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Parity Violation
Weak Force
Running and Unification
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E-158 has made the first observation of Parity Violation in electron-electron (Mller) scattering. This precise Parity Violation measurement gives the best determination of the electron's weak charge at low energy (low momentum transfer between interacting particles).  E158's measurement  tests the predicted running (or evolution) of this weak charge with energy, and searches for new phenomena at TeV energy scales (one thousand times the proton-mass energy scale).

diagram of the e158 experiment

SLAC E-158 uses SLAC's polarized electron beam.  The beam is accelerated in the 2-Mile Linear Accelerator and then transported through a 24.5-degree bend into End Station A, where it strikes a Liquid Hydrogen Target. The experiment measures the (parity-violating) asymmetry in scattering rate of left- versus right-polarized beam electrons from the unpolarized atomic electrons in the Liquid Hydrogen Target.

At high energy, the electron's weak charge (closely related to the weak mixing angle) has been accurately measured by SLAC's SLD experiment and the LEP experiments at CERN.  Lowering the energy scale is equivalent to increasing the length scale at which the weak charge is measured.   At longer length scales, a cloud of particle-antiparticle pairs forms a "screen" that effectively reduces the charge of each interacting particle. The Standard Model makes precise predictions for this evolution with change of energy (equivalently length) scale. While this prediction has previously been verified for the electric charge of the electron, E-158 has verified for the first time the equivalent prediction for evolution (running) of the electron's weak charge.

The E158 Parity Violation measurement  determines the weak mixing angle parameter, sin2qWeff, at low momentum transfer, Q.  The result is shown here, together with results from other experiments and a theoretical prediction (black curve) from Czarnecki and Marciano.  The gray region about the black theory curve represents the theoretical uncertainty in the "running" (evolution) of the weak mixing angle from the Z0 mass energy scale of the SLD and LEP experiments to low energy.   E158 measures the weak mixing angle parameter to be 4% greater than the result at high energy obtained from SLD and LEP, demonstrating the "running" of the weak mixing angle with a significance of 6.2 standard deviations.  The electron's weak charge is approximately given by QW(e) = -(1-4sin2qWeff).  E158 finds QW(e) = -0.041 0.006, approximately the value expected if there were no running!

The Standard Model of Particle Physics has had outstanding success for describing the fundamental particles and their interactions.  Yet it is incomplete and there remain many puzzles.  New Physics at the Energy Frontier, probed by today's highest energy accelerators, is predicted by many theorists to resolve some of these puzzles.  The precision achieved by E-158 make it sensitive to some of the new physics that may occur at TeV (Tera, or a thousand billion, electron-volt) energy scales now being directly probed at Fermilab's Tevatron, and in the next few years at CERN's Large Hadron Collider.  The E-158 result is consistent with Standard Model predictions and puts significant limits on new physics that may occur.

E158 performed 3 physics runs in 2002 and 2003. Details on the experiment, apparatus and analysis can be found in the E158 Ph.D. theses.

Last Update: 27 Jun 2005