*metrology*, which is easily confused with meteorology. So it's worth commenting on why it could be interesting. Using interferometry techniques, the Berkeley group measured the mass of the cesium-133 atom with the relative accuracy of 4*10^-10, that is 0.4 parts par billion (ppb). From that, α can be determined after a cavalier rewriting of the high-school formula for the Rydberg constant:

Everybody knows the first 3 digits of the Rydberg constant, Ry≈13.6 eV, but actually it is experimentally known with the fantastic accuracy of 0.006 ppb, and the electron-to-atom mass ratio has also been determined precisely. Thus the measurement of the cesium mass can be translated into a 0.2 ppb measurement of the fine structure constant: 1/α=137.035999046(27).

You may think that this kind of result could appeal only to a Pythonesque chartered accountant. But you would be wrong. First of all, the new result excludes α = 1/137 at 1 million sigma, dealing a mortal blow to the field of epistemological numerology. Perhaps more importantly, the result is relevant for testing the Standard Model. One place where precise knowledge of α is essential is in calculation of the magnetic moment of the electron. Recall that the

*g-factor*is defined as the proportionality constant between the magnetic moment and the angular momentum. For the electron we have

Experimentally, g

*e*is one of the most precisely determined quantities in physics, with the most recent measurement quoting

*ae*= 0.00115965218073(28), that is 0.0001 ppb accuracy on g

*e,*or 0.2 ppb accuracy on

*ae*. In the Standard Model, g

*e*is calculable as a function of α and other parameters. In the classical approximation g

*e*=2, while the one-loop correction proportional to the first power of α was already known in prehistoric times thanks to Schwinger. The dots above summarize decades of subsequent calculations, which now include O(α^5) terms, that is 5-loop QED contributions! Thanks to these heroic efforts (depicted in the film

*For a Few Diagrams More*- the sequel to Kurosawa's

*Seven Samurai*), the main theoretical uncertainty for the Standard Model prediction of g

*e*is due to the experimental error on the value of α. The Berkeley measurement allows one to reduce the relative theoretical error on

*ae*down to 0.2 ppb:

*ae*= 0.00115965218161(23), which matches in magnitude the experimental error and improves by a factor of 3 the previous prediction based on the α measurement with rubidium atoms.

At the spiritual level, the comparison between the theory and experiment provides an impressive validation of quantum field theory techniques up to the 13th significant digit - an unimaginable theoretical accuracy in other branches of science. More practically, it also provides a powerful test of the Standard Model. New particles coupled to the electron may contribute to the same loop diagrams from which g

*e*is calculated, and could shift the observed value of

*ae*away from the Standard Model predictions. In many models, corrections to the electron and muon magnetic moments are correlated. The latter famously deviates from the Standard Model prediction by 3.5 to 4 sigma, depending on who counts the uncertainties. Actually, if you bother to eye carefully the experimental and theoretical values of

*ae*beyond the 10th significant digit you can see that they are also discrepant, this time at the 2.5 sigma level. So now we have

**two**g-2 anomalies! In a picture, the situation can be summarized as follows:

If you're a member of the Holy Church of Five Sigma you can now preach an unambiguous discovery of physics beyond the Standard Model. However, for most of us this is not the case yet. First, there is still some debate about the theoretical uncertainties entering the muon g-2 prediction. Second, while it is quite easy to fit each of the two anomalies separately, there seems to be no appealing model to fit both of them at the same time. Take for example the very popular model with a new massive spin-1 Z' boson (aka the dark photon) kinetically mixed with the ordinary photon. In this case Z' has, much like the ordinary photon, vector-like and universal couplings to electron and muons. But this leads to a

*positive*contribution to g-2, and it does not fit well the

*ae*measurement which favors a new negative contribution. In fact, the

*ae*measurement provides the most stringent constraint in part of the parameter space of the dark photon model. Conversely, a Z' boson with purely axial couplings to matter does not fit the data as it gives a negative contribution to g-2, thus making the muon g-2 anomaly

*worse*. What might work is a hybrid model with a light Z' boson having lepton-flavor violating interactions: a vector coupling to muons and a somewhat smaller axial coupling to electrons. But constructing a consistent and realistic model along these lines is a challenge because of other experimental constraints (e.g. from the lack of observation of μ→eγ decays). Some food for thought can be found in this paper, but I'm not sure if a sensible model exists at the moment. If you know one you are welcome to drop a comment here or a paper on arXiv.

More excitement on this front is in store. The muon g-2 experiment in Fermilab should soon deliver first results which may confirm or disprove the muon anomaly. Further progress with the electron g-2 and fine-structure constant measurements is also expected in the near future. The biggest danger is that, if the accuracy improves by another two orders of magnitude, we will need to calculate six loop QED corrections...