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Results and Physics Plots

The primary physics result for the E158 experiment is the measurement of parity violation in electron-electron scattering.  This measurement is used to determine the electron's weak charge and the weak mixing angle parameter, sin2qWeff, at low momentum transfer, Q.  E158 measures the weak mixing angle parameter to be 0.2397 0.0014.  This is 4% greater than the result at high energy obtained from the SLD and LEP experiments, 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 following plot summarizes E158's measurement of the weak mixing angle parameter and its "running" with momentum scale.
More details of E158's measurement and analysis can be found here.


The E158 weak mixing angle 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.   The result is consistent with the Standard Model (SM) Prediction.  In addition to the results from SLAC E158 and the SLD and LEP experiments, we also plot low energy results from an Atomic Parity Violation (APV) experiment using Cesium, QW(Cs), and a neutrino-nucleon scattering experiment, NuTeV.  The APV experiment was carried out by a group at Boulder, Colorado and the NuTeV experiment was performed at Fermilab.  The APV result is from data taken in the 1990s, and as recently as 2000 indicated a discrepancy of 2.5 standard deviations with the Standard Model predictions.  Since 2000, improved atomic theory calculations indicate the discrepancy to be within 1 standard deviation of the SM prediction.  The NuTeV result, however, disagrees with the SM prediction by 3 standard deviations.  Studies are ongoing to check for contributions and uncertainties from "conventional" effects -- strong and electroweak radiative corrections, isospin symmetry violation or an asymmetric strange sea.  A distinct advantage for the E158 experiment is that the theoretical prediction is quite precise because of the simple electron-electron scattering process.


Implications and Limits on New Physics, Beyond the Standard Model
The parity-violating asymmetry measured by E158 arises from interference between photon and Z exchange interactions in electron-electron scattering (see illustration).  The precision of the experimental result makes it sensitive to additional interference effects from Z' particles postulated in theories with supersymmetry, extra dimensions and unified forces.  The consistency of E158's result with SM predictions imply that the mass of such Z' particles be greater than 1000 GeV, more than 10 times the mass of the Z particle.  Stringent limits can be also be set on a compositeness scale describing the binding energy of new constituent particles that comprise the known leptons (such as electrons) and quarks.  The E158 result sets a limit on the compositeness scale describing electron substructure to be 10 TeV (10, 000 GeV) or greater.

Last Update: 27 Apr 2005