The field of particle physics is populated with believers and skeptics. The believers will try to convince you that new physics is about to be discovered, or that is anyway at close reach. The skeptics will on the other hand look at the mass of confirmations of the current theory -the Standard Model- and claim that any speculation about the existence of discoverable new phenomena has no basis.
Believers are fun to argue with, and often I manage to bring them to put their money where their mouth is, by offering to take bets on the fact that the LHC will discover new particles or forces in the forthcoming years, or on the origin of some statistical fluctuation in the data already collected. I won over 1250 $ that way in the past few years, plus a bottle of very fine wine.

A common trait of all discussions with believers, when we get down to examining the evidence, is the appearance of the topic of the anomalous magnetic moment of the muon. This is a quantity which insiders address to with the term "muon G minus 2". The muon G-2 has been measured with extreme precision by experiments that watched muons spinning inside a small accelerator decay, yielding electrons going one way or the other. From asymmetries in the decay kinematics one may infer the size of the magnetic moment that muons possess, to which a theoretical calculation of the same quantity can be compared. 

Theory and experiment currently differ by 3.6 standard deviations, making this one of the most striking and long-standing anomalies of the Standard Model. What is more, the anomaly directly points to the possibility that new particles of mass too high to have been discovered by the LHC yet are affecting the measured value. Their contribution, not contained in the theoretical calculation, would thus "explain" the discrepancy. Hence the interest for this anomaly by believers.

The history of the muon G-2 is full of twists and turns - the theoretical calculation in itself is mind-boggling; but experimental setups that determine the anomalous moment are also of incredible complexity and sophistication. The quantity was precisely determined at Brookhaven, and now a new experiment has been in construction at Fermilab specifically to increase the experimental accuracy.

On the theoretical side, a new idea has lately came about, to try and reduce the uncertainty on what is called "hadronic contribution" to the measured value. In practice, the latter depends on a quantum property of photons, that may "fluctuate" for unmeasurably small instants into a mess of hadronic particles before returning to be photons. Determining that contribution requires to compute complex integrals that sum over all the possible ways that photons fluctuate to hadrons. A comparison with experimental input is also needed, and this calls in the process when an electron and a positron produce a muon-antimuon pair. 

So, it was recently understood that one can get the same information from a "crossed diagram" where instead of having electrons and positrons on one side and muon pair on the other, you take a muon and collide it with an electron, getting again a muon and an electron in the final state. The crossed diagram is much easier to compute, so once one compares theory with experiment on that process one should be able to strongly reduce the uncertainty of the hadronic contribution to the muon magnetic moment. I wrote about that idea here recently - there is a whole collaboration of theorists and experimentalists who are working toward performing an accurate muon-electron scattering experiment at CERN.

But I am divagating. The news are that a set of three articles were published on the Cornell ArXiv today. They consider gravitational effects due to the Earth on the magnetic moment of the electron and the muon. The effect of the curvature of spacetime is of two parts in a billion, and should be the same for all fermions. The authors computed the effect, finding that for the electron the modification can be reabsorbed in the calculation, while for the muon a modification arises. The difference has roots in the fact that the measurement in the muon case is made in a highly relativistic regime.

It all boils down to this: if the three theorists will be shown to have made no mistake in their calculation, the infamous "G-2 anomaly" of the magnetic muon moment will exist no more. The idea that classical gravitational effects affect its value in a way quite consistent with the observed departure is extremely surprising and exhilarating, if you ask me. As for the fate of the experiments that are setting up to measure more precisely the discrepancy, I let you fantasize... 

What I would like to see now, however, is a set of phenomenological papers that recompute the favourite space of SUSY instantiations under the hypothesis that the muon G-2 is exactly at its SM expectation. Is that too much to wish for in the matter of one week ? You SUSY pheno enthusiasts, show me you're fast!


Tommaso Dorigo is an experimental particle physicist who works for the INFN at the University of Padova, and collaborates with the CMS experiment at the CERN LHC. He coordinates the European network AMVA4NewPhysics as well as research in accelerator-based physics for INFN-Padova, and is an editor of the journal Reviews in Physics. In 2016 Dorigo published the book “Anomaly! Collider physics and the quest for new phenomena at Fermilab”. You can get a copy of the book on Amazon.