Strong hints of new physics at Fermilab

particle physics

Apr 12

The Fermilab Muon g-2 Experiment announced their results of their first data taking run this past Wednesday. While not the direct confirmation of new physics we'd all been dreaming about for almost a decade now, the newest version of the long-studied precision test of quantum electrodynamics has added serious optimism to a community frustrated by years of the null result.

The Muon g-2 Experiment aims to measure the magnetic field generated by the muon, a fat version of the electron which have discussed extensively. The strength of that magnetic field, is parametrized in calculations by the letter g, and its "classically" expected value is 2. Whence "g-2" aims to measure quantum corrections to that tiny magnetic field.

*from Muon g-2 (https://arxiv.org/abs/2104.03281)*

As discussed in the seminar linked above, the experiment used some clever tricks to maximize the precision of the measurement.

Any confirmed deviation from the standard model implies as-yet unknown quantum effects, which would probably be best explained by the existence of a new particle (or family of particles!) at masses beyond the reach of current experiments. Analogous corrections would be a great help to explaining the suspiciously small mass of the Higgs Boson.

In a live broadcast recoded on YouTube, details of the experiment were recounted with clarity and boatloads of dry humor and dad jokes. Here are some important highlights:

While an unfortunate malfunction introduced greater than expected systematic error, the measurement was still statistics limited.
Fermilab's run 1 took twice as much data as Brookhaven's g-2 experiment. As a result, the errors on the measurements were smaller, although not quite at the experiment's target precision.
Fermilab's run 1 results are actually more in agreement with the standard model than Brookhaven's final result: (compare 3.3 vs 3.7 standard deviations or "sigma")
Because the margin of error of the two measurements strongly overlapped, the data sets could be combined. The resulting error from the combined data suggests a 4.2-sigma deviation from the standard model, with a median value in between the two results.
Fermilab g-2 will continue to take data and beat back the experimental error. Run 2 is already complete and the improved results are expected in 12-18 months.

While it is true that results short of the "5-sigma" standard aren't considered "official", no sane person would chide you for being excited about the result. Although, maybe not as excited as Overbye's headline editor seems to be.