Pion

In particle physics, pion (short for the Greek language ''pi meson'' = 'P middle') is the collective name for three subatomic particles discovered in 1947: π⁰, π⁺ and π⁻. Pions are the lightest mesons. ==Basic properties== Pions have zero spin (physics) 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. Combinations of up and anti-up, or down and anti-down, would both be neutral, but because they have the same quantum numbers are only found in superpositions. The lowest energy superposition is the π⁰, which is its own antiparticle. The π^± mesons have a mass of 139.6 MeV/''c''² and a half-life of 2.6 × 10⁻⁸ seconds. The main decay mode is into a muon and its neutrino or antineutrino: :

\pi^+\to\mu^++\nu_\mu \,

\pi^-\to\mu^-+\bar{\nu}_\mu \,

The π⁰ has as slightly smaller mass of 135.0 MeV/''c''² and a ''much'' shorter mean life of 8.4 × 10⁻¹⁷ seconds. The main decay mode is into two photons: :

\pi^0\to2\gamma \,

==History== After theoretical work by Hideki Yukawa in 1935 had predicted the existence of mesons as the carrier particles of the strong nuclear force, charged pions were found experimentally in 1947 by a team led by Cecil Powell. The age of particle accelerators had yet to arrive in those days. Instead, the team lifted photographic emulsions by balloon to high altitude, where they were exposed to cosmic rays. After recovery of the balloon, microscopic inspection of the emulsion revealed the tracks of charged particles, among which the pions were discovered. Their achievements earned Yukawa in 1949 and Powell in 1950 the Nobel Prize in Physics. The π⁰ is more difficult to observe than π^±; being electrically neutral it doesn't leave a track in an emulsion. The π⁰ was finally identified in 1950 by its decay products. The pions are considered to be the pseudo Nambu-Goldstone bosons of a spontaneously broken symmetry. This is also the reason why the pion masses are considerably lighter than the masses of other mesons like the

\eta

-Meson (547.75 MeV/''c''²). ==Theoretical overview== The pion can be thought of as the particle that mediates the interaction between a pair of nucleons. This interaction is attractive, that is, 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 general terms of quantum field theory, the Lagrangian describing the pion-nucleon interaction is called the Yukawa interaction. The nearly identical masses of

\pi^\pm

and

\pi^0

imply that there must be a symemtry at play; this symmetry is called the SU(2) flavour symmetry or isospin. The reason that there are three pions,

\pi^+

\pi^-

and

\pi^0

is that these are understood to belong to the triplet representation or the adjoint representation of SU(2). By contrast, the up and down quarks transform accodring to the irreducible, semi-simple fundamental representation of SU(2), whereas the the anti-quarks transform according to the conjugate representation. Thus, one has :

SU(2) \otimes \overline{SU(2)} \approx SO(3) \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) symmetry with the strange paritcles, belonging to the octect representation (the adjoint representation of SU(3)). With the addition of the charmed, top, and bottom quarks, one can argue that the full flavour symmetry is SU(6) but the argument is strained because the large masses of these other quarks implies that the masses of the corresponding particles are nowhere near the pion mass. Pions are pseudoscalars under a parity (physics) 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 link== * [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 Meson Hadron Nuclear physics Quantum chromodynamics

Pion

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Pion

:zh:User:Pion

Pion

==ToDo List== The following article enhancements should be dealt with at some point: # pions mediate the strong (nuclear) force, # The Yukawa potential is a non-relativistic description for them, # The relativistic equation for pions is the Klein-Gordon equation # They are pseudoscalars under parity inversion, # They couple to the axial vector current, the couple is the axial coupling constant which is a "fundamental"/"important" parameter of the nucleon. # pion is part of the triplet representation aka adjoint representation of SU(2) flavour symmetry, because the up and down quarks belong doublet rep of SU(2) and a pion is a quark and anti-quark, and su(2) cross su(2)-bar is the adjoint rep. # The SU(2) flavour symmetry allows fun games with chiral symmetry and in particular, since su(2) also describes spatial rottations, one can play games with non-trivial topological mappings of the embedding of pion fields into 3D space. i.e. mappings because of the non-trivial fundamental group. # Skyrme's topological soliton aka skyrmion aka 'cloud of pions' aka 'chiral model' is a decent one-parameter model of the nucleon (proton/neutron), which predicts a variety of nuclear properties fairly accurately with only one free parameter. User:Linas 01:00, 24 May 2005 (UTC)

See other meanings of words starting from letter:

P

PA | PB | PC | PD | PE | PF | PG | PH | PI | PJ | PK | PL | PM | PN | PO | PR | PS | PT | PU | PW | PX | PY | PZ |

Words begining with Pion:

Pion
Pion
Pion
Pion
Pioneer
Pioneer,_IA
Pioneer,_Iowa
Pioneer,_LA
Pioneer,_Louisiana
Pioneer,_OH
Pioneer,_Ohio
Pioneer-12
Pioneer-12
Pioneer-12/Archive0000001
Pioneer-12/demos/page_concatenation
Pioneering
Pioneering_(Bah�'�)
Pioneers
Pioneers_by_field
Pioneers_of_photography
Pioneers_of_rail_transport
Pioneertown
Pioneertown,_CA
Pioneertown,_California
Pioneer_(company)
Pioneer_(military)
Pioneer_(military)
Pioneer_0
Pioneer_0
Pioneer_1
Pioneer_10
Pioneer_10
Pioneer_11
Pioneer_12
Pioneer_13
Pioneer_2
Pioneer_3
Pioneer_4
Pioneer_5
Pioneer_6
Pioneer_6,_7,_8_and_9
Pioneer_7
Pioneer_8
Pioneer_9
Pioneer_A
Pioneer_A,_B,_C,_D_and_E
Pioneer_anomaly
Pioneer_anomaly
Pioneer_Aviation
Pioneer_Award
Pioneer_B
Pioneer_C
Pioneer_Camps_of_Canada
Pioneer_Chicken
Pioneer_Chicken
Pioneer_Column
Pioneer_Corporation
Pioneer_Courthouse_Square
Pioneer_D
Pioneer_Day
Pioneer_E
Pioneer_effect
Pioneer_F
Pioneer_Football_League
Pioneer_Fund
Pioneer_Fund
Pioneer_G
Pioneer_HLD-X0
Pioneer_HLD-X0
Pioneer_I
Pioneer_II
Pioneer_III
Pioneer_Institute
Pioneer_Island
Pioneer_IV
Pioneer_IX
Pioneer_Jazz
Pioneer_League
Pioneer_Movement
Pioneer_Movement
Pioneer_movement
Pioneer_P
Pioneer_P-2
Pioneer_P-3
Pioneer_P-30
Pioneer_P-31
Pioneer_P-31
Pioneer_Park
Pioneer_Peaks_National_Park
Pioneer_Plant
Pioneer_plant
Pioneer_plaque
Pioneer_plaque
Pioneer_Playhouse
Pioneer_Press
Pioneer_program
Pioneer_program
Pioneer_program
Pioneer_program
Pioneer_Public_Television
Pioneer_PureVision_Las_Vegas_Bowl
Pioneer_Purevision_Las_Vegas_Bowl
Pioneer_Sergeant
Pioneer_species
Pioneer_species
Pioneer_Square
Pioneer_Square,_Seattle,_WA
Pioneer_Square,_Seattle,_Washington
Pioneer_Square_South_and_Pioneer_Square_North_(MAX_stations)
Pioneer_Total_Abstinence_Association
Pioneer_Total_Abstinence_Association
Pioneer_Township,_MI
Pioneer_Township,_Michigan
Pioneer_USA
Pioneer_V
Pioneer_Valley
Pioneer_Valley
Pioneer_Valley_Transit_Authority
Pioneer_Venus
Pioneer_Venus_1
Pioneer_Venus_2
Pioneer_Venus_Multiprobe
Pioneer_Venus_Orbiter
Pioneer_Venus_Probe_1
Pioneer_Venus_Probe_2
Pioneer_Venus_Probe_3
Pioneer_Venus_Probe_4
Pioneer_Venus_project
Pioneer_VI
Pioneer_VII
Pioneer_VIII
Pioneer_Village
Pioneer_Village,_Kentucky
Pioneer_Village,_KY
Pioneer_Women's_Memorial_Gardens
Pioneer_X
Pioneer_XI
Pioneer_XII
Pioneer_XIII
Pioneer_Y
Pioneer_Z
Pioneer_Zephyr
Pioneer_Zephyr
Pioneer_Zephyr/to_do
Pioner
Pionerskaya_Pravda
Pioniere
Pionium
Pionium
Pionki
Pions
Pionus
Pion_coiffé
Pion_coiff�