiquark}} / {{SubatomicParticle|Down quark}}{{SubatomicParticle|Up antiquark}}
| [[Pseudoscalar (mathematics)|Pseudoscalar]]
| 139.6
| 0
| 0
| 0
| 2.60×10<sup>-8</sup>
| {{SubatomicParticle|Antimuon}} + {{SubatomicParticle|Muon neutrino}}
|-
| Neutral<br>Pion
| {{SubatomicParticle|Pion0}}
| Self
| <math>\mathrm{\frac{u\bar{u} - d \bar{d}}{\sqrt{2}}}</math>
| Pseudoscalar
| 135.0
| 0
| 0
| 0
| 0.84×10<sup>-16</sup>
| 2{{SubatomicParticle|gamma}}
| Makeup inexact due to non-zero quark masses
|}
==History==
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 [[atomic nucleus|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 accelerator]]s had yet to arrive, high energies were only accessible from atmospheric [[cosmic ray]]s. [[Photographic emulsion]]s 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"; note that the muon is not classified as a meson in modern particle physics). In 1948, Lattes and [[Eugene Gardner]] first achieved artificial production of pion particles at the [[University of California, Berkeley]] [[cyclotron]] by bombarding [[carbon]] atoms with [[alpha particle]]s.
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 {{SubatomicParticle|Pion0}} was identified at the Berkeley cyclotron in 1950 by its decay into two photons and the same year in cosmic ray balloon experiments at Bristol University, England.
In the modern understanding of the strong interaction ([[quantum chromodynamics]]), pions are considered to be the pseudo Nambu-[[Goldstone boson]]s of [[spontaneous symmetry breaking|spontaneously broken]] [[chiral symmetry]]. This explains why the pion masses are considerably lighter than the masses of other mesons like the {{SubatomicParticle|eta prime}} meson (958 MeV). If their constituent [[quark]]s 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.
The use of pions in radiation therapy was explored at a number of institutions, including the [[Los Alamos National Laboratory]] Meson Physics Facility, which treated 228 patients between 1974 and 1981 [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3114189&dopt=Abstract], and [[TRIUMF]] in British Columbia, Canada [http://triumf.ca/welcome/pion_trtmt.html].
==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 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 {{SubatomicParticle|Pion+-}} and {{SubatomicParticle|Pion0}} 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, {{SubatomicParticle|Pion+}}, {{SubatomicParticle|Pion-}} and {{SubatomicParticle|Pion0}}, 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 [[kaon]]s and the [[eta meson]].
Pions are [[pseudoscalar (physics)|pseudoscalar]]s under a [[parity (physics)|parity]] transformation. Pion currents thus couple to the [[axial vector current]] and pions participate in the [[chiral anomaly]].
==See also==
*[[Pionium]]
*[[List of particles]]
*[[Quark model]]
==References==
* [[Gerald Edward Brown]] and A. D. Jackson, ''The Nucleon-Nucleon Interaction'', (1976) North-Holland Publishing, Amsterdam ISBN 0-7204-0335-9
==External links==
* [http://pdg.lbl.gov/2004/tables/mxxx.pdf Mesons] at the Particle Data Group
* [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/hadron.html Mesons] at Hyperphysics
{{Particles}}
[[Category:Mesons]]
[[bg:Пион]]
[[ca:Pió]]
[[cs:Pion]]
[[de:Pion]]
[[es:Pión]]
[[fa:پیون]]
[[fr:Pion (particule)]]
[[hr:Pion]]
[[it:Pione]]
[[he:פאיון]]
[[lt:Pionas]]
[[lv:Pions]]
[[hu:Pion]]
[[nl:Pion (natuurkunde)]]
[[ja:パイ中間子]]
[[no:Pi-meson]]
[[pl:Pion (cząstka)]]
[[pt:Píon]]
[[ru:Пион (частица)]]
[[sk:Pión]]
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[[uk:Піони]]