An elementary particle is a particle with no measurable internal structure, that is, it is not a composite of other particles. In 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 ˝. 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.
Supersymmetric theories predict the existence of more fermions. Their existence has not been confirmed experimentally.
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.
New theories predict the existence of other bosons.
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 111 have been officially named. Refer to the periodic table for an overview.
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 are defined as strongly interacting composite particles. Hadrons are either:
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.
Ordinary baryons (fermions) contain three valence quarks or three valence antiquarks each.
- Nucleons are the fermionic constituents of normal atomic nuclei:
- 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.
First hints at the existence of Exotic baryons have been found only recently.
- Pentaquarks consist of four valence quarks and one valence antiquark.
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 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.
- 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)