New Muon g-2 Measurement Coming Soon

One of the most important discrepancies between theory and experiment in the Standard Model is the muon g-2 anomaly, a roughly 3 sigma tension. The last precision measurement of muon g-2 was done by the E821 Muon g-2 Experiment at Brookhaven National Laboratory which finished collecting data in 2001 and issued its final report analyzing that data in 2006. 

Two more experiments are underway to make a new more precise measurement. The first to produce results will be the E989 Muon g-2 Experiment at Fermilab which is projected to obtain ∼20 times more data and a ∼3-fold reduction of systematic errors compared to E821. The relative error in the new measurement of muon g-2 at E989 will be about 150 parts per billion and will make the experimental error smaller than the uncertainty in theoretical prediction.

Like all major government projects, it is behind schedule. But a Conference paper released today at arXiv discussing the technical details of one part of the E989 experimental setup also included a hint about how far along E989 is on data collection and when the first results will be announced. According to this paper:

The Muon g-2 Experiment (E989) at Fermilab has a goal of measuring the muon anomaly (a(μ)) with unprecedented precision . . . the experiment has recently commenced Run-3, and the release of Run-1 physics results is planned for 2020.

So, the nearly two decades long wait for new experimental data related to this issue is almost over. Another experiment (the J-PARC E34 experiment see also here) measuring Muon g-2 will be releasing its new measurement result a year or two or so after the E989 results are released, providing a quick replication or non-replication of whatever measurement emerges from the E989 experiment.

More background is available in a June 11, 2020 post at this blog. In a real nutshell, the theoretical side is summed up in this chart from a Fermilab website article:

The theoretical value of the anomalous magnetic moment of the muon is:

a = (g-2)/2 (theory) = 116 591 810(43) x 10-11

The most precise experimental result available so far is:

a = (g-2)/2 (experiment) = 116 592 089(63) x 10-11

The experimental value exceeds the theoretical value by:

279(76) x 10-11

This discrepancy is about 2 parts per million, which is a 3.7 sigma tension.

If the experimental measurement from E989 is 1.6 parts per million lower than it was in the E821 experiment, the new measurement would be considered to be "consistent" with the theoretical prediction. 

The muon g-2 measurement is of great interest to scientists because almost all components of the Standard Model contribute to it to some extent.

If the new measurement is consistent with the theoretical prediction, this would be strong evidence that the Standard Model is globally consistent with reality at the electroweak scale plus or minus several orders of magnitude, and that the Standard Model is a complete low energy effective description of the laws of Nature (although, it is not inconceivable that there could be more than one kind of beyond the Standard Model physics and that these new physics contributions offset each other in their contribution to muon g-2). In the event that the results were consistent, the discrepancy would be less than or equal to:

93 x 10-11

On the other hand, if the discrepancy between theory and prediction increased to 5 sigma, this would be considered a scientific discovery of unknown new physics at the same energy scale, but an incomplete one, because the discrepancy tells us only the magnitude of the undiscovered new physics and not its source. A 5 sigma discrepancy would be equal to or more than:

233 x 10-11

For discrepancies between those values, we would be left in more or less the same place where we are now, with a tension between theory and experiment, but not a big enough tension to constitute a definitive scientific discovery of new physics.

But, the reduced magnitude of the absolute size of the discrepancy between theory and experiment in a significantly more accurate measurement of muon g-2 would still probably tend to favor the conventional wisdom that the muon g-2 anomaly is due to some combination of theoretical and experimental error, and not due to new physics.