“Do We Understand All Of Nature’s Basic Ingredients?” Muon Experiment Wins Breakthrough Prize for efforts to advance the Standard Model of Particle Physics

On Saturday 18 April, the 2026 Breakthrough Prize in Fundamental Physics was awarded to the Muon g-2 Collaboration at CERN, Brookhaven National Laboratory, and Fermilab. Last year, the collaboration reached unprecedented precision in measuring the properties of muons and tested the limits of the standard model of particle physics like never before.The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.The muon is one of the less common fundamental particles. It’s like a heavier cousin of the electron and can form as a product of a bunch of particle collisions. Due to cosmic rays falling on our heads all the time, you get hit by a muon every second without noticing.What this experiment is trying to determine is [...] do we understand all of nature's basic ingredients?Dr Chris PollyBoth electrons and muons possess an electric charge and produce a magnetic field. Fundamental particles also have a property called spin, which while it doesn't mean the particles are physically spinning like a top, does cause them to behave in many ways as if they were. Spin produces a magnetic moment, and, in the formulation of that very moment, we find the g-factor.The g-factor describes the strength of this magnetic moment, and if we weren’t under the yoke of quantum mechanics, its value would be 2. Unfortunately, reality is more complex than that.According to quantum field theory, particles interact with “virtual particles” that pop in and out of existence in a vacuum. This phenomenon is also described as quantum foam, due to how these virtual particles appear and disappear like transient froth. Some virtual particles have charges, and these affect the true value of g.           For the theoreticians, the challenge is to calculate the probability and strength of all these interactions and produce a high-precision value; for the experimenters, the challenge is actually measuring it. The race between the two has been going on for decades: from CERN in the 1960s to Fermilab today, via the Brookhaven experiment.“What motivated each generation of this experiment has changed a little bit over time, but fundamentally, what this experiment is trying to determine is, by looking at the quantum foam that surrounds the particles of the universe, do we understand all of nature's basic ingredients?” Chris Polly, a physicist at the US Department of Energy's Fermilab and co-spokesperson for the Muon g-2 project, told IFLScience.The standard model of particle physics underpins our best understanding of the building blocks of matter and three out of four of the universe’s fundamental forces. Gravity doesn’t fit into it (yet), and hypothetical but important features of the universe like dark matter and dark energy also elude this theory. Still, it has been extremely successful, passing many, many tests and predicting the existence of several particles, including the Higgs boson.The experiment during the last leg of its trip from Long Island to Illinois in 2013.Image credit: FermilabThe limitations imply that there’s more out there not yet captured in the theory, and the measurement of g-2 has historically been a crucial battleground for this quest. This is why the experiment has persisted for so long, and why it is important enough that a crucial component was moved from Brookhaven, New York, to Fermilab, Illinois, in an adventurous journey in 2013. Worth the trip, for sure!“The Fermilab experiment is 34,000 times more precise than the g-2 experiment in 1965 at CERN,” David Hertzog, former co-spokesperson for the Muon g-2 experiment at Fermilab, told IFLScience. “That's how far we've come. And each time, we've had to build up more, and more, and more of the catalog of thousands and thousands of little processes that can happen.”The final result agrees with previous experimental results and improves on them massively. They were aiming for a precision of 140 parts per billion; they got to 127 parts per billion. The theory is now the side that needs to catch up.“For the experiment to really say something meaningful, you need to be able to compare the experimental measurement to the theoretical prediction, because it's thanks to the extent that they differ that we know there might be new particles entering that quantum foam that we haven't yet discovered at high-energy colliders,” Polly told IFLScience.A recent approach to understanding everything that goes on at the particle level is known as lattice quantum chromodynamics (lattice QCD). This is the study of the strong nuclear force present in the nucleus of atoms and in interactions that involve quarks or particles made of quarks (the hadrons – a group of particles that includes protons and neutrons).A big source of uncertainty in the g-2 theoretical value is called hadronic vacuum polarization (HVP). Basically, you have a virtual photon that turns briefly into some sort of hadron, then back to a photon, and is reabsorbed. Calculating this tiny quantum correction before lattice QCD was extremely hard and had to be estimated from experiments that involved collisions between electrons and positrons (their antimatter counterparts), also known as e⁺e⁻.           The development of lattice QCD is extremely exciting because the g-2 value has been different from the theoretical prediction for decades. Now lattice-QCD comes along and the Fermilab results reach their highest precision, and the two of them agree with each other. Problem solved? Not so fast. Lattice-QCD does not agree with most of the electron–positron experiments that got the HVP value.“What [the g-2 result] means still remains to be seen,” Polly told IFLScience, “because we have this body of data from e⁺e⁻ predicting how often quarks appear, which is very different than when we ask the lattice QCD to calculate how often we should expect those quarks to appear. And the question right now really is, who's correct? The experimental inputs or the lattice QCD?”“We won't know for sure if g-2 at Fermilab has discovered new physics until we understand what the difference is between these two determinations of the theory.”While we wait on the theoretical physicists to straighten out this issue, don’t think that the experimentalists are sitting idly by. There are experiments across the world that will continue to probe muonic physics. At Fermilab, Mu2e, the muon-to-electron conversion experiment, aims to test the standard model prediction for these particles in a different way.The Breakthrough Prize awards $3 million to be shared among the members of the collaboration. Both Polly and Hertzog stressed how good it is that the award recognizes the collective and international nature of this groundbreaking experiment.“To imagine that hundreds of people from all over the world can focus their energy and training to work collectively to produce one and only one result to the highest level of precision, and then this result has a deep impact on understanding the subatomic world that we live in, it's pretty impressive,” Hertzog told IFLScience. “They've, in their hearts, just understood the importance of getting it right."“It's very gratifying that the Breakthrough Prize Foundation is recognizing us, because this really is recognizing the beauty of this experiment, regardless of the theoretical interpretation at this exact snapshot in time,” echoed Polly.This year, the foundation awarded six Breakthrough Prizes, each worth $3 million, as well as recognizing 15 early-career physicists and mathematicians, who share six $100,000 New Horizons Prizes. Three $50,000 Maryam Mirzakhani New Frontiers Prizes were awarded to three women mathematicians who recently completed PhDs.