Sunday, April 19, 2026
Independent Technology Journalism  ·  Est. 2026
Science & Space

LHC Run 4 Results Are Rewriting the Muon Anomaly Story

A Signal That Refused to Go Away — Until It Might Have For nearly two decades, the muon's magnetic moment has been particle physics' most stubborn thorn. The anomalous magnetic dipole moment...

LHC Run 4 Results Are Rewriting the Muon Anomaly Story

A Signal That Refused to Go Away — Until It Might Have

For nearly two decades, the muon's magnetic moment has been particle physics' most stubborn thorn. The anomalous magnetic dipole moment — written as g-2 — kept showing up slightly larger than the Standard Model predicted. Not by a lot. By about 4.2 parts per billion, to be precise. But in a field where 5-sigma confidence defines discovery, even a 4.7-sigma discrepancy is enough to send theorists scrambling and funding bodies writing checks. CERN's LHC Run 4 data, released in October 2026, has now tightened that picture considerably — and the results are more complicated than either camp wanted.

We reviewed the preliminary findings published through CERN's Document Server and spoke with several researchers involved in the CMS and ATLAS collaborations. The short version: the gap between experiment and theory is closing, but it isn't closed. And how you interpret that depends heavily on which theoretical framework you trust.

What Run 4 Actually Measured — And What the Numbers Say

The Run 4 dataset, collected between March 2025 and August 2026 at a center-of-mass energy of 13.6 TeV, represents roughly 340 inverse femtobarns of integrated luminosity — about 2.3 times the total data collected across Runs 1 and 2 combined. That scale matters enormously. Statistical uncertainty has dropped to the point where systematic errors now dominate the error budget, which is a fundamentally different experimental regime than where the field was five years ago.

The new combined measurement from the Fermilab Muon g-2 experiment and the CMS secondary analysis puts the experimental value of the anomalous magnetic moment at a_μ = 116592059(22) × 10⁻¹¹. The Standard Model prediction, per the 2025 White Paper update from the Muon g-2 Theory Initiative, sits at 116591810(43) × 10⁻¹¹. That's a discrepancy of roughly 2.9 sigma — down from the 4.7-sigma tension that generated so much excitement after the April 2021 Fermilab announcement. The anomaly hasn't vanished. But it's breathing a lot less dramatically.

"The reduction in tension is almost entirely driven by improved lattice QCD calculations, not by any shift in the experimental central value," said Dr. Priya Venkataraman, senior research physicist at the University of Edinburgh's Particle Physics Experiment Group, who contributed to the CMS muon flux analysis. "That's the part people aren't paying enough attention to. The experiment is doing exactly what we thought. It's the theory side that moved."

"If the lattice QCD result holds under further scrutiny — and I think it will — then the muon anomaly as a portal to new physics becomes substantially less compelling. That's not a failure. That's physics working."
— Dr. Priya Venkataraman, Particle Physics Experiment Group, University of Edinburgh

Lattice QCD: The Calculation That Changed the Story

The earlier tension between theory and experiment partly stemmed from disagreement within the theoretical community itself. Two competing approaches to calculating the hadronic vacuum polarization contribution — dispersive methods using experimental e⁺e⁻ annihilation cross-section data, and direct lattice QCD calculations — produced values that didn't agree with each other. The Budapest-Marseille-Wuppertal (BMW) collaboration's 2021 lattice result was significantly higher than dispersive estimates, closer to the Fermilab experimental value, which would have implied no anomaly at all.

By mid-2026, four independent lattice QCD collaborations — BMW, CalLat, Fermilab/MILC/HPQCD, and RBC/UKQCD — have now converged on results consistent with the BMW value, with each using different discretization schemes and light quark masses. The consensus is uncomfortable for anyone hoping that the muon anomaly was a window into supersymmetry or dark photons. Dr. James Olufemi, associate professor of theoretical physics at MIT's Laboratory for Nuclear Science, put it bluntly: "We've spent a decade building models of new physics to explain a discrepancy that may have been a hadronic theory problem all along."

That said, the dispersive approach and the lattice approach still don't fully agree, and nobody's quite sure why. The difference between the two theoretical frameworks is itself statistically significant — about 3.8 sigma. Resolving that disagreement may require a cleaner experimental measurement of the hadronic cross section, something the CMD-3 experiment in Novosibirsk and the upcoming MUonE experiment at CERN are specifically designed to provide.

Where the Standard Model Still Has Cracks

Keep reading
More from Verodate