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| | I'm Stacy and I live with my husband and our three children in Olivette, in the MO south part. My hobbies are Dancing, Leaf collecting and pressing and Locksport.<br><br>Feel free to visit my web site: [http://gbk.tianjinmama.com/plus/guestbook.php Fifa 15 coin Generator] |
| {{Infobox Particle
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| | bgcolour =
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| | name = Pion
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| | image = [[Image:Quark structure pion.svg|250px]]
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| | caption = The quark structure of the pion.
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| | num_types = 3
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| | composition = {{SubatomicParticle|Pion+}}: {{SubatomicParticle|Up quark}}{{SubatomicParticle|Down antiquark}}<br />{{SubatomicParticle|Pion0}}: {{SubatomicParticle|Up quark}}{{SubatomicParticle|Up antiquark}} or {{SubatomicParticle|Down quark}}{{SubatomicParticle|Down antiquark}}<br />{{SubatomicParticle|Pion-}}: {{SubatomicParticle|Down quark}}{{SubatomicParticle|Up antiquark}}
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| | statistics = [[Boson]]ic
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| | group = [[Meson]]s
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| | generation =
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| | interaction = [[Strong interaction|Strong]]
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| | antiparticle=
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| | theorized = [[Hideki Yukawa]] (1935)
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| | discovered = [[César Lattes]], [[Giuseppe Occhialini]] (1947) and [[Cecil Powell]]
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| | symbol = {{SubatomicParticle|Pion+}}, {{SubatomicParticle|Pion0}}, and {{SubatomicParticle|Pion-}}
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| | mass = {{SubatomicParticle|Pion+-}}: {{val|139.57018|(35)|ul=MeV/c2}}<br />{{SubatomicParticle|Pion0}}: {{val|134.9766|(6)|u=MeV/c2}}
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| | decay_time = {{SubatomicParticle|Pion+-}}: {{val|2.6|e=-8|ul=s}}, {{SubatomicParticle|Pion0}}: {{val|8.4|e=-17|u=s}}
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| | decay_particle =
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| | electric_charge = {{SubatomicParticle|Pion+}}: +1 [[Elementary charge|e]]<br>{{SubatomicParticle|Pion0}}: 0 e<br>{{SubatomicParticle|Pion-}}: −1 e
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| | color_charge=
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| | spin = 0
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| | parity = −1
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| | num_spin_states =
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| }}
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| In [[particle physics]], a '''pion''' (short for '''pi meson''', denoted with {{SubatomicParticle|Pion}}) is any of three [[subatomic particle]]s: {{SubatomicParticle|Pion0}}, {{SubatomicParticle|Pion+}}, and {{SubatomicParticle|Pion-}}. 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]].
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| Pions are unstable, with the charged pions {{SubatomicParticle|Pion+}} and {{SubatomicParticle|Pion-}} decaying with a mean life time of 26 nanoseconds and the neutral pion {{SubatomicParticle|Pion0}} decaying with an even shorter lifetime. Charged pions tend to decay into [[muon]]s and muon neutrinos, and neutral pions into [[gamma ray]]s.
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| Pions are not produced in [[radioactive decay]], but are produced commonly in high energy accelerators in collisions between [[hadron]]s. 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.<ref name=ackermann-2013>
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| {{cite journal
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| | author1=M. Ackermann
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| | coauthors=''et al.''
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| | date=2013
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| | title=Detection of the Characteristic Pion-Decay Signature in Supernova Remnants
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| | journal=[[Science (journal)|Science]]
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| | volume=339 | issue=6424 | pages=807–811
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| | arxiv = 1302.3307
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| | bibcode = 2013Sci...339..807A
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| | doi =10.1126/science.1231160
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| }}</ref>
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| ==Basic properties==
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| Pions are [[meson]]s with zero [[spin (physics)|spin]], and they are composed of first-[[generation (particle physics)|generation]] [[quark]]s. In the [[quark model]], an [[up quark]] and an anti-down quark make up a {{SubatomicParticle|Pion+}}, whereas a [[down quark]] and an anti-up quark make up the {{SubatomicParticle|Pion-}}, and these are the [[antiparticle]]s of one another. The neutral pion {{SubatomicParticle|Pion0}} is a combination of an up quark with an anti-up quark or a down quark with an anti-down quark. The two combinations have identical [[quantum number]]s, and hence they are only found in [[quantum superposition|superposition]]s. The lowest-energy superposition of these is the {{SubatomicParticle|Pion0}}, which is its own antiparticle. Together, the pions form a triplet of [[isospin]]. Each pion has isospin (''I'' = 1) and third-component [[Gell-Mann–Nishijima formula|isospin equal to its charge]] (''I''<sub>z</sub> = +1, 0 or −1).
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| ===Charged-pion decays===
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| [[Image:PiPlus-muon-decay.png|right|thumb|300px|[[Feynman diagram]] of the dominating leptonic pion decay.]]
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| The {{SubatomicParticle|Pion+-}} mesons have a [[mass]] of {{val|139.6|ul=MeV/c2}} and a [[mean life]]time of {{val|2.6|e=-8|ul=s}}. They decay due to the [[weak force|weak interaction]]. The primary decay mode of a pion, with probability 0.999877, is a purely [[lepton]]ic decay into an [[anti-muon]] and a [[muon neutrino]]:
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| :{|
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| | {{SubatomicParticle|Pion+}} || → || {{SubatomicParticle|link=yes|Muon+}} || + || {{SubatomicParticle|link=yes|Muon neutrino}}
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| |--
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| | {{SubatomicParticle|Pion-}} || → || {{SubatomicParticle|link=yes|Muon-}} || + || {{SubatomicParticle|link=yes|Muon antineutrino}}
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| |}
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| 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:
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| :{|
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| | {{SubatomicParticle|Pion+}} || → || {{SubatomicParticle|link=yes|Positron}} || + || {{SubatomicParticle|link=yes|Electron neutrino}}
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| |--
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| | {{SubatomicParticle|Pion-}} || → || {{SubatomicParticle|link=yes|Electron}} || + || {{SubatomicParticle|link=yes|Electron antineutrino}}
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| |}
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| The suppression of the electronic mode, with respect to the muonic one, is given approximately (to within radiative corrections) by the ratio of the half-widths of the pion–electron and the pion–muon decay reactions:
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| :<math> 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}</math>
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| and is a [[spin (physics)|spin]] effect known as the [[helicity (particle physics)|helicity]] suppression. Measurements of the above ratio have been considered for decades to be tests of the ''V − A structure'' ([[vector (geometry)|vector]] minus [[axial vector]] or left-handed [[lagrangian]]) of the charged [[weak interaction|weak current]] and of [[lepton universality]]. Experimentally this ratio is {{val|1.230|(4)|e=-4}}.<ref name=PDGPion>C. Amsler ''et al.''. (2008): [http://pdg.lbl.gov/2008/listings/s008.pdf Particle listings – {{SubatomicParticle|Pion+-}}]</ref>
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| Besides the purely leptonic decays of pions, some structure-dependent radiative leptonic decays (that is, decay to the usual leptons plus a gamma ray) have also been observed.
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| Also observed, for charged pions only, is the very rare "pion [[beta decay]]" (with probability of about 10<sup>−8</sup>) into a neutral pion plus an electron and electron antineutrino (or for positive pions, a neutral pion, positron, and electron neutrino).
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| :{|
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| | {{SubatomicParticle|Pion-}} || → || {{SubatomicParticle|Pion0}} || + ||{{SubatomicParticle|link=yes|Electron}} || + || {{SubatomicParticle|link=yes|Electron antineutrino}}
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| |--
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| | {{SubatomicParticle|Pion+}} || → || {{SubatomicParticle|Pion0}} || + ||{{SubatomicParticle|link=yes|Positron}} || + || {{SubatomicParticle|link=yes|Electron neutrino}}
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| |}
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| ===Neutral pion decays===
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| The {{SubatomicParticle|Pion0}} meson has a slightly smaller mass of {{val|135.0|u=MeV/c2}} and a much shorter mean lifetime of {{val|8.4|e=-17|u=s}}. This pion decays in an [[electromagnetic force]] process. The main decay mode, with probability 0.98798, is into two [[photon]]s (two [[gamma ray]] photons in this case):
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| :{|
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| | {{SubatomicParticle|Pion0}} || → || 2 {{SubatomicParticle|link=yes|Gamma}}
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| |}
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| Its second most common decay mode, with probability 0.01198, is the [[Richard Dalitz|Dalitz]] decay into a photon and an [[electron]]–[[positron]] pair:
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| :{|
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| | {{SubatomicParticle|Pion0}} || → || {{SubatomicParticle|Gamma}} || + || {{SubatomicParticle|link=yes|Electron}} || + || {{SubatomicParticle|link=yes|Positron}}
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| |}
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| 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]] (''ƒ''<sub>π</sub>), which is about {{val|90|u=MeV}}.
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| {| class="wikitable sortable" style="text-align: center;"
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| |+Pions
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| |-
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| ! class=unsortable|Particle name
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| ! Particle <br>symbol
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| ! Antiparticle <br>symbol
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| ! class=unsortable|Quark<br>content<ref name=PDGQuarkmodel>C. Amsler ''et al.''. (2008): [http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf Quark Model]</ref>
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| ! [[Rest mass]] ([[electron volt|MeV]]/[[speed of light|c]]<sup>2</sup>)
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| ! width="50"|[[Isospin|I]]<sup>[[G parity|G]]</sup>
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| ! width="50"|[[Total angular momentum|J]]<sup>[[Parity (physics)|P]][[C parity|C]]</sup>
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| ! width="50"|[[strangeness|S]]
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| ! width="50"|[[charm (quantum number)|C]]
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| ! width="50"|[[bottomness|B']]
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| ! [[Mean lifetime]] ([[second|s]])
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| ! class=unsortable|Commonly decays to<br>
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| (>5% of decays)
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| |-
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| | Pion<ref name="PDGPion"/>
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| |align="center"| {{SubatomicParticle|Pion+}}
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| |align="center"| {{SubatomicParticle|Pion-}}
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| |align="center"| {{SubatomicParticle|link=yes|Up quark}}{{SubatomicParticle|link=yes|Down antiquark}}
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| |align="center"| {{nowrap|139.570 18 ± 0.000 35}}
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| |align="center"| 1<sup>−</sup>
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| |align="center"| 0<sup>−</sup>
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| |align="center"| 0
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| |align="center"| 0
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| |align="center"| 0
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| |align="center"| {{nowrap|2.6033 ± 0.0005 × 10<sup>−8</sup>}}
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| |align="center"| {{nowrap|{{SubatomicParticle|link=yes|Antimuon}} + {{SubatomicParticle|link=yes|Muon neutrino}}}}
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| |-
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| | Pion<ref name=PDGPion0>C. Amsler ''et al.''. (2008): [http://pdg.lbl.gov/2008/listings/s009.pdf Particle listings – {{SubatomicParticle|Pion0}}]</ref>
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| |align="center"| {{SubatomicParticle|Pion0}}
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| |align="center"| Self
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| |align="center"| <math>\tfrac{\mathrm{u\bar{u}} - \mathrm{d\bar{d}}}{\sqrt 2}</math><sup>{{ref|quarkcontent|[a]}}</sup>
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| |align="center"| {{nowrap|134.976 6 ± 0.000 6}}
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| |align="center"| 1<sup>−</sup>
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| |align="center"| 0<sup>−+</sup>
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| |align="center"| 0
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| |align="center"| 0
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| |align="center"| 0
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| |align="center"| {{nowrap|8.4 ± 0.6 × 10<sup>−17</sup>}}
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| |align="center"| {{nowrap|{{SubatomicParticle|link=yes|Photon}} + {{SubatomicParticle|link=yes|Photon}}}}
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| |-
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| |}
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| <sup>[a]</sup> {{note|quarkcontent}} Make-up inexact due to non-zero quark masses.<ref>
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| {{cite book
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| |author1=D. J. Griffiths
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| |authorlink=David J. Griffiths
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| |year=1987
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| |title=Introduction to Elementary Particles
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| |publisher=[[John Wiley & Sons]]
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| |isbn=0-471-60386-4
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| }}</ref>
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| ==History==
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| Theoretical work by [[Hideki Yukawa]] in 1935 had predicted the existence of [[meson]]s 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.
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| 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 accelerator]]s had not yet come, high-energy subatomic particles were only obtainable from atmospheric [[cosmic ray]]s. [[Photographic emulsion]]s, 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.
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| After the development of the [[photographic plate]]s, [[microscope|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 at Berkeley|University of California]]'s [[cyclotron]] in [[Berkeley, California]], by bombarding [[carbon]] atoms with high-speed [[alpha particle]]s. Further advanced theoretical work was carried out by [[Riazuddin (physicist)|Riazuddin]], who in 1959, used the [[dispersion relation]] for [[Compton scattering]] of [[virtual photon]]s on pions to analyze their charge radius.<ref>
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| {{Cite journal
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| | author = Riazuddin
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| | year = 1959
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| | title = Charge Radius of Pion
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| | journal = [[Physical Review]]
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| | volume = 114 | issue = 4 | pages = 1184–1186
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| | bibcode = 1959PhRv..114.1184R
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| | doi =10.1103/PhysRev.114.1184
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| }}</ref>
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| [[Nobel Prize in Physics|Nobel Prizes in Physics]] were 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 emulsion]]s.
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| Since the neutral pion is not [[electric charge|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 chamber]]s. The existence of the neutral pion was inferred from observing its decay products from [[cosmic ray]]s, a so-called "soft component" of slow electrons with photons. The {{SubatomicParticle|Pion0}} was identified definitively at the University of California's cyclotron in 1950 by observing its decay into two photons.<ref>
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| {{cite journal
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| |author1=R. Bjorklund
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| |author2=W. E. Crandall
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| |author3=B. J. Moyer
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| |author4=H. F. York
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| |year=1950
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| |title=High Energy Photons from Proton-Nucleon Collisions
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| |journal=[[Physical Review]]
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| |volume=77 |issue=2 |pages=213–218
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| |bibcode= 1950PhRv...77..213B
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| |doi= 10.1103/PhysRev.77.213
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| }}</ref> Later in the same year, they were also observed in cosmic-ray balloon experiments at Bristol University.
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| The pion also plays a crucial role in cosmology, by imposing an upper limit on the energies of cosmic rays surviving collisions with the [[cosmic microwave background]], through the [[Greisen–Zatsepin–Kuzmin limit]].
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| In the standard understanding of the [[strong force]] interaction (called QCD, "[[quantum chromodynamics]]"), pions are understood to be the pseudo-Nambu-[[Goldstone boson]]s of [[spontaneous symmetry breaking|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 [[quark]]s 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 mass]]es, albeit [[Chiral_symmetry_breaking|''almost an order of magnitude smaller'']] than that of the nucleons, roughly<ref>{{cite doi|10.1103/PhysRev.175.2195|noedit}}</ref> ''m''<sub>π</sub> ≈ √{{overline|''v m''}}<sub>q</sub> / ''f''<sub>π</sub> ≈ √{{overline|''m''}}<sub>q</sub> 45 MeV, where ''m'' are the relevant current quark masses in MeV, 5−10 MeVs.
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| 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]],<ref>
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| {{cite journal
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| |author1=C. F. von Essen
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| |author2=M. A. Bagshaw
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| |author3=S. E. Bush
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| |author4=A. R. Smith
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| |author5=M. M. Kligerman
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| |year=1987
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| |title=Long-term results of pion therapy at Los Alamos
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| |journal=[[International Journal of Radiation Oncology, Biology, Physics]]
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| |volume=13 |issue=9 |pages=1389–98
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| |bibcode=
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| |doi=10.1016/0360-3016(87)90235-5
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| |pmid=3114189
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| }}</ref> and the [[TRIUMF]] laboratory in [[Vancouver, British Columbia]].<ref>
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| {{cite web
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| |title=TRIUMF: Cancer Therapy with Pions
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| |url=http://legacyweb.triumf.ca/welcome/pion_trtmt.html
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| }}</ref>
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| ==Theoretical overview==
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| The pion can be thought of as one of the particles that mediate 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]].
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| 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 of a Lie group|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*'''.
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| 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]].
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| 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]].
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| ==See also==
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| *[[Pionium]]
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| *[[List of particles]]
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| *[[Quark model]]
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| *[[Static forces and virtual-particle exchange]]
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| *[[César Lattes]]
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| ==References==
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| {{reflist}}
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| == Further reading ==
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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
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| ==External links==
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| * [http://pdg.lbl.gov/2004/tables/mxxx.pdf Mesons] at the Particle Data Group
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| * [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/hadron.html Mesons] at Hyperphysics
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| {{Particles}}
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| [[Category:Mesons]]
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