High-Precision Tests of the Standard Model

High-precision tests of the Standard Model have been reported this past week in two areas: CP-violation in B mesons (experiments at the KEK lab in Japan and the SLAC lab in California) and the magnetic moment of the muon (an experiment at the Brookhaven lab in New York).

The standard model, trying to explain the forces of nature through the exchange of particles, consists of the electroweak framework (force exchanged by photons and by Z and W bosons) plus the quantum chromodynamic (QCD) framework for quarks (force exchanged by gluons).

The model has been highly successful in accounting for the behavior of electrons in atoms (in the case of some transition frequencies, theory and experiment agree at the parts-per-trillion level or better) and does a good job of predicting other phenomena as well, such as CP violation. The model does not include, but can accommodate, neutrino oscillation.

Extensions of the standard model, such as superstring theory--which pictures all matter as consisting of tiny strings or membranes--can (unlike the standard model) account for the force of gravity, the existence of extra spatial dimensions, and the proposition (known as supersymmetry, or SUSY) that all fermion particles have boson counterparts and vice versa.

SUSY is by now an acceptable idea for many particle physicists but it would necessitate an overhaul of the standard model since the existence of superparticles would entail a whole new force, one which transforms fermions into bosons and back again.

The new CP violation tests were reported at the International Conference on High Energy Physics in Amsterdam. Both the Belle detector group at KEK and the BaBar detector group at SLAC observed subtleties in the decays of B mesons and measured a parameter called sine two beta.

The value measured for both groups, with much better precision than ever before, is approaching the value predicted by the standard model, thus erasing past discrepancies. (See SLAC news release.)

Meanwhile, at Brookhaven the g-2 collaboration seeks to observe a departure of the muon's magnetic moment (related to the muon's spin by the g parameter) from 2, the value it would have in the absence of interactions between the muon and virtual particles in the universal vacuum, including possible exotica outside the standard model such as the supersymmetric entities. Although the SUSY particles are rare and unstable their mere existence in the vacuum would modify observable quantities such as the muon magnetic moment.

Thus a measurement of the magnetic moment, by watching muons decay even as they wobble about in a strong magnetic field, would give indirect evidence for the extra particles. Moderate evidence in this direction was previously reported by the g-2 team; the new results, reported also in Amsterdam (and submitted to Physical Review Letters), follow suit but with twice the precision of the last report. (See Brookhaven news release.)