## Monday, April 05, 2021

### Place your bets. Muon g-2....

Back in the early 20th century, there was a major advance in physics when people realized that particles like the electron have intrinsic angular momentum, spin, discussed here a bit.  The ratio between the magnetic dipole moment of a particle (think of this like the strength of a little bar magnet directed along the direction of the angular momentum) and the angular momentum is characterized by a dimensionless number, the g-factor.  (Note that for an electron in a solid, the effective g-factor is different, because of the coupling between electron spin and orbital angular momentum, but that's another story.)

For a free electron, the g-factor is a little bit larger than 2, deviating from the nice round number due to contributions of high-order processes.  The idea here is that apparently empty space is not so empty, and there are fluctuating virtual particles of all sorts, the interactions of which with the electron leading to small corrections related to high powers of (m/M), where m is the electron mass and M is the mass of some heavier virtual particle.   The "anomalous" g-factor of the electron has been measured to better than one part in a trillion and is in agreement with theory calculations involving contributions of over 12000 Feynman diagrams, including just corrections due to the Standard Model of particle physics.

A muon is very similar to an electron, but 220 times heavier.  That means that the anomalous g-factor of the muon is a great potential test for new physics, because any contributions from yet-undiscovered particles are larger than the electron case.  Technique-wise, measuring the g-factor for the muon is complicated by the fact that muons aren't stable and each decays into an electron (plus a muon neutrino and an electron antineutrino).  In 2006, a big effort at Brookhaven reported a result (from a data run that ended in 2001) that seems to deviate from Standard Model calculations by around 3 $\sigma$.

The experiment was moved from Brookhaven to Fermilab and reconstituted and improved, and on Wednesday the group will report their latest results from a new, large dataset.  The big question is, will that deviation from Standard Model expectations grow in significance, indicating possible new physics?  Or will the aggregate result be consistent with the Standard Model?   Stay tuned.

UpdateHere is the FNAL page that includes a zoom link to the webinar, which will happen at 10 AM CST on Wednesday, April 7.

Anonymous said...

From the general attitude around UChicago, my bet is yes they have a 5 sigma (or better) deviation from SM.

But I admit I am biased. I really hope that they find something new here, the proton mass puzzle in recent years was a bit of a letdown. Precision measurements are a lot more promising to me at least than higher energy collisions, so i personally want to seee this continue.

Anonymous said...

Kind of disappointing that after so many years the result is still not conclusive enough to say much.

Douglas Natelson said...

So, the experimental value seems to be consistent with the BNL number, despite numerous changes in the experimental setup, which is good. The data collection from runs 2-4 will shrink the statistical uncertainties by quite a bit. The question is then "what is the correct theoretical prediction for comparison?" As mentioned in this typically high quality article by Natalie Wolchover, and further in this blog post, there is tension between theory calculations of the hadron contribution that are computed using "R-ratios" (which feeds in experimentally measured cross-sections plus the optical theorem), and pure theory calculations based on lattice QCD methods (which include certain approximations). The "BMW" result is a new version of the latter, and it's closer to the experimental value, though quite far from all of the R-ratio approaches.

Even aside from the experiment, if the R-ratio analysis is all correct, and the lattice QCD calculations are also all correct and complete, and they still disagree, then there has to be missing physics somewhere....

Anonymous said...

Perhaps useful to quote Peter Woit on this:

"The problem is that while the situation with the experimental value is pretty clear (and uncertainties should drop further in coming years as new data is analyzed), the theoretical calculation is a different story. It involves hard to calculate strong-interaction contributions, and the muon g-2 Theory Initiative number quoted above is not the full story. The issues involved are quite technical and I certainly lack the expertise to evaluate the competing claims. To find out more, I’d suggest watching the first talk from the FNAL seminar today, by Aida El-Khadra, who lays out the justification for the muon g-2 Theory Initiative number, but then looking at a new paper out today in Nature from the BMW collaboration. They have a competing calculation, which gives a number quite consistent with the experimental result [...]"

Anonymous said...