List of particles

Elementary particles

An elementary particle is a particle with no measurable internal structure, that is, it is not a composite of other particles. In a quantum field theory, these are the particles which are created and annihilated by the field operators in the Lagrangian. Elementary particles can be classified according to their spin.

Fermions (half-integral spin)

Fermions have half-integral spin; for all known elementary particles this is 1/2. 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 color force or not. According to the Standard Model there are 12 flavors of elementary fermions, six quarks and six leptons.

• Quarks can interact via the color force. They exist in six flavors:
  • down
  • up
  • strange
  • charm
  • bottom
  • top
• Leptons do not interact via the color force. They also exist in six flavors:
  • electron
  • muon
  • tauon
  • electron-neutrino
  • muon-neutrino
  • tauon-neutrino

Supersymmetric theories predict the existence of more fermions. Their existence has not been confirmed experimentally.

• The neutralino (spin 1/2) is a superposition of the superpartners of several neutral standard model particles. It is a leading candidate for dark matter. The partners of charged bosons are called charginos.
• The photino (spin 1/2) is the superpartner of the photon.
• The gravitino (spin 3/2) is the superpartner of the graviton boson in supergravity theories.

Gauge Bosons (integral spin)

Bosons have integral spin. The fundamental forces of nature are mediated by gauge bosons. According to the Standard Model there are 13 elementary bosons.

• The photon (spin 1) mediates the electromagnetic force.
• The W⁺, W^- and Z⁰ bosons (spin 1) mediate the weak nuclear force.
• The eight gluons (spin 1) mediate the strong nuclear force.
• The Higgs boson (spin 0) is predicted by standard model electroweak theory. Physicists expect the Higgs to be discovered at the Large Hadron Collider (LHC) particle accelerator now under construction at CERN.

New theories predict the existence of other bosons.

• The graviton (spin 2) has been proposed to mediate gravity in a theory of quantum gravity.
• The supersymmetric partners of the standard model fermions (sleptons and squarks) would also be bosons.
• The graviscalar (spin 0).
• The graviphoton (spin 1).
• The Goldstone boson.
• The X boson and the anti X boson. (GUT theories?)

Composite particles

Molecules

Molecules are the smallest particles into which a 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.

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, of which 110 have been named. Refer to the periodic table for an overview.

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 Isotope table (complete) for a list of isotopes.

Hadrons

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

• Fermions, in which case they are called baryons.
• 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 color force, which is mediated by gluons. A "sea" of virtual quark-antiquark pairs is also present in each hadron.

Baryons (Fermions)

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

• Nucleons are the fermionic constituents of normal atomic nuclei:
  • proton
  • neutron
• Hyperons such as the Δ, Λ, Ξ and Ω particles are generally short-lived and heavier than nucleons. They do not normally appear in atomic nuclei.

Exotic baryons

Exotic baryons have been discovered only recently.

• Pentaquarks consist of four valence quarks and one valence antiquark.

Mesons (Bosons)

Ordinary mesons (bosons) contain a valence quark and a valence antiquark, and include the pions, the kaons and many other types of mesons. In quantum hadrodynamic models the strong force between nucleons is mediated by mesons.

Exotic mesons

Exotic mesons are predicted by new theories.

• Tetraquarks consist of two valence quarks and two valence antiquarks.
• Glueballs are bound states of two or more real gluons.
• Hybrids consist of one or more valence quark-antiquark pairs and one or more real gluons.

Other

• The Oh-My-God particle is an extremely high-energy proton, which has been observed several times.

See also

• List of compounds for a list of molecules.
• Periodic table for an overview of atoms.
• Isotope table (complete) for a list of isotopes.

External links

• Particle Data Guide K. Hagiwara et al., Phys. Rev. D66, 010001 (2002) (http://pdg.lbl.gov/)
• Elementary Particles by Joseph F. Alward, PhD, Department of Physics, University of the Pacific (http://sol.sci.uop.edu/~jfalward/elementaryparticles/elementaryparticles.html)
• elementary particles, The Columbia Encyclopedia, Sixth Edition. 2001. (http://www.bartleby.com/65/el/elementr-p.html)

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