List of particles

Composite particles

• 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 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 consist of protons and neutrons. Each type of nucleus (depending on the number of neutrons) is called a nuclide or isotope. Nuclear reactions can change one nuclide into another. See Isotope table (complete) for a list of isotopes.

• Hadrons are built from quarks and/or antiquarks, tightly bound by the strong nuclear force. Hadrons are further classified by their quark content.
  • Baryons contain three quarks each.
    • Nucleons are the proton and the neutron, both part of atomic nuclei. When a neutron exists outside a nucleus it is classified as either a fast neutron or a thermal 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.
  • Mesons are built from a quark and an antiquark, and include the pions, the kaons and many other types of mesons. The strong force between the protons and neutrons in an atomic nucleus is mediated by mesons.
  • Exotic baryons have been discovered only recently.
    • Tetraquark particles consist of two quarks and two antiquarks.
    • Pentaquark particles consist of four quarks and one antiquark.

Elementary particles

An elementary particle refers to a particle of which other, larger particles are composed. Elementary particles are classified according to their 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 feel the strong nuclear force or not.
  • According to the Standard Model there are 12 flavors of elementary fermions, six quarks and six leptons.
    • Quarks interact strongly. They exist in six flavors: up, down, strange, charm, bottom and top.
    • Leptons do not interact strongly. They also exist in six flavors: electron, muon, tauon, electron-neutrino, muon-neutrino and tauon-neutrino.
  • Supersymmetry theories predict the existence of more fermions.
    • 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 gravitino (spin 3/2) is the superpartner of the graviton boson in supergravity theories.

• Bosons have integral spin. The fundamental forces of nature are mediated by gauge bosons.
  • According to the Standard Model there are 12 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.

Oddities

• The Oh-My-God particle has been observed several times, but its nature is as yet unknown.

External links

• Particle Data Guide K. Hagiwara et al., Phys. Rev. D66, 010001 (2002)
• Elementary Particles by Joseph F. Alward, PhD, Department of Physics, University of the Pacific
• elementary particles, The Columbia Encyclopedia, Sixth Edition. 2001.