In particle physics, pion (short for pi meson) is the collective name for 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. Particles erupt from the collision point of two relativistic (100 GeV per nucleon) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... A subatomic particle is a particle smaller than an atom: it may be elementary or composite. ... In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ...

The quark model of π⁺ (positive pion): it consists of one up quark and one anti-down quark

Image File history File links Quark_structure_pion. ... Image File history File links Quark_structure_pion. ...

Basic properties

Pions have zero spin 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. The neutral combinations of up with anti-up and down with anti-down have identical quantum numbers, so they are only found in superpositions. The lowest-energy superposition is the π⁰, 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_z = +1, 0 or −1). In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is generated by the motion of its center of mass about an external point. ... According to the standard model of particle physics, all the elementary particles seen in particle collision experiments can be divided into three generations. ... Quarks are one of the two basic constituents of matter in the Standard Model of particle physics. ... In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks, ie, the quarks (and antiquarks) which give rise to the quantum numbers of the hadrons. ... Corresponding to each kind of particle, there is an associated antiparticle with the same mass and spin. ... A quantum number describes the energies of electrons in atoms. ... Quantum superposition is the application of the superposition principle to quantum mechanics. ... Isospin (isotopic spin, isobaric spin) is a physical quantity which is mathematically analogous to spin. ...

The mesons have a mass of 139.6 MeV/c² and a mean life of 2.6×10⁻⁸ seconds. They decay due to weak processes. The main decay mode (99.9877%) is into a muon and its neutrino: Mass is a property of a physical object that quantifies the amount of matter it contains. ... Given an assembly of elements, the number of which decreases ultimately to zero, the lifetime (also called the mean lifetime) is a certain number that characterizes the rate of reduction (decay) of the assembly. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... The moons shadow, as seen in muons 700m below ground at the Soudan 2 detector. ... The neutrino is an elementary particle. ...

The second largest decay mode (0.0123%) is into an electron and the corresponfing neutrino: Properties The electron is a lightweight fundamental subatomic particle that carries a negative electric charge. ...

The π⁰ meson has as slightly smaller mass of 135.0 MeV/c² and a much shorter mean life of 8.4×10⁻¹⁷ seconds. It decays due to electromagnetic force. The main decay mode (98.798%) is into two photons: Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... In physics, the photon (from Greek φως, phōs, meaning light) is the quantum of the electromagnetic field; for instance, light. ...

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Its second largest decay mode (1.198%) is the so-called Dalitz decay into a photon and an electron-positron pair: Richard Henry Dalitz (28 February 1925 – 13 January 2006) was an Australian physicist known for his work in quantum mechanics. ... Properties The electron is a lightweight fundamental subatomic particle that carries a negative electric charge. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ...

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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_π), which is about 90 MeV. Chiral perturbation theory is an effective field theory constructed on a lagrangian consistent with the (approximate) chiral symmetry of quantum chromodynamics. ... In particle physics, the pion decay constant is the square root of the coefficient in front of the kinetic term for the pion in the low-energy effective action. ...

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 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. Hideki Yukawa Hideki Yukawa (湯川 秀樹, January 23, 1907 - September 8, 1981) was a Japanese theoretical physicist and the first Japanese person to win the Nobel prize. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... A semi-accurate depiction of the helium atom. ... The moons shadow, as seen in muons 700m below ground at the Soudan 2 detector. ... In physics, a particle is a lepton if it has a spin of 1/2 and does not experience the strong nuclear force. ...

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 accelerators had yet to arrive in those days, high energies were only accessible from atmospheric cosmic rays. Photographic emulsions 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). In 1948, Lattes and Eugene Gardner first achieved artificial production of pion particles at the University of California at Berkeley cyclotron by bombarding carbon atoms with alpha particles. Cecil Frank Powell (December 5, 1903 _ August 9, 1969) was a British physicist, awarded the Nobel Prize for Physics in 1950 for his development of the photographic method of studying nuclear processes and for the resulting discovery of the pion (pi-meson), a heavy subatomic particle. ... César Lattes in 1987 Cesare Mansueto Giulio Lattes (b. ... Giuseppe Occhialini (b. ... The University of Bristol is a university in Bristol in the United Kingdom. ... A 1960s single stage 2MeV linear Van de Graaff accelerator, here opened for maintenance A particle accelerator is a device that uses electric and/or magnetic fields to propel electrically charged particles to high speeds. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... The Gelatin-silver process is the photographic process used with currently available black and white films and printing papers. ... Another Pic du Midi is the Pic du Midi dOssau. ... Central Pyrenees. ... Chacaltaya is a mountain in Bolivia with an elevation of 5421 m (17,785 feet). ... The Andes between Chile and Argentina Computer generated image of the Andes, made from a digital elevation model with a resolution of 30 arcseconds The Andes is a vast mountain range forming a continuous chain of highland along the western coast of South America. ... The University of California, Berkeley (also known as Cal, UC Berkeley, UCB, or simply Berkeley) is a prestigious, public, coeducational university situated in the foothills of Berkeley, California to the east of San Francisco Bay, overlooking the Golden Gate and its bridge. ... A pair of Dee electrodes with loops of coolant pipes on their surface at the Lawrence Hall of Science. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... An alpha particle is deflected by a magnetic field Alpha particles (named after the first letter in the Greek alphabet, α) are a highly ionizing form of particle radiation which have low penetration. ...

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). Hannes Alfvén (1908–1995) accepting the Nobel Prize for his work on magnetohydrodynamics [1]. List of Nobel Prize laureates in Physics from 1901 to the present day. ...

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 π⁰ was identified at the Berkeley cyclotron in 1950 by its decay into two photons.

In the modern understanding of the strong interaction (quantum chromodynamics), pions are considered to be the pseudo Nambu-Goldstone bosons of spontaneously broken chiral symmetry. This explains why the pion masses are considerably lighter than the masses of other mesons like the meson (958 MeV). If their constituent quarks 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. Quantum chromodynamics (QCD) is the theory of the strong interaction, a fundamental force describing the interactions of the quarks and gluons found in nucleons (such as the proton and neutron). ... In particle physics, Goldstone bosons are bosons that appear in models with spontaneously broken symmetry. ... Spontaneous symmetry breaking in physics takes place when a system that is symmetric with respect to some symmetry group goes into a vacuum state that is not symmetric. ... A phenomenon is said to be chiral if it is not identical to its mirror image (see Chirality (mathematics)). The spin of a particle may be used to define a handedness for that particle. ... Quarks are one of the two basic constituents of matter in the Standard Model of particle physics. ...

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. A Yukawa potential (also called a screened Coulomb potential) is a potential of the form Hideki Yukawa showed in the 1930s that such a potential arises from the exchange of a massive scalar field such as the field of the pion whose mass is . ... This article or section may be confusing for some readers, and should be edited to be clearer or more simplified. ... The Klein-Gordon equation (Klein-Fock-Gordon equation or sometimes Klein-Gordon-Fock equation) is the relativistic version of the Schrödinger equation. ... Quantum field theory (QFT) is the application of quantum mechanics to fields. ... In physics, an effective field theory is an approximate theory (usually a quantum field theory) that contains the appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale, but ignores the substructure and the degrees of freedom at shorter distances (or, equivalently, higher energies). ... A Lagrangian of a dynamical system, named after Joseph Louis Lagrange, is a function of the dynamical variables and concisely describes the equations of motion of the system. ... In particle physics, Yukawa interaction, named after Hideki Yukawa, is an interaction between a scalar field and a Dirac field of the type . The Yukawa interaction can be used to describe the strong nuclear force between nucleons (which are fermions), mediated by pions (which are scalar mesons). ...

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 irreducible, semi-simple fundamental representation 2 of SU(2), whereas the anti-quarks transform according to the conjugate representation 2*. Thus, one has In mathematics, the special unitary group of degree n is the group of n by n unitary matrices with determinant 1 and entries from the field C of complex numbers, with the group operation that of matrix multiplication. ... Flavour (or flavor) is a quantum number of elementary particles related to their weak interactions. ... Isospin (isotopic spin, isobaric spin) is a physical quantity which is mathematically analogous to spin. ... The adjoint representation of a Lie group G is the linearized version of the action of G on itself by conjugation. ... In mathematics, a fundamental representation is a representation of a mathematical structure, such as a group, that satisfies the following condition: All other irreducible representations of the group can be found in the tensor products of the fundamental representation with many copies of itself. ...

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) flavour symmetry, belonging to the adjoint representation 8 of SU(3). The other members of this octet are the four kaons and the eta. For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the Σc++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... In particle physics, Kaons (also called K-mesons and denoted K) are a group of four mesons distinguished by the fact that they carry a quantum number called strangeness. ...

Pions are pseudoscalars under a parity transformation. Pion currents thus couple to the axial vector current and pions participate in the chiral anomaly. In mathematics, a pseudoscalar in a geometric algebra is the highest-grade basis element of the algebra. ... In physics, a parity transformation (also called parity inversion) is the simultaneous flip in the sign of all spatial coordinates: A 3×3 matrix representation of P would have determinant equal to –1, and hence cannot reduce to a rotation. ... A chiral anomaly is the anomalous nonconservation of a chiral current. ...

See also

• Pionium
• List of particles
• Quark model

Pionium is an exotic atom consisting of a π+ and a π− meson. ... This is a list of particles in particle physics, including currently known and hypothetical elementary particles, as well as the composite particles that can be built up from them. ... In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks, ie, the quarks (and antiquarks) which give rise to the quantum numbers of the hadrons. ...

References

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

External links

• Mesons at the Particle Data Group
• Mesons at Hyperphysics