Muon result may rewrite standard model of physics

A new measurement of the magnetic moment of the muon may draw into question the standard model of physics, the reigning theoretical construct describing all known fundamental forces and particles of physics. The new result was released in a paper dated today (7 April 2021) with 240 authors, led by researchers at Fermilab in the U.S., but also including researchers from Italy, Germany, United Kingdom, Russia, South Korea, China and Croatia.

The standard model of physics is arguably is the most successful physical theory ever devised, explaining all known fundamental particles and all known forces between these particles, except for gravity. It has reigned supreme since it was first formulated in the 1970s. However, physicists have long recognized that it cannot be the final answer. For one thing, it is incompatible with general relativity. Further, it says nothing about the identity of dark matter, which consists of at least 75% of known matter, nor does it identify dark energy that pervades the cosmos. At several junctures physicists have thought they found experimental evidence for physics beyond the standard model, but in each case these results evaporated in the wake of additional, more carefully analyzed data.

Muons are similar to electrons in several ways (negative electrical charge, and a spin), but their mass is approximately 207 times that of an electron. In part due to their much larger mass, muons are very short-lived, decaying into electrons and neutrinos with a half-life of roughly 2.2 microseconds. The Fermilab experiment measures the magnetic moment of the muon, known as g. In 1928, Paul Dirac, one of the founders of quantum mechanics, showed that g should equal 2. But to this value must be added the contribution of a sea of virtual particles.

A careful tabulation of these particles and their forces, done via massive calculations on supercomputers, yields the current best theoretical result that the magnetic moment of the muon should be 2.00233183620(86), where (86) denotes the uncertainty in the last digits of the calculation.

The discrepancy reported in the paper certainly isn’t great. The latest Fermilab experimental value is 2.00233184122(82), yielding a difference of 5.02 x 10^-9, or 4.2 standard deviations from the theoretical value. This does not yet meet the 5.0 standard deviation level that most physics experiments strive for, but it is enough to start raising the possibility of a major issue with the theoretical model. Additional runs are planned on the Muon g-2 experimental facility in the coming months that should further clarify the matter.

Theoreticians are already speculating as to the identity of other particles and forces that may be the source of the discrepancy. One possibility is a lightweight particle known as Z’ (Z prime). If such a particle exists, it could explain another nagging anomaly, namely the fact that the universe appears to be expanding slightly faster than standard cosmological models (based on the standard model) predict.

Gordon Krnjaic of Fermilab says the the g-2 result could set the agenda for particle physics for the next generation. “If the central value of the observed anomaly stays fixed, the new particles can’t hide forever. … We will learn a great deal more about fundamental physics going forward.” Fermilab Deputy Director of Research Joe Lykken adds, “this is an exciting time for particle physics research.”

However, physicist Sabine Hossenfelder recommends holding the champagne. After all, as mentioned above the result still does not meet the 5.0 standard deviation level recommended for most major physics experiments. She notes that the Higgs boson was “discovered” in 1996, in the wake of a 4 sigma result at the Large Electron-Positron (LEP) facility at CERN (near Geneva, Switzerland) but then subsequently disappeared after more data was obtained (the Higgs boson remained elusive until 2012, when experiments at the Large Hadron Collider definitely established its existence). Similarly, supersymmetric particles were “detected” at LEP in 2003, but again the evidence later evaporated. So let’s be careful before we jump to conclusions.

What’s more, another paper, also published today (7 Apr 2021), presents results of a different theoretical calculation (see also this report). Its results are closer to the new experimental value, meaning that the muon g-2 experiments might not represent a departure from the standard model. Additional calculations are planned to refine these values.

For additional details, see the researchers’ research paper, this Fermilab press report, and news reports from BBC, Nature, New York Times, Quanta Magazine and Scientific American.