Pion

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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.

Image:Pi-plus.jpg

1 Basic properties
2 History
3 Theoretical overview
4 See also
5 References
6 External links

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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).

The π^± mesons have a mass of 139.6 MeV/c² and a mean life of 2.6×10⁻⁸ seconds. Their main decay mode is into a muon and its neutrino:

The π⁰ has as slightly smaller mass of 135.0 MeV/c² and a much shorter mean life of 8.4×10⁻¹⁷ seconds. Its main decay mode is into two photons:

<math>\pi^0\to2\gamma</math>

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.

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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.

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.

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 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 <math>\eta^\prime</math> 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.

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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 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

<math>SU(2) \otimes \overline{SU(2)} \approx SO(3) \oplus U(1)</math>

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.

Pions are pseudoscalars under a parity transformation. Pion currents thus couple to the axial vector current and pions participate in the chiral anomaly.

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