In particle physics, a pion (or a pi meson,
denoted with the Greek letter pi: π) is any
of three subatomic particles: π0, π+, and
π−. Each pion consists of a quark and an
antiquark and is therefore a meson. Pions
are the lightest mesons and, more generally,
the lightest hadrons. They are unstable, with
the charged pions π+ and π− decaying with
a mean lifetime of 26.033 nanoseconds (2.6033×10−8
seconds), and the neutral pion π0 decaying
with a much shorter lifetime of 8.4×10−17
seconds. Charged pions most often decay into
muons and muon neutrinos, while neutral pions
generally decay into gamma rays.
The exchange of virtual pions, along with
the vector, rho and omega mesons, provides
an explanation for the residual strong force
between nucleons. Pions are not produced in
radioactive decay, but are commonly produced
in high energy accelerators in collisions
between hadrons. 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, the detection
of characteristic gamma rays originating from
the decay of neutral pions in two supernova
remnants has shown that pions are produced
copiously after supernovas, most probably
in conjunction with production of high energy
protons that are detected on Earth as cosmic
rays.The concept of mesons as the carrier
particles of the nuclear force was first proposed
in 1935 by Hideki Yukawa. While the muon was
first proposed to be this particle after its
discovery in 1936, later work found that it
did not participate in the strong nuclear
interaction. The pions, which turned out to
be examples of Yukawa's proposed mesons, were
discovered later: the charged pions in 1947,
and the neutral pion in 1950.
== 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 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 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 meson. However,
some communities of astrophysicists continue
to call the muon a "mu-meson".
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 accelerators
had not yet come, high-energy subatomic particles
were only obtainable from atmospheric cosmic
rays. Photographic emulsions based on 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 the plates were
struck by cosmic rays.
After the development of the photographic
plates, 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 a putative meson. The particle
was identified as a muon, which is not typically
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's cyclotron in
Berkeley, California, by bombarding carbon
atoms with high-speed alpha particles. Further
advanced theoretical work was carried out
by Riazuddin, who in 1959, used the dispersion
relation for Compton scattering of virtual
photons on pions to analyze their charge radius.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 emulsions.
Since the neutral pion is not electrically
charged, it is more difficult to detect and
observe than the charged pions are. Neutral
pions do not leave tracks in photographic
emulsions or Wilson cloud chambers. The existence
of the neutral pion was inferred from observing
its decay products from cosmic rays, a so-called
"soft component" of slow electrons with photons.
The π0 was identified definitively at the
University of California's cyclotron in 1950
by observing its decay into two photons. Later
in the same year, they were also observed
in cosmic-ray balloon experiments at Bristol
University.
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.
In the standard understanding of the strong
force interaction as defined by quantum chromodynamics,
pions are loosely portrayed as Goldstone bosons
of spontaneously broken chiral symmetry. That
explains why the masses of the three kinds
of pions are considerably less than that of
the other mesons, such as the scalar or vector
mesons. If their current quarks were massless
particles, it could make the chiral symmetry
exact and thus the Goldstone theorem would
dictate that all pions have a zero mass. Empirically,
since the light quarks actually have minuscule
nonzero masses, the pions also have nonzero
rest masses. However, those weights are almost
an order of magnitude smaller than that of
the nucleons, roughly mπ ≈ √v mq / fπ
≈ √mq 45 MeV, where m are the relevant
current quark masses in MeV, 5−10 MeVs.
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, and the TRIUMF
laboratory in Vancouver, British Columbia.
== Theoretical overview ==
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.
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 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.
== Basic properties ==
Pions, which are mesons with zero spin, are
composed of first-generation quarks. In the
quark model, an up quark and an anti-down
quark make up a π+, whereas a down quark
and an anti-up quark make up the π−, and
these are the antiparticles of one another.
The neutral pion π0 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 numbers, and hence
they are only found in superpositions. The
lowest-energy superposition of these is the
π0, which is its own antiparticle. Together,
the pions form a triplet of isospin. Each
pion has isospin (I = 1) and third-component
isospin equal to its charge (Iz = +1, 0 or
−1).
=== Charged pion decays ===
The π± mesons have a mass of 139.6 MeV/c2
and a mean lifetime of 2.6033×10−8 s. They
decay due to the weak interaction. The primary
decay mode of a pion, with a branching fraction
of 0.999877, is a leptonic decay into a muon
and a muon neutrino:
The second most common decay mode of a pion,
with a branching fraction of 0.000123, is
also a leptonic decay into an electron and
the corresponding electron antineutrino. This
"electronic mode" was discovered at CERN in
1958:
The suppression of the electronic decay mode
with respect to the muonic one is given approximately
(up to a few percent effect of the radiative
corrections) by the ratio of the half-widths
of the pion–electron and the pion–muon
decay reactions:
R
π
=
(
m
e
/
m
μ
)
2
(
m
π
2
−
m
e
2
m
π
2
−
m
μ
2
)
2
=
1.283
×
10
−
4
{\displaystyle R_{\pi }=(m_{e}/m_{\mu })^{2}\left({\frac
{m_{\pi }^{2}-m_{e}^{2}}{m_{\pi }^{2}-m_{\mu
}^{2}}}\right)^{2}=1.283\times 10^{-4}}
and is a spin effect known as helicity suppression.
Its mechanism is as follows: The negative
pion has spin zero, therefore the lepton and
antineutrino must be emitted with opposite
spins (and opposite linear momenta) to preserve
net zero spin (and conserve linear momentum).
However, because the weak interaction is sensitive
only to the left chirality component of fields,
the antineutrino has always chirality left,
which means it is right-handed, since for
massless anti-particles the helicity is opposite
to the chirality. This implies that the lepton
must be emitted with spin in the direction
of its linear momentum (i.e., also right-handed).
If, however, leptons were massless, they would
only interact with the pion in the left-handed
form (because for massless particles helicity
is the same as chirality) and this decay mode
would be prohibited. Therefore, suppression
of the electron decay channel comes from the
fact that the electron's mass is much smaller
than the muon's. The electron is thus relatively
massless compared with the muon, and thus
the electronic mode is almost prohibited.
Although this explanation suggests that parity
violation is causing the helicity suppression,
it should be emphasized that the fundamental
reason lies in the vector-nature of the interaction
which demands a different handedness for the
neutrino and the charged lepton. Thus, even
a parity conserving interaction would yield
the same suppression.
Measurements of the above ratio have been
considered for decades to be a test of lepton
universality. Experimentally, this ratio is
1.230(4)×10−4.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.
Also observed, for charged pions only, is
the very rare "pion beta decay" (with branching
fraction of about 10−8) into a neutral pion,
an electron and an electron antineutrino (or
for positive pions, a neutral pion, a positron,
and electron neutrino).
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
(ƒπ), related to the wave function overlap
of the quark and antiquark, which is about
130 MeV.
=== Neutral pion decays ===
The π0 meson has a mass of 135.0 MeV/c2 and
a mean lifetime of 8.4×10−17 s. It decays
via the electromagnetic force, which explains
why its mean lifetime is much smaller than
that of the charged pion (which can only decay
via the weak force). The main π0 decay mode,
with a branching ratio of BR=0.98823, is into
two photons:
The decay π0 → 3γ (as well as decays into
any odd number of photons) is forbidden by
the C-symmetry of the electromagnetic interaction.
The intrinsic C-parity of the π0 is +1, while
the C-parity of a system of n photons is (−1)n.
The second largest π0 decay mode (BR=0.01174)
is the Dalitz decay (named after Richard Dalitz),
which is a two-photon decay with an internal
photon conversion resulting a photon and an
electron-positron pair in the final state:
The third largest established decay mode (BR=3.34×10−5)
is the double Dalitz decay, with both photons
undergoing internal conversion which leads
to further suppression of the rate:
The fourth largest established decay mode
is the loop-induced and therefore suppressed
(and additionally helicity-suppressed) leptonic
decay mode (BR=6.46×10−8):
The neutral pion has also been observed to
decay into positronium with a branching fraction
of the order of 10−9. No other decay modes
have been established experimentally. The
branching fractions above are the PDG central
values, and their uncertainties are not quoted.
[a] ^ Make-up inexact due to non-zero quark
masses.
== See also ==
Pionium
List of particles
Quark model
Static forces and virtual-particle exchange
César Lattes
