List of particles

2008/9 Schools Wikipedia Selection. Related subjects: Physics

For a chronological listing of subatomic particles by discovery date, see Timeline of particle discoveries.

This is a list of particles in particle physics, including currently known and hypothetical elementary particles, as well as the composite particles that can be built up from them.

Elementary particles

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. Elementary particles can be classified according to their spin, with fermions having half-integer spin and bosons integer spin.

Standard Model

The Standard Model of particle physics is the current understanding of the physics of elementary particles. All Standard Model particles except the Higgs boson have been observed.

Fermions (half-integer spin)

Fermions have half-integer spin; for all known elementary fermions this is ½. Each fermion has its own distinct antiparticle. Fermions are the basic building blocks of all matter. They are classified according to whether they interact via the colour force or not. In the Standard Model, there are 12 types of elementary fermions: six quarks and six leptons.

Quarks

Quarks interact via the colour force. Their respective antiparticles are known as antiquarks. Quarks exist in six flavours:

• Up
• Down
• Strange
• Charm
• Bottom
• Top

Leptons

Leptons do not interact via the colour force. Their respective antiparticles are known as antileptons. (The antiparticle of the electron is called the positron for historical reasons.) There are six leptons, listed here with its corresponding antiparticle:

• Electron and Positron
• Electron neutrino and Electron antineutrino
• Muon and Antimuon
• Muon neutrino and Muon antineutrino
• Tau lepton and Antitauon
• Tau neutrino and Tau antineutrino

Bosons (integer spin)

Bosons have whole number spins. The fundamental forces of nature are mediated by gauge bosons, and mass is hypothesized to be created by the Higgs boson. According to the Standard Model (and to both linearized general relativity and string theory, in the case of the graviton) the elementary bosons are:

Name	Symbol	Charge ( e)	Spin	Mass ( GeV)	Force mediated	Existence
Photon	γ	0	1	0	Electromagnetism	Confirmed
W boson	W±	±1	1	80.4	Weak	Confirmed
Z boson	Z	0	1	91.2	Weak	Confirmed
Gluon	g	0	1	0	Strong	Confirmed
Graviton	-	0	2	0	Gravity	Unconfirmed
Higgs boson	H0	0	0	>112	See below	Unconfirmed

The Higgs boson (spin-0) is necessitated by electroweak theory primarily to explain the origin of particle masses. Following a process known as the Higgs mechanism, the Higgs boson, and the other fermions in the Standard Model acquire mass via spontaneous symmetry breaking of the SU(2) gauge symmetry. It should be noted that in some theories, the Higgs mechanism, which explains the origin of mass, does not require the existence of a Higgs boson. It is also the only Standard Model particle not yet observed; note that the graviton is not a standard model particle. Assuming that the Higgs boson exists, it is expected to be discovered at the Large Hadron Collider particle accelerator now running at CERN.

Hypothetical particles

Supersymmetric theories predict the existence of more particles, none of which have been confirmed experimentally as of 2008:

• The photino (spin-½) is the superpartner of the photon.
• The gluino (spin-½) is the superpartner of the gluon.
• The gravitino (spin-³⁄₂) is the superpartner of the graviton boson in supergravity theories.
• The neutralino (spin-½) is a superposition of the superpartners of several neutral Standard Model particles. The lightest neutralino is a leading candidate for dark matter. The partners of charged bosons are called charginos.
• Sterile neutrinos are introduced by many extensions to the Standard Model, and may be needed to explain the LSND results.
• Sleptons and squarks (spin-0) are the supersymmetric partners of the Standard Model fermions. The stop squark (superpartner of the top quark) is thought to have a low mass and is often the subject of experimental searches.

Other theories predict the existence of additional bosons:

• The Higgs (spin-0) has been proposed to explain the origin of mass by the spontaneous symmetry breaking of the SU(2) gauge symmetry.
• The graviton (spin-2) has been proposed to mediate gravity in theories of quantum gravity.
• The graviscalar (spin-0) and graviphoton (spin-1).
• The axion (spin-0) is a pseudoscalar particle introduced in Peccei-Quinn theory to solve the strong-CP problem.
• The axino and saxion form together with the axion a supermultiplet in supersymmetric extensions of Peccei-Quinn theory.
• The branon is predicted in brane world models.
• The X and Y bosons are predicted by GUT theories to be heavier equivalents of the W and Z.
• The magnetic photon.
• The Majoron is predicted to understand neutrino masses by the seesaw mechanism.

Mirror particles are predicted by theories that restore Parity symmetry.

Magnetic monopole is a generic name for particles with non-zero magnetic charge. They are predicted by some GUT theories.

Tachyon is a generic name for hypothetical particles that travel faster than the speed of light and have an imaginary rest mass.

The preon was a suggested substructure for both quarks and leptons, but modern collider experiments have all but disproven their existence.

Composite particles

Hadrons

Hadrons are defined as strongly interacting composite particles. Hadrons are either:

• 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 valence quarks and/or antiquarks, tightly bound by the colour force, which is mediated by gluons. A "sea" of virtual quark-antiquark pairs is also present in each Hadron.

Notice that mesons are composite bosons, but not composed of bosons. All hadrons, including mesons, are composed of quarks (which are fermions).

Baryons (fermions)

Ordinary baryons (composite fermions) contain three valence quarks or three valence antiquarks each.

• Nucleons are the fermionic constituents of normal atomic nuclei:
  • Protons, composed of two up and one down quark (uud)
  • Neutrons, composed of two down and one up quark (ddu)
• 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.

Mesons (bosons)

Ordinary mesons (composite bosons) contain a valence quark and a valence antiquark, and include the pion, kaon, the J/ψ, and many other types of mesons. In quantum hadrodynamic models, the strong force between nucleons is mediated by mesons.

Exotic mesons may also exist. Positive signatures have been reported for all of these particles at some time, but their existence is still somewhat uncertain.

• Tetraquarks consist of two valence quarks and two valence antiquarks.
• Glueballs are bound states of gluons with no valence quarks.
• Hybrids consist of one or more valence quark-antiquark pairs and one or more real gluons.

Atomic nuclei

Atomic nuclei consist of protons and neutrons. Each type of nucleus contains a specific number of protons and a specific number of neutrons, 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

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, 117 elements have been discovered (atomic numbers 1-116 and 118), and the first 111 have received official names. Refer to the periodic table for an overview. Atoms consist of protons and neutrons within the nucleus. Within these particles, there are smaller particles still which are then made up of even smaller particles still.

Molecules

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 composites of one or more atoms. See list of compounds for a list of molecules.

Condensed matter

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.

Other

• 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 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.
• A UHECR is an ultra-high energy cosmic ray (probably a proton) falling well beyond the GZK cutoff, the energy limit beyond which virtually no cosmic rays should be detected.
• A spurion is the name given to a "particle" inserted mathematically into a Lagrangian. It is a non-propagating field that can be given different symmetry properties to the other fields in the Lagrangian and thus may be used to (softly) break (or re-form a broken) symmetry.
• An inflaton is the generic name for an unidentified scalar particle responsible for the cosmic inflation.
• A chronon is a proposed quantum of time.

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.