Pushing the limits of particle physics
For over three decades, a single theoretical model has described all experimental results in particle physics. It is called the Standard Model and is one of the most successful models of modern physics. (Actually, there was a modification to allow neutrinos to have mass but that doesn't affect the rest of the model.)
However, there are now tests so precise that the validity of the Standard Model is being called into question. An experimental result announced earlier this week by the "muon (g-2)" collaboration is consistent with physics beyond the standard model but does not yet prove anything conclusively.
Many of the popular theories of particle physics such as supersymmetric theories and string theories go beyond the Standard Model but they have no way of being tested yet. The result presented earlier this week places tight constraints on what sort of new models might be correct. However, if something beyond the Standard Model is required, these experiments do not indicate what sort of theory will fit the bill.
Perhaps surprisingly, the greatest uncertainty at the moment is in the theoretical models. The calculations are extremely hard to do and rely on some experimentally determined parameters. Although the theoretical models agree with each other, there is still quite a spread of uncertainty in them. As more experiments are done, parameters can be tightened and calculations improved.
The particular quantity of interest in these measurements is called the muon g-2 value. It is essentially a measurement of the magnetic strength of the muon (which is like an overgrown electron). The first theories of the electron and muon indicated that g=2 exactly but better theories modified the value to something slightly larger. The difference from 2 is now measured to better than one part per million.
