US Muon-g2 experiment - reading recommendations

[Milsomonk]

TL;DR Summary

Theory papers relating to the Muon g-2 experiment.

Hi folks,
Hope everyone is keeping well. I've recently read the headlines of "strong evidence for new force of nature" pertaining to the Muon g-2 experiment. Interesting as I find the article in the BBC linked below:

https://www.bbc.co.uk/news/56643677

I'd love to read some background theory in this area and wondered if anyone had any suggestions of papers related to this?
Many thanks,

 

[George Jones]

I'd love to read some background theory in this area and wondered if anyone had any suggestions of papers related to this?

You might want to take at look this thread in the PF High Energy, Nuclear, Particle Physics forum,
https://www.physicsforums.com/threa...ial-muon-g-2-measurement.998446/#post-6478176

 

[ohwilleke]

I've collected links to all of the commentary I could find that was not duplicative, as well as links to many relevant journal articles, the Fermilab presentation available on YouTube, and many (at least 38) related pre-prints See some in the next post.

 

[ohwilleke]

Background

The pre-print for the Nature paper by the BMW group was placed on arXiv in 2020 around the same time as the muon g-2 Theory Initiative paper (submitted on 8 Jun 2020 and last revised 13 Nov 2020, published here) that the Fermilab collaboration referenced in its announcement. The BMW pre-print was posted February 27, 2020, and last revised on August 18, 2020. The 14-person BMW team is named after Budapest, Marseille and Wuppertal, the three European cities where most team members were originally based.

Compared to the combined results used for the 4.2 sigma compared to the Theory Initiative paper, the BMW number is consistent with the combined experimental result at the 1.6 sigma level. The BMW paper claims 27% more uncertainty in its theoretical result than the Theory Initiative paper and the Theory Initiative result has been replicated (something much more difficult to do in the case of the much more computation resource intensive BMW calculation), which is part of the reasons that Fermilab decided to go with the Theory Initiative benchmark (another is that the Fermilab theory director is part of the Theory Initiative collaboration, while the BMW collaborators aren't affiliated with Fermilab).

A new pre-print making a more precise calculation of the hadronic light by light contribution (HLbL) to muon g-2 (reducing the relative error in that component of the calculation from 20% to 14%) was also strategically released on arXiv today, and would further reduce the experiment-SM prediction variant in the BMW prediction to 1.3 sigma (the two innovations do not overlap). Compared solely to the new Fermilab result alone, the absolute magnitude of the discrepancy is even smaller and the deviation is less than 1 sigma.

Suppose, for the sake of argument that at a big QCD conference held sometime later this year the scientists get together, discuss these papers and determine unanimously that the LO-HVP calculations and HLbL calculations of the BMW collaboration and this new HLbL paper are correct, and that the calculations of these quantities by the Theory Initiative were wrong. Suppose further that as the accuracy of the Fermilab muon g-2 experiment improves with Runs 2, 3 and 4 (increasing its accuracy by a factor of four) that the fit remains within 1 sigma of the new SM prediction, and that the E34 experiment at J-PARC a few years from now confirms the Fermilab measurement of muon g-2.

What BSM physics are left that aren't ruled out by other data?

What kind of BSM physics are still possible if muon g-2 is consistent to the limits of experimental accuracy and theoretical accuracy with each other, say, for the sake of argument, that they are within exactly 1 sigma of each other measured by margins of error, and within 50 x 10^-11 of each other in terms of the predicted and measured values of muon g-2, which could be seen at a proposed next generation collider?

One of the attractions of muon g-2 is that it is a high precision global measurement of the consistency of observation with the Standard Model. It receives contributions from electromagnetism, the weak force, and the strong force, and almost all particles in the Standard Model, in principle, interact with it via loop contributions even if there is not tree level interaction with a particle at tree level. So, in principle, it measures any new physics.

I've seen statements in coverage of muon g-2 saying that a match between expectation and experiment basically rules out all new physics that could be detected at a next generation collider. Of course, that isn't literally and absolutely true.

While this measurement is ultra-precise, it still has uncertainty. A tremendously massive BSM particle (say 10¹⁴ GeV) will probably have such a low contribution to the total muon g-2 calculation that it effective "decouples" from the "low energy" effective theory (and couldn't be seen at any foreseeable next generation collider anyway).

Some of the other contributions are very small. I can't imagine that a fourth generation active neutrino with a 1 eV mass, even if it was allowed by other experimental measurements (which is probably isn't) would make much of a dent in muon g-2.

But, the LHC and other experiments have also already ruled out myriad possible sources of new physics at hundreds to low thousands of GeVs energy scales, so lots of new physics that would have made a small contribution are already ruled out. Likewise, other tests confirm that there is very little room for new physics in the electroweak sector that contributions to muon g-2.

As I see it, any new physics either has to be (1) decoupled due to its limitation to high energies, (2) have offsetting contributions to muon g-2, or (3) be very small in its contribution to the calculation, so as not to be ruled out by muon g-2.

In short, what is left if the muon g-2 anomaly really doesn't exist?

If it is hard to answer in that form, what kinds of existing popular BSM theories would be ruled out if muon g-2 is measured to match the SM prediction to the highest available precision?