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The Standard Model


     The Standard Model (SM) is conceptually simple
and contains a description of elementary particles and forces.

The SM particles are 12 spin-1/2 fermions (6 quarks and 6 leptons), 4 spin-1 ‘gauge’ bosons and a spin-0 Higgs boson. These are shown in the figure above and constitute the (known) building blocks of the universe. The 6 quarks include the up and down quarks that make up the neutron and proton. The 6 leptons include the electron and its partner, the electron neutrino. The 4 bosons are particles that transmit forces and include the photon, which transmits the electromagnetic force. With the recent observation of the tau neutrino at Fermilab, all 12 fermions and all 4 gauge bosons have been observed. Seven of these 16 particles (charm, bottom, top, tau neutrino, W, Z, gluon)   were predicted by the Standard Model before they were observed experimentally!   There is one additional particle predicted by the Standard Model called the Higgs, which has not yet been observed. It is needed in the model to give mass to the W and Z bosons, consistent with experimental observations. While photons and gluons have no mass, the W and Z are quite heavy. The W weighs 80.3 GeV (80 times as much as the proton) and the Z weighs 91.2 GeV. The Higgs is expected to be heavy as well. Direct searches for it at CERN dictate that it must be heavier than 110 GeV.

The SM forces result from the exchange of gauge bosons (the force particles) between interacting quarks and leptons.  Electromagnetic forces occur via exchange of photons; weak nuclear forces occur via exchange of W and Z particles; and strong nuclear forces occur via exchange of gluons. Electromagnetic forces and interactions are familiar to everyone. They are responsible for visible light and radiowaves, and are the physics behind the electronics and telecommunications industries. All quarks and leptons can interact electromagnetically. Strong nuclear forces are responsible for holding protons and neutrons together inside the nucleus, and for fueling the power of the sun. Only quarks interact via the strong interaction. Weak nuclear forces are responsible for radioactivity and also for exhibiting some peculiar symmetry features not seen with the other forces. In contrast to electromagnetic and strong forces, the strength of  the weak force is different for particles and anti-particles, for a scattering process and its mirror image (Parity Violation), and for a scattering process and the time reversal of that scattering process. All quarks and leptons can interact via the weak interaction.

The Standard Model provides much more than simply a description of electromagnetic, strong and weak interactions. Its mathematics provides explicit and accurate calculations for the rates at which these processes take place and relative probabilities for decays of unstable particles into other lower mass particles.

Useful links and references for the Standard Model:

  1. The Particle Adventure's Chart of Particles and Forces   
  2. The Particle Adventure's Timeline about development of the SM
  3. SLAC's Virtual Visitor Center's description of the SM
  4. The 1979 Nobel Prize in Physics, awarded to Sheldon L. Glashow, Abdus Salam, Steven Weinberg for their contributions to developing the Standard Model.
  5. Deep Down Things:  The Breathtaking Beauty of Particle Physics,
    B. Schumm and John Hopkins University Press.



Last Update: 29 Apr 2005