Please see the updates in each category below.
This is a big if but if there is an unknown force that interacts with muons but not electrons then the following anomalies could be explained in one sweep:
- broken lepton universality
- proton size being slightly larger when it is orbited by an electron than when it is orbited by a muon
- muon’s magnetic moment being larger than the theoretically predicted value
These anomalies are statistically significant but more data are needed to confirm them as discoveries. Once the anomalies are confirmed then we will be certain that there is new physics beyond SM (Standard Model) – current theory of particle physics – but then the challenge will be to find an explanation for the anomalies. One possibility is that there is a particle/force that interacts with muons but not electrons.
Tests of lepton universality
Electron, muon and tau are identical except for their invariant masses. Muon is 206.85 times heavier than electron and tau is 16.8 times heavier than muon.
Electron, muon and tau are identical with respect to the electromagnetic force because they carry the same electrical charge (-1). They are also identical with respect to the weak nuclear force because electrons, muons and tau particles are produced (after adjustment for the masses ) equally often in weak decays. We don’t understand why there are 3 copies of the same particle with different masses but physicists refer to this puzzling similarity of electron, muon and tau as “lepton universality.”
Experimental evidence is groving that electron, muon and tau are not exactly identical with respect to the weak nuclear force. SLAC’s Vera Luth explains the experimental side of this story here.
Experiments measuring the size of a proton
Good writing makes a difference. I wish I could write as well as Natalie Wolchover.
“The puzzle is that the proton — the positively charged particle found in atomic nuclei, which is actually a fuzzy ball of quarks and gluons — is measured to be ever so slightly larger when it is orbited by an electron than when it is orbited by a muon, a sibling of the electron that’s 207 times as heavy but otherwise identical. It’s as if the proton tightens its belt in the muon’s presence. And yet, according to the reigning theory of particle physics, the proton should interact with the muon and the electron in exactly the same way. As hundreds of papers have pointed out since the proton radius puzzle was born in 2010, a shrinking of the proton in the presence of a muon would most likely signify the existence of a previously unknown fundamental force — one that acts between protons and muons, but not between protons and electrons. (Interestingly, this new physics could also explain a long-standing discrepancy in the measurement of the muon’s anomalous magnetic moment.)” – Natalie Wolchover
There is another article that packs a lot of information regarding the proton radius measurements.
“There’s still a lot we don’t know about the proton” by Emily Conover
Update (June 6, 2020): Natalie Wolchover wrote another article summarizing the latest experimental results. You can also look at this Nature news article. The most informative article on this subject, however, has to be this CERN Courier article. “We can no longer speak about a discrepancy between measurements of the proton radius in muonic and electronic spectroscopy” says Krzysztof Pachucki.
Muon g-2 experiments
As I indicated in my post titled “Muon g-2 mystery“, the final report of the E821 experiment at BNL (Brookhaven National Laboratory) showed that the muon’s magnetic moment was higher than the theoretically predicted value. Fermilab will repeat this experiment with better statistics. Fermilab muon g-2 experiment will measure muon’s magnetic moment to a precision of 140 parts per billion. This will be a factor of 4 improvement over E821 experiment’s precision. Fermilab muon g-2 experiment (E989) has started its operations.