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Parity Violation
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Running Strengths of Interactions

The demonstration that the electron’s weak charge varies with distance, a trait called running, complements previous experiments that established running in electromagnetism and the strong force. Running means that the effective (electromagnetic, weak or strong) charge of a particle varies with energy scale, or distance. And it means that the force strength, characterized by a force coupling constant between interacting particles, also varies with energy scale and distance.  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.



Running of the Strong Force Strength

  The plot at left shows the running, or evolution, of the strength of the strong interaction.  This is characterized by the evolution of the strong coupling constant aS, plotted here versus  energy scale in GeV. This plot is taken from the Particle Data Group.



Unification of Strong, Electromagnetic and Weak Forces?    
Grand Unified Theories (GUTs)

     In the 1800's, the phenomena of electricity and magnetism were demonstrated to be two aspects of a unified electromagnetic force.  This was a spectacular achievement and is well described theoretically by Maxwell's equations and quantum electrodynamics.  In the 1960's and 1970's, a similarly spectacular achievement was the demonstration that electromagnetic and weak interactions are two aspects of a unified electroweak force.  Many believe that there should also be unification of the strong and electroweak forces, described by a Grand Unified Theory (GUT) at a very high energy scale of ~1016 GeV.  At even higher energy, at the Planck scale of 1019 GeV, a quantum theory of gravity might lead to a further unification of all 4 forces.  GUT theories require that the running strengths of the Strong, Electromagnetic and Weak forces become the same at the GUT energy scale.  The Standard Model makes precise predictions for these running strengths, plotted below for a1, a2 and a3;  these coupling constants are related to the electromagnetic coupling constant, weak mixing angle and strong coupling constant by the relations given below (see also Peskin reference).  As seen in this figure, the running strengths for the 3 forces never coincide.  However, if one makes the same plot for a supersymmetric (SUSY) extension of the Standard Model, one does find an energy scale with unified couplings!  But does SUSY really exist??  It postulates a doubling of the known particles, with a  SUSY counterpart to the electron called a selectron for example.  Stable SUSY particles may also be responsible for the large abundance of Dark Matter in the Universe.  Experiments at CERN's Large Hadron Collider (LHC) and the proposed International Linear Collider will tell us in the coming decade if nature is supersymmetric..

(plot taken from M. Peskin; see reference below)


1.  Measurements of running of the electromagnetic force strength
     i) TOPAZ experiment at TRISTAN,
    ii) VENUS experiment at TRISTAN,
   iii) L3 experiment at LEP,
    iv) OPAL experiment at LEP, hep-ex/0505072.
   iv) Muon (g-2) experiment at Brookhaven.
2.  Measurements of running of the strong force strength are summarized by the
     Particle Data Group.
3.  Unification -- see (for example)
Beyond the Standard Model, by M. Peskin,
     SLAC-PUB-7479, 1997.  Lectures given at The 1996 European School of High-Energy

Last Update: 01 Jun 2005