Elementary particles are particles with no measurable internal structure; that is, they are not composed of other particles. They are the fundamental objects of quantum field theory. Many families and sub-families of elementary particles exist. Elementary particles are classified according to their spin. Fermions have half-integer spin while bosons have integer spin. All the particles of the Standard Model have been experimentally observed, recently including the Higgs boson.
Fermions are one of the two fundamental classes of particles, the other being bosons. Fermion particles are described by Fermi–Dirac statistics and have quantum numbersdescribed by the Pauli exclusion principle. They include the quarks and leptons, as well as any composite particles consisting of an odd number of these, such as all baryons and many atoms and nuclei.
Fermions have half-integer spin; for all known elementary fermions this is 1⁄2. All known fermions are also Dirac fermions; that is, each known fermion has its own distinctantiparticle. It is not known whether the neutrino is a Dirac fermion or a Majorana fermion. Fermions are the basic building blocks of all matter. They are classified according to whether they interact via the color force or not. In the Standard Model, there are 12 types of elementary fermions: six quarks and six leptons.
Quarks are the fundamental constituents of hadrons and interact via the strong interaction. Quarks are the only known carriers of fractional charge, but because they combine in groups of three (baryons) or in groups of two with antiquarks (mesons), only integer charge is observed in nature. Their respective antiparticles are the antiquarks which are identical except for the fact that they carry the opposite electric charge (for example the up quark carries charge +2⁄3, while the up antiquark carries charge −2⁄3), color charge, and baryon number. There are six flavors of quarks; the three positively charged quarks are called up-type quarks and the three negatively charged quarks are called down-type quarks.
Leptons do not interact via the strong interaction. Their respective antiparticles are the antileptons which are identical except for the fact that they carry the opposite electric charge and lepton number. The antiparticle of the electron is the antielectron, which is nearly always called positron for historical reasons. There are six leptons in total; the three charged leptons are called electron-like leptons, while the neutral leptons are called neutrinos. Neutrinos are known to oscillate, so that neutrinos of definite flavour do not have definite mass, rather they exist in a superposition of mass eigenstates. The hypothetical heavy right-handed neutrino, called a sterile neutrino, has been left off the list.
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Bosons are one of the two fundamental classes of particles, the other being fermions. Bosons are characterized by Bose–Einstein statistics and all have integer spins. Bosons may be either elementary, like photons and gluons, or composite, like mesons.
The fundamental forces of nature are mediated by gauge bosons, and mass is believed to be created by the Higgs Field. According to the Standard Model (and to both linearizedgeneral relativity and string theory, in the case of the graviton) the elementary bosons are:
|Name||Symbol||Antiparticle||Charge (e)||Spin||Mass (GeV/c2)||Interaction mediated||Existence|
|W boson||W−||W+||−1||1||80.4||Weak interaction||Confirmed|
|Z boson||Z||Self||0||1||91.2||Weak interaction||Confirmed|
The graviton is added to the list although it is not predicted by the Standard Model, but by other theories in the framework of quantum field theory. Furthermore, gravity is non-renormalizable. There are a total of 8 independent gluons. The Higgs boson is postulated by the electroweak theory primarily to explain the origin of particle masses. In a process known as the Higgs mechanism, the Higgs boson and the other gauge bosons in the Standard Model acquire mass via spontaneous symmetry breaking of the SU(2) gauge symmetry. The Minimal Supersymmetric Standard Model (MSSM) predicts several Higgs bosons. A new particle expected to be the Higgs boson was observed at theCERN/LHC on March 14th of 2013 around the energy of 126.5GeV with the accuracy of close to five sigma (99.9999% that is accepted as definitive). The Higgs mechanism giving mass to other particles has not been observed yet.
Supersymmetric theories predict the existence of more particles, none of which have been confirmed experimentally as of 2014:
|neutralino||neutral bosons||1⁄2||The neutralinos are superpositions of the superpartners of neutral Standard Model bosons: neutral higgs boson, Z boson and photon.
The lightest neutralino is a leading candidate for dark matter.
The MSSM predicts 4 neutralinos.
|chargino||charged bosons||1⁄2||The charginos are superpositions of the superpartners of charged Standard Model bosons: charged higgs boson and W boson.
The MSSM predicts two pairs of charginos.
|photino||photon||1⁄2||Mixing with zino and neutral Higgsinos for neutralinos.|
|wino, zino||W± and Z0 bosons||1⁄2||The charged wino mixing with the charged Higgsino for charginos, for the zino see line above.|
|Higgsino||Higgs boson||1⁄2||For supersymmetry there is a need for several Higgs bosons, neutral and charged, according with their superpartners.|
|gluino||gluon||1⁄2||Eight gluons and eight gluinos.|
|gravitino||graviton||3⁄2||Predicted by Supergravity (SUGRA). The graviton is hypothetical, too – see next table.|
|sleptons||leptons||0||The superpartners of the leptons (electron, muon, tau) and the neutrinos.|
|sneutrino||neutrino||0||Introduced by many extensions of the Standard Supermodel, and may be needed to explain the LSND results.
A special role has the sterile sneutrino, the supersymmetric counterpart of the hypothetical right-handed neutrino, called sterile neutrino.
|squarks||quarks||0||The stop squark (superpartner of the top quark) is thought to have a low mass and is often the subject of experimental searches.|
Note: Just as the photon, Z boson and W± bosons are superpositions of the B0, W0, W1, and W2 fields – the photino, zino, and wino± are superpositions of the bino0, wino0, wino1, and wino2 by definition.
No matter if one uses the original gauginos or this superpositions as a basis, the only predicted physical particles are neutralinos and charginos as a superposition of them together with the Higgsinos.
Other theories predict the existence of additional bosons:
|graviton||2||Has been proposed to mediate gravity in theories of quantum gravity.|
|graviscalar||0||Also known as radion.|
|graviphoton||1||Also known as gravivector.
|axion||0||A pseudoscalar particle introduced in Peccei–Quinn theory to solve the strong-CP problem.|
|axino||1⁄2||Superpartner of the axion. Forms, together with the saxion and axion, a supermultipletin supersymmetric extensions of Peccei–Quinn theory.|
|branon||?||Predicted in brane world models.|
|dilaton||0||Predicted in some string theories.|
|dilatino||1⁄2||Superpartner of the dilaton.|
|X and Y bosons||1||These leptoquarks are predicted by GUT theories to be heavier equivalents of the W and Z.|
|W’ and Z’ bosons||1|
|magnetic photon||?||A. Salam (1966). “Magnetic monopole and two photon theories of C-violation”. Physics Letters 22 (5): 683–684.|
|majoron||0||Predicted to understand neutrino masses by the seesaw mechanism.|
|majorana fermion||1⁄2 ; 3⁄2 ?…||gluino, neutralino, or other – is its own antiparticle.|
|Chameleon particle||0||a possible candidate for dark energy and dark matter, and may contribute to cosmic inflation.|
Kaluza–Klein towers of particles are predicted by some models of extra dimensions. The extra-dimensional momentum is manifested as extra mass in four-dimensional spacetime.
- Composite fermions, in which case they are called baryons.
- Composite bosons, in which case they are called mesons.
Quark models, first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks “aces”), describe the known hadrons as composed of valencequarks and/or antiquarks, tightly bound by the color force, which is mediated by gluons. A “sea” of virtual quark-antiquark pairs is also present in each hadron.
- Nucleons are the fermionic constituents of normal atomic nuclei:
- Hyperons, such as the Λ, Σ, Ξ, and Ω particles, which contain one or more strange quarks, are short-lived and heavier than nucleons. Although not normally present in atomic nuclei, they can appear in short-lived hypernuclei.
- A number of charmed and bottom baryons have also been observed.
Some hints at the existence of exotic baryons have been found recently; however, negative results have also been reported. Their existence is uncertain.
- Pentaquarks consist of four valence quarks and one valence antiquark.
Ordinary mesons are made up of a valence quark and a valence antiquark. Because mesons have spin of 0 or 1 and are not themselves elementary particles, they are composite bosons. Examples of mesons include the pion, kaon, the J/ψ. In quantum hydrodynamic models, mesons mediate the residual strong force between nucleons.
- A tetraquark consists of two valence quarks and two valence antiquarks;
- A glueball is a bound state of gluons with no valence quarks;
- Hybrid mesons consist of one or more valence quark-antiquark pairs and one or more real gluons.
Atomic nuclei consist of protons and neutrons. Each type of nucleus contains a specific number of protons and a specific number ofneutrons, and is called a nuclide or isotope. Nuclear reactions can change one nuclide into another. See table of nuclides for a complete list of isotopes.
Atoms are the smallest neutral particles into which matter can be divided by chemical reactions. An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. Each type of atom corresponds to a specific chemical element. To date, 118 elements have been discovered, while only the elements 1-112,114, and 116 have received official names. Refer to the periodic table for an overview.
The atomic nucleus consists of protons and neutrons. Protons and neutrons are, in turn, made of quarks.
Molecules are the smallest particles into which a non-elemental substance can be divided while maintaining the physical properties of the substance. Each type of molecule corresponds to a specific chemical compound. Molecules are a composite of two or more atoms. See list of compounds for a list of molecules.
The field equations of condensed matter physics are remarkably similar to those of high energy particle physics. As a result, much of the theory of particle physics applies to condensed matter physics as well; in particular, there are a selection of field excitations, called quasi-particles, that can be created and explored. These include:
- Phonons are vibrational modes in a crystal lattice.
- Excitons are bound states of an electron and a hole.
- Plasmons are coherent excitations of a plasma.
- Polaritons are mixtures of photons with other quasi-particles.
- Polarons are moving, charged (quasi-) particles that are surrounded by ions in a material.
- Magnons are coherent excitations of electron spins in a material.
- An anyon is a generalization of fermion and boson in two-dimensional systems like sheets of graphene that obeysbraid statistics.
- A plekton is a theoretical kind of particle discussed as a generalization of the braid statistics of the anyon to dimension > 2.
- A WIMP (weakly interacting massive particle) is any one of a number of particles that might explain dark matter (such as the neutralino or the axion).
- The pomeron, used to explain the elastic scattering of Hadrons and the location of Regge poles in Regge theory.
- The skyrmion, a topological solution of the pion field, used to model the low-energy properties of the nucleon, such as the axial vector current coupling and the mass.
- A genon is a particle existing in a closed timelike world line where spacetime is curled as in a Frank Tipler or Ronald Mallett time machine.
- A goldstone boson is a massless excitation of a field that has been spontaneously broken. The pions are quasi-Goldstone bosons (quasi- because they are not exactly massless) of the broken chiral isospin symmetry of quantum chromodynamics.
- A goldstino is a Goldstone fermion produced by the spontaneous breaking of supersymmetry.
- An instanton is a field configuration which is a local minimum of the Euclidean action. Instantons are used in nonperturbative calculations of tunneling rates.
- A dyon is a hypothetical particle with both electric and magnetic charges.
- A geon is an electromagnetic or gravitational wave which is held together in a confined region by the gravitational attraction of its own field energy.
- An inflaton is the generic name for an unidentified scalar particle responsible for the cosmic inflation.
- A spurion is the name given to a “particle” inserted mathematically into an isospin-violating decay in order to analyze it as though it conserved isospin.
- What is called “true muonium”, a bound state of a muon and an antimuon, is a theoretical exotic atom which has never been observed.
Classification by speed
- A tardyon or bradyon travels slower than light and has a non-zero rest mass.
- A luxon travels at the speed of light and has no rest mass.
- A tachyon (mentioned above) is a hypothetical particle that travels faster than the speed of light and has an imaginary rest mass.