Experiment E-158 at SLAC hopes to force a revision of the Standard Model—one of the most elegant and powerful predictive tools in science.

The experiment seeks to measure the electroweak mixing angle—the proportions in which the weak and electromagnetic forces combine to form the electroweak interaction—with the best accuracy ever achieved at low energies.

Four forces call the shots in the subatomic realm: gravity, the electromagnetic force and the weak and strong forces. Physicists and the Standard Model are pretty clear on the weak, strong, and electromagnetic forces, though gravity still has everyone flummoxed.

"If we don’t get the measure for the electroweak mixing angle predicted by the Standard Model," E-158 spokesperson Krishna Kumar (University of Massachusetts, Amherst) said, "it could point to new physics at high energies."

The SLD experiment has already made an accurate measurement of the electroweak mixing angle at the 100 GeV energy scale. Now, E-158 is going after an accurate measurement at a lower energy. Lowering the energy scale is equivalent to increasing the length scale at which the electroweak force is measured.

At longer scales, an ephemeral cloud of particle-antiparticle pairs forms a "screen" that effectively reduces the charge of each interacting particle, thus reducing the strength of the electroweak force. The Standard Model makes precise predictions for this evolution with change of energy (equivalently length) scale. While this prediction has been verified for the electromagnetic force between two electrons, E-158’s measurement of the mixing angle would verify the equivalent prediction for the weak force.

There have been two previous efforts elsewhere to measure the weak mixing angle at low energy, but the SLAC measurement hopes to be the first to compellingly affirm or reject how the parameter evolves as the interaction energy scale is varied. E-158 is a collaboration of 60 scientists from 11 institutions now conducting an experiment at SLAC’s End Station A.

Making the measurement is like drawing a line from an object’s shadow to the object itself, in order to point directly at the light source. Some researchers try to directly map those points of verification by building bigger, more powerful instruments; other researchers use existing instruments to carry out ultra-precise measurements in experiments such as E-158.

The researchers collected roughly 20 percent of the data they need during the first phase of their work, from May to June. The second phase will commence in October.