The nature of the experiment has been discussed here, but my main question is this: is there any theory that has predicted the results of this experiment or are we completely clueless about what's happening? In other words, have we come up with a new hypothetical interaction that could explain the results?
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1Related: https://physics.stackexchange.com/q/627849/50583 – ACuriousMind Apr 09 '21 at 16:15
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6There are 137.5 speculative "theories". The archives are ablaze with them. None work. – Cosmas Zachos Apr 09 '21 at 16:54
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https://arxiv.org/search/?query=Muon&searchtype=title&source=header – G. Smith Apr 09 '21 at 17:35
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@CosmasZachos "None work". Why do you say that? I thought we had a surfeit of models that work... – Mitchell Porter Apr 10 '21 at 05:36
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1@CosmasZachos apart from that printed in a Nature paper on the very same day? – ProfRob Apr 10 '21 at 06:54
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@ProfRob Naturally. I was hinting at Zsa Zsa Gabor’s endless parade of fabulously successful marriages... – Cosmas Zachos Apr 10 '21 at 10:08
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In the standard model the $α=(g-2)/2$ of an elementary particle should be calculable , the calculations as accurate as the higher orders are computed.
For the electron the calculations coincide with the experimental value to great accuracy
The muon $α=(g-2)/2$ has different diagrams dominant so the theoretical value will be different, but it was seen ,first at the Brookhaven data, that the theory differed , not statistically significantly, with the data and so the Fermi lab experiment was designed at great effort experimentally and expense, because the difference would mean that a new interaction should add to the effect, not in the standard model that the calculations use.
As the comments say there are many extensions of the standard model that can try to fit the new results and thus choose the extension or the new theory. Hit the link given by G.Smith to get a large number of papers using different theories.
anna v
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3The muon does not "have different diagrams". All leptons have the exact same diagrams, by lepton universality. The electron couples to the weak interaction too. The value of the diagrams is different because these particles have different masses. – AccidentalFourierTransform Apr 10 '21 at 12:24
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@AccidentalFourierTransform thanks, also to the hadronic interactions discussed in the other answers for the muons duo to a new paper claiming there is no need for extra exchanges, https://www.nature.com/articles/s41586-021-03418-1 – anna v Apr 10 '21 at 12:33
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So would one of these standard model extensions have to make a new testable prediction for it to be accepted? One can always come up with an extension to fit the data – user9343456 Apr 13 '21 at 14:24
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@user9343456 yes, for a new theory or an extension of a theory to be validated , predictions should be made to be tested or falsified, If this use of lattice QCD to explain the numbers is valid, no need of an extension. – anna v Apr 13 '21 at 14:43
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A Nature paper was published on the same day, which seems to have attracted a lot less press. This presents a recalculation of the muon $g-2$ value, using standard model physics and their value is consistent with the new experimental value (Borsanyi et al. 2021). So there's one theoretical explanation of the result!
ProfRob
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glanced through the paper, I cannot judge it of course, but it nowhere is discussing of why the electron g-2 works so well. Should not QCD enter there and spoil the good aggreement? (checked for the worrd "electron") – anna v Apr 10 '21 at 07:48
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@anna v . I would assume the hadronic corrections to the electron are much smaller on account of the smaller energy scales involved. – Cosmas Zachos Apr 10 '21 at 10:23
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1@CosmasZachos OK, I found this https://arxiv.org/abs/1208.6583 , where they are discussing hadronic corrections to the electron case, they are small. ευχαριστω Κοσμα – anna v Apr 10 '21 at 11:08
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Yes there is a theory that explains the results ... the Standard Model. In other words, the claim is that the Standard Model already is consistent with the experimental data, and the original "prediction" was calculated wrong.
Check out the paper published in Nature together with the muon g-2 results, or the writeup at popular level.
Allure
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This answer was already suggested above. This is another source that has some really nice embedded links on the authors and the university that hosted the study. Very nice reading some might enjoy so I thought it would be nice to share. Assuming I am allowed to put in a link. Not sure it is allowed by stack exchange.