New Muon g-2 Measurement Expected February 2021

FNAL-E989 first announcement in February 2021

Per Asian Twitter via Physics Forums.

This is arguably the most important fundamental physics measurement since the discovery of the Higgs boson. It has the potential to either (1) unambiguously establish that the Standard Model of Particle Physics is missing undiscovered new physics of some well quantified type (still only enough to generate parts be million irregularities on the measured value), if the consensus theoretical estimate and the experimental measurement of muon g-2 differ by five sigma or more, or instead, (2) profoundly limit any form of low energy new physics, if the consensus theoretical estimate and the experimental measurement of muon g-2 differ by two sigma or less.

In the second case, any impact of new fundamental particles or new forces in the Standard Model on muon g-2 must be almost exactly offsetting, and/or any undiscovered new physics must be extremely slight.

This is because the theoretical value of muon g-2 is a function of the strong coupling constant, the weak coupling constant, the strong force coupling constant, and the properties of essentially all of the fundamental particles of the Standard Model, through intermediate loops included in the calculation of this derived muon property. Essentially anything that can interact with a muon in the Standard Model, and anything that interacts with something that interacts with a muon in the Standard Model, ever so slightly tweaks the value of muon g-2.

For example, as one June 2020 paper explains:

The longstanding muon g-2 anomaly may indicate the existence of new particles that couple to muons, which could either be light (< GeV) and weakly coupled, or heavy (>> 100 GeV) with large couplings. If light new states are responsible, upcoming intensity frontier experiments will discover further evidence of new physics. However, if heavy particles are responsible, many candidates are beyond the reach of existing colliders. We show that, if the g-2 anomaly is confirmed and no explanation is found at low-energy experiments, a high-energy muon collider program is guaranteed to make fundamental discoveries about our universe. New physics scenarios that account for the anomaly can be classified as either "Singlet" or "Electroweak" (EW) models, involving only EW singlets or new EW-charged states respectively. We argue that a TeV-scale future muon collider will discover all possible singlet model solutions to the anomaly. If this does not yield a discovery, the next step would be a O(10 TeV) muon collider. Such a machine would either discover new particles associated with high-scale EW model solutions to the anomaly, or empirically prove that nature is fine-tuned, both of which would have profound consequences for fundamental physics.

Rodolfo Capdevilla, David Curtin, Yonatan Kahn, Gordan Krnjaic, "A Guaranteed Discovery at Future Muon Colliders" arXiv (June 29, 2020).

In contrast, if the theoretical prediction for muon g-2 is confirmed, even very weakly coupled sub-GeV particles, and also high mass new particles (above 100 GeV up to tens of TeV) with anything more than extremely weak couplings are largely ruled out. Thus, any new fundamental particles would have to be confined to a scale that would not be detectable at a next generation collider.

In the intermediate range, where a tension between the new experimentally measured value of muon g-2 and the theoretically predicted value of muon g-2 continues to be more than two sigma but less than five sigma, the case for new physics relative to merely something like underestimated error bars in either the experimental measurement or the theoretical prediction or both, is still diminished. 

This is because that can only happen if the best fit value of the experimental measurement becomes significantly closer to the theoretical prediction than it was fifteen years ago, even though a tension remains.

If that happened, the source of any new physics leading to a tension would also be expected to be smaller than the previous tension would have predicted, implying either a much weaker coupling to Standard Model physics, or a much higher mass scale for new physics, than previously surmised. 

An ongoing tension would still motivate searches for new physics by providing some observational motivation for them. But a lot of new physics models proposed to explain the tension observed fifteen years ago would end up on the cutting room floor. 

Anything other than a definitive confirmation of the necessity of new physics to explain the muon g-2 anomaly would, in particular, be a huge blow to the prospects of supersymmetry (SUSY) models (see, e.g., here) or models with additional types of Higgs bosons (see, e.g. here) at scales potentially discoverable by a next generation particle collider (i.e. with new particles having masses of tens of TeVs or less).

UPDATE January 31, 2021:

A periodical reports a March 2021 date, which would be another modest postponement.

Locked cabinets, sealed envelopes, and secret codes surround a big question in particle physics: Could the magnetism of a particle called the muon point to new vistas in physics?

Behind the scenes drama here.