 The quark structure of the positively charged pion. | |
| Composition |
|
|---|---|
| Statistics | Bosonic |
| Family | Mesons |
| Interactions | Strong,weak,electromagnetic, andgravity |
| Symbol | π+ , π0 , and π− |
| Antiparticle |
|
| Theorized | Hideki Yukawa (1935) |
| Discovered |
|
| Types | 3 |
| Mass | |
| Mean lifetime |
|
| Electric charge |
|
| Charge radius | π± :±0.659(4) fm[1] |
| Color charge | 0 |
| Spin | 0 ħ |
| Isospin |
|
| Hypercharge | 0 |
| Parity | −1 |
| C parity | +1 |
Inparticle physics, apion (/ˈpaɪ.ɒn/,PIE-on) orpi meson, denoted with theGreek letterpi (
π
), is any of threesubatomic particles:
π0
,
π+
, and
π−
. Each pion consists of aquark and anantiquark and is therefore ameson. Pions are the lightest mesons and, more generally, the lightesthadrons. They are unstable, with the charged pions
π+
and
π−
decaying after amean lifetime of 26.033 nanoseconds (2.6033×10−8 seconds), and the neutral pion
π0
decaying after a much shorter lifetime of 85 attoseconds (8.5×10−17 seconds).[1] Charged pions most oftendecay intomuons andmuon neutrinos, while neutral pions generally decay intogamma rays.
The exchange ofvirtual pions, along withvector,rho andomega mesons, provides an explanation for theresidual strong force betweennucleons. Pions are not produced inradioactive decay, but commonly are in high-energy collisions betweenhadrons. Pions also result from some matter–antimatterannihilation events. All types of pions are also produced in natural processes when high-energycosmic-ray protons and other hadronic cosmic-ray components interact withmatter in Earth'satmosphere. In 2013, the detection of characteristic gamma rays originating from the decay of neutral pions in twosupernova 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.[2]
The pion also plays a crucial role in cosmology, by imposing an upper limit on the energies of cosmic rays surviving collisions with thecosmic microwave background, through theGreisen–Zatsepin–Kuzmin limit.[3]
Theoretical work byHideki Yukawa in 1935 had predicted the existence ofmesons as the carrier particles of thestrong nuclear force. From the range of the strong nuclear force (inferred from the radius of theatomic nucleus), Yukawa predicted the existence of a particle having a mass of about100 MeV/c2. Initially after its discovery in 1936, themuon (initially called the "mu meson") was thought to be this particle, since it has a mass of106 MeV/c2. However, later experiments showed that the muon did not participate in the strong nuclear interaction. In modern terminology, this makes the muon alepton, and not a meson. However, some communities of astrophysicists continue to call the muon a "mu-meson".[according to whom?] 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.
In 1947, the first true mesons, the charged pions, were found by the collaboration led byCecil Powell at theUniversity of Bristol, in England. The discovery article had four authors:César Lattes,Giuseppe Occhialini,Hugh Muirhead and Powell.[4] Since the advent ofparticle accelerators had not yet come, high-energy subatomic particles were only obtainable from atmosphericcosmic rays.Photographic emulsions based on thegelatin-silver process were placed for long periods of time in sites located at high-altitude mountains, first atPic du Midi de Bigorre in thePyrenees, and later atChacaltaya in theAndes Mountains, where the plates were struck by cosmic rays.After development, thephotographic plates were inspected under amicroscope by a team of about a dozen women.[5]Marietta Kurz was the first person to detect the unusual "double meson" tracks, characteristic for a pion decaying into amuon, but they were too close to the edge of the photographic emulsion and deemed incomplete. A few days later, Irene Roberts observed the tracks left by pion decay that appeared in the discovery paper. Both women are credited in the figure captions in the article.
In 1948,Lattes,Eugene Gardner, and their team first artificially produced pions at theUniversity of California'scyclotron inBerkeley, California, by bombardingcarbon atoms with high-speedalpha particles. Further advanced theoretical work was carried out byRiazuddin, who in 1959 used thedispersion relation forCompton scattering ofvirtual photons on pions to analyze their charge radius.[6]
Since the neutral pion is notelectrically charged, it is more difficult to detect and observe than the charged pions are. Neutral pions do not leave tracks in photographic emulsions or Wilsoncloud chambers. The existence of the neutral pion was inferred from observing its decay products fromcosmic rays, a so-called "soft component" of slow electrons with photons. The
π0
was identified definitively at the University of California's cyclotron in 1949 by observing its decay into two photons.[7] Later in the same year, they were also observed in cosmic-ray balloon experiments at Bristol University.
... Yukawa choose the letterπ because of its resemblance to theKanji character for介 [kai], which means "to mediate", based on the idea that the meson works as a strong force mediator particle between hadrons.[8]
The use of pions in medical radiation therapy, such as for cancer, was explored at a number of research institutions, including theLos Alamos National Laboratory'sMeson Physics Facility, which treated 228 patients between 1974 and 1981 inNew Mexico,[9] and theTRIUMF laboratory inVancouver, British Columbia.
In the standard understanding of thestrong force interaction as defined byquantum chromodynamics, pions are loosely portrayed asGoldstone bosons of spontaneouslybroken 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 currentquarks were massless particles, it could make the chiral symmetry exact and thus the Goldstone theorem would dictate that all pions have a zero mass.
In fact, it was shown by Gell-Mann, Oakes and Renner (GMOR)[10] that the square of the pion mass is proportional to the sum of the quark masses times thequark condensate: withB the quark condensate: This is often known as theGMOR relation and it explicitly shows that in the massless quark limit. The same result also follows fromLight-front holography.[11]
Empirically, since the light quarks actually have minuscule nonzero masses, the pions also have nonzerorest masses. However, those masses arealmost an order of magnitude smaller than that of the nucleons, roughly[10] 45 MeV, wheremq are the relevant current-quark masses in MeV, around 5−10 MeV.
The pion is one of the particles that mediate the residual strong interaction between a pair ofnucleons. This interaction is attractive: it pulls the nucleons together. Written in a non-relativistic form, it is called theYukawa potential. The pion, being spinless, haskinematics described by theKlein–Gordon equation. In the terms ofquantum field theory, theeffective field theoryLagrangian describing the pion-nucleon interaction is called theYukawa interaction.
The nearly identical masses of
π±
and
π0
indicate that there must be a symmetry at play: this symmetry is called theSU(2)flavour symmetry orisospin. The reason that there are three pions,
π+
,
π−
and
π0
, is that these are understood to belong to the triplet representation or theadjoint representation3 of SU(2). By contrast, the up and down quarks transform according to thefundamental representation2 of SU(2), whereas the anti-quarks transform according to the conjugate representation2*.
With the addition of thestrange quark, the pions participate in a larger, SU(3), flavour symmetry, in the adjoint representation,8, of SU(3). The other members of thisoctet are the fourkaons and theeta meson.
Pions arepseudoscalars under aparity transformation. Pion currents thus couple to the axial vector current and so participate in thechiral anomaly.
Pions, which aremesons with zerospin, are composed of first-generationquarks. In thequark model, anup quark and an anti-down quark make up a
π+
, whereas adown quark and an anti-up quark make up the
π−
, and these are theantiparticles 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 identicalquantum numbers, and hence they are only found insuperpositions. The lowest-energy superposition of these is the
π0
, which is its own antiparticle. Together, the pions form a triplet ofisospin. Each pion has overallisospin (I = 1) and third-componentisospin equal to its charge (Iz = +1, 0, −1).
The
π±
mesons have amass of139.6 MeV/c2 and amean lifetime of2.6033×10−8 s. They decay due to theweak interaction. The primary decay mode of a pion, with abranching fraction of 0.999877, is aleptonic decay into amuon and amuon neutrino:
The second most common decay mode of a pion, with a branching fraction of 0.000123, is also a leptonic decay into anelectron and the correspondingelectron antineutrino. This "electronic mode" was discovered atCERN in 1958:[12]
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,
and is aspin effect known ashelicity suppression.
Its mechanism is as follows: The negative pion has spin zero; therefore the lepton and the 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 leftchirality component of fields, the antineutrino has always left chirality, 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 relatively massless compared with the muon, and thus the electronic mode is greatly suppressed relative to the muonic one, virtually prohibited.[13]
Although this explanation suggests that parity violation is causing the helicity suppression, the fundamental reason lies in the vector-nature of the interaction which dictates 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 oflepton universality. Experimentally, this ratio is1.233(2)×10−4.[1]
Beyond 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 "pionbeta 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 aschiral perturbation theory. This rate is parametrized by thepion decay constant (fπ), related to thewave function overlap of the quark and antiquark, which is about130 MeV.[14]
The
π0
meson has a mass of135.0 MeV/c2 and a mean lifetime of8.5×10−17 s.[1] It decays via theelectromagnetic force, which explains why its mean lifetime is much smaller than that of the charged pion (which can only decay via theweak force).
The dominant
π0
decay mode, with abranching ratio ofBRγγ = 0.98823, is into twophotons:
The decay
π0
→ 3
γ
(as well as decays into any odd number of photons) is forbidden by theC-symmetry of the electromagnetic interaction: The intrinsic C-parity of the
π0
is +1, while the C-parity of a system ofn photons is(−1)n.
The second largest
π0
decay mode (BRγee = 0.01174) is the Dalitz decay (named afterRichard Dalitz), which is a two-photon decay with an internal photon conversion resulting in a photon and anelectron-positron pair in the final state:
The third largest established decay mode (BR2e2e = 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 theloop-induced and therefore suppressed (and additionallyhelicity-suppressed) leptonic decay mode (BRee = 6.46×10−8):
The neutral pion has also been observed to decay intopositronium with a branching fraction on the order of 10−9. No other decay modes have been established experimentally. The branching fractions above are thePDG central values, and their uncertainties are omitted, but available in the cited publication.[1]
| Particle name | Particle symbol | Antiparticle symbol | Quark content[15] | Rest mass (MeV/c2) | IG | JPC | S | C | B' | Mean lifetime (s) | Commonly decays to (>5% of decays) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Pion[1] | π+ | π− | u d | 139.57039 ±0.00018 | 1− | 0− | 0 | 0 | 0 | 2.6033 ± 0.0005 × 10−8 | μ+ + ν μ |
| Pion[1] | π0 | Self | [a] | 134.9768 ±0.0005 | 1− | 0−+ | 0 | 0 | 0 | 8.5 ± 0.2 × 10−17 | γ + γ |
[a]^ The quark composition of the
π0
is not exactly divided between up and down quarks, due to complications from non-zero quark masses.[16]
{{cite journal}}: CS1 maint: multiple names: authors list (link)| Elementary |
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