Everything about Pion totally explained
In
particle physics,
pion (short for
pi meson) is the collective name for three
subatomic particles:, and . Pions are the lightest
mesons (excluding the misnamed "Mu Meson" or
muon) and play an important role in explaining low-energy properties of the
strong nuclear force.
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, while a down and an anti-up quark compose the, its
antiparticle. The neutral combinations of up with anti-up and down with anti-down have identical
quantum numbers, so they're only found in
superpositions. The lowest-energy superposition is the, 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 (
Iz = +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. They decay due to
weak processes. The main decay mode (99.9877%) is into a
muon and its
neutrino:
» → +
→ +
The second largest decay mode (0.0123%) is into an
electron and the corresponding neutrino:
» → +
→ +
The meson has a 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:
» → 2
Its second largest decay mode (1.198%) is the so-called
Dalitz decay into a photon and an
electron-
positron pair:
» → + +
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 (
fπ), which is about 90 MeV.
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 didn't 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, 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"; note that the muon isn't 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 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 isn't 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 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 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.
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
(External Link
), and
TRIUMF in British Columbia, Canada
(External Link).
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's 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 and 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,, and, 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
kaons and the
eta meson.
Pions are
pseudoscalars under a
parity transformation. Pion currents thus couple to the
axial vector current and pions participate in the
chiral anomaly.
Further Information
Get more info on 'Pion'.
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