The Muon Surprise Melts Away express
The muon is a heavy cousin of the electron that lives for just two millionths of a second. In a magnetic field, it spins like a top, but slightly faster than formulas predict. Twenty years ago, this 'slightly faster' was noticed at Brookhaven, and physicists have been arguing ever since: either the instruments were wrong, or unknown particles are hiding in the vacuum.
Now, at Fermilab, the muon's spin was measured four times more precisely, and again it turned out 'too fast.' But in parallel, theorists on supercomputers recalculated the vacuum's contribution: it turns out the vacuum isn't empty but teems with virtual particles that give the muon a slight extra twist. New calculations, accounting for the mixing of two types of particles, raised the theoretical value almost to the experimental one. The discrepancy melted away.
The Standard Model has held up again. It doesn't explain dark matter or the tiny neutrino masses, but for the muon, its predictions now match experiment. This isn't a failure but a triumph of precision: we've learned to calculate the vacuum better.
Original abstract
as written by the authors arXiv:2601.08101
Recently, the exciting new Fermilab (FNAL) Muon g-2 measurement impressively confirmed the final Brookhaven (BNL) result from 2004, and with a result four times more precise, has launched a new serious attack on the Standard Model (SM). On the theoretical side, ab initio lattice QCD (LQCD) calculations of hadronic vacuum polarization have made remarkable progress. They are now the new standard for studying the leading non-perturbative contributions, which have previously hindered matching with the precision required for full exploitation of the experimental results. The lattice results affected both leading hadronic contributions the hadronic vacuum polarization (HVP) and the hadronic light-by-light (HLbL) contributions by increasing the previously generally accepted $$e^+e^-$$ to hadrons based dispersion relation results. The shifts reduced the discrepancy between theory and experiment, leaving nothing missing. One of the most prominent signs of Beyond the Standard Model (BSM) physics has disappeared: the SM appears validated more than ever, in agreement with what other searches at the Large Hadron Collider (LHC) at CERN tell us! A triumph of the SM, even though the SM cannot explain known cosmological puzzles like dark matter or baryogenesis, and why neutrino masses are so tiny, the absence of strong CP violation, for example. I also argue that the discrepancy between the data-driven dispersive result and the lattice QCD results for the hadronic vacuum polarization can be largely explained by correcting the $$e^+e^-$$ data for 'rho-gamma' mixing effects.
The muon is so heavy that at the Large Hadron Collider, its production is recorded as a 'shower' of dozens of particles, yet it lives for less time than it takes light to cross a grain of sand.