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The Weak Force

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 (Charge Violation), for a scattering process and its mirror image (Parity Violation), and for a scattering process and the time reversal of that scattering process (Time Violation).

The weak force drives radioactive decays that:

  • help generate sunlight,
  • enable advanced medical diagnosis and treatment,
  • help determine the age of organic materials from carbon isotope abundances,
  • help determine the age of the earth from uranium isotope abundances
  • provide heating for the Earth and an energy source for plate tectonics, through the decays of Uranium, Thorium and Potassium.

Some description and note of the significance of the weak interaction is given in the following quote taken from the press release for the 1979 Nobel Prize in Physics, awarded to Sheldon L. Glashow, Abdus Salam and Steven Weinberg for their contributions to developing the Standard Model:
     "Although the weak interaction is much weaker than both the strong and the electromagnetic interactions, it is of great importance in many connections. The actual strength of the weak interaction is also of significance. The energy of the sun, all-important for life on earth, is produced when hydrogen fuses or burns into helium in a chain of nuclear reactions occurring in the interior of the sun. The first reaction in this chain, the transformation of hydrogen into heavy hydrogen (deuterium), is caused by the weak force. Without this force solar energy production would not be possible. Again, had the weak force been much stronger, the life span of the sun would have been too short for life to have had time to evolve on any planet. The weak interaction finds practical application in the radioactive elements used in medicine and technology, which are in general beta-radioactive, and in the beta-decay of a carbon isotope into nitrogen, which is the basis for the carbon-14 method for dating of organic archaeological remains. ... Of special interest is a result, published in the summer of 1978, of an experiment at the electron accelerator at SLAC in Stanford, USA. In this experiment the scattering of high energy electrons on deuterium nuclei was studied and an effect due to a direct interplay between the electromagnetic and weak parts of the unified interaction could be observed."

 The Weak Charge

The strength of the weak force between interacting quarks and leptons can be characterized by their weak charge (distinct from their electric charge). The weak charges of quarks and leptons are comparable to their electromagnetic charges, a manifestation of how electromagnetism and the weak force are components of a unified electroweak force. At “long” distances approximately the width of a proton, the weak charge looks smaller because of quantum fluctuations in the vacuum—every particle is surrounded by an ephemeral cloud of particles that effectively form a screen between interacting electrons.  A primary purpose of the E158 experiment has been to establish the variation (running) of the electron's weak charge with energy scale, or distance.  The running of interaction strengths (electric charges, weak and strong nuclear charges) has previously been established for the electromagnetic and strong forces, but not for the weak force.
The electron's weak charge, QW(e), is closely related to a quantity called the weak mixing angle,
qW, that describes the relative strengths of the electromagnetic and weak interactions.  The relation between the two is approximately given by QW(e) = -(1-4sin2qW).

Last Update: 28 Jun 2005