sldlogo.jpg (6106 bytes)  Standard Model of Particle Physics     

     Chemistry can be understood in the physics of 3 particles (proton, neutron and electron), and the influence of the electromagnetic force. Nuclear physics can be understood in the physics of 4 particles (proton, neutron, electron and electron neutrino), and the influence of the strong and weak nuclear forces together with the electromagnetic force.  The Standard Model Theory (SM) of particle physics provides a framework for explaining chemistry and nuclear physics (low energy processes). It additionally provides an explanation for sub-nuclear physics and some aspects of cosmology in the earliest moments of the universe (high energy processes).

     The Standard Model is conceptually simple and contains a description of the 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 below and constitute the 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 matter and force particles of the Standard Model.  Up and down quarks were observed for the first time in electron-scattering experiments at  SLAC in the late 1960s.  The 1990 Nobel Prize in physics for this discovery was awarded to SLAC's Richard Taylor and to Jerome Friedman and Henry Kendall from MIT.   The charm quark was discovered simultaneously in experiments at SLAC and at Brookhaven in 1974.  SLAC's Burton Richter and MIT's Samuel Ting shared the 1976 Nobel Prize in physics for this discovery.   The tau lepton was discovered at SLAC in 1975, for which SLAC's Martin Perl was awarded the 1995 Nobel Prize in physics.

     The SM particles are considered to be point-like, but contain an internal ‘spin’ (angular momentum) degree of freedom which is quantized and can have values of 0, or 1. Spin-1/2 particles obey Fermi statistics, which have as a consequence that no 2 electrons can be in the same quantum state. This feature is necessary for forming atoms more complex than hydrogen. Spin-1 and spin-0 particles obey Bose-Einstein statistics, which prefer to have many particles in the lowest energy or ground state. This phenomenon is responsible for superconductivity.

     The Standard Model says that forces are the exchange of gauge bosons (the force particles) between interacting quarks and leptons. Feynman diagrams are useful to describe this pictorially. As illustrated in the figures below, two electrons may interact by scattering and exchanging a photon; or an electron and positron may collide and annihilate to form a Z particle, which then decays into a quark and anti-quark. 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 laws of physics (ie. the strengths of the forces) for the weak force are different for particles and anti-particles (C Violation), for a scattering process and its mirror image (P Violation), and for a scattering process and the time reversal of that scattering process (T Violation). 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 (such as for a Z particle to decay into different types of quarks and leptons).
wpe1.jpg (7700 bytes) wpe2.jpg (8002 bytes)
Feynman diagram for an electromagnetic interaction in electron-electron scattering by exchange of a photon. Feynman diagram for a weak interaction in electron-positron annihilation to a Z0, which then decays to a quark-antiquark pair.

Last updated:  04-09-2001 by M. Woods