Muon g-2 Experiment Results – Profound Implications for the History of the Universe

Particle Storage Ring at Fermilab’s Muon g 2 Experiment

Peering down a row of magnets leading to the particle storage ring at Fermilab’s Muon g-2 experiment. The results have theoretical physicists around the world frantically working through ideas for explanations. Credit: Photo by Cindy Arnold/Fermilab

Experiment opens up field for new physics, say Fermilab, UChicago scientists.

The news that muons have a little extra wiggle in their step sent word buzzing around the world this spring.

The Muon g-2 experiment hosted at Fermi National Accelerator Laboratory announced on April 7 that they had measured a particle called a muon behaving slightly differently than predicted in their giant accelerator. It was the first unexpected news in particle physics in years.

Everyone’s excited, but few more so than the scientists whose job it is to spitball theories about how the universe is put together. For these theorists, the announcement has them dusting off old theories and speculating on new ones.

“To a lot of us, it looks like and smells like new physics,” said Prof. Dan Hooper. “It may be that one day we look back at this and this result is seen as a herald.”

Gordan Krnjaic, a fellow theoretical physicist, agreed: “It’s a great time to be a speculator.”

The two scientists are affiliated with the University of Chicago and Fermilab; neither worked directly on the Muon g-2 experiment, but both were elated by the results. To them, these findings could be a clue that points the way to unraveling the last mysteries of particle physics—and with it, our understanding of the universe as a whole.

Muon g-2 Experiment at Fermilab

The Muon g-2 ring sits in its detector hall amidst electronics racks, the muon beamline, and other equipment. This impressive experiment operates at negative 450 degrees Fahrenheit and studies the precession, or “wobble,” of particles called muons as they travel through the magnetic field. Credit: Reidar Hahn/Fermilab

Setting the Standard

The problem was that everything was going as expected.

Based on century-old experiments and theories going back to the days of Albert Einstein’s early research, scientists have sketched out a theory of how the universe—from its smallest particles to its largest forces—is put together. This explanation, called the Standard Model, does a pretty good job of connecting the dots. But there are a few holes—things we’ve seen in the universe that aren’t accounted for in the model, like dark matter.

No problem, scientists thought. They built bigger experiments, like the Large Hadron Collider in Europe, to investigate the most fundamental properties of particles, sure that this would yield clues. But even as they looked more deeply, nothing they found seemed out of step with the Standard Model. Without new avenues to investigate, scientists had no idea where and how to look for explanations for the discrepancies like dark matter.

Then, finally, the Muon g-2 experiment results came in from Fermilab (which is affiliated with the University of Chicago). The experiment reported a tiny difference between how muons should behave according to the Standard Model, and what they were actually doing inside the giant accelerator.

What is a muon, and how does the Muon g-2 experiment work? Fermilab scientists explain the significance of the result.

Murmurs broke out around the world, and the minds of Hooper, Krnjaic and their colleagues in theoretical physics began to race. Almost any explanation for a new wrinkle in particle physics would have profound implications for the history of the universe.

That’s because the tiniest particles affect the largest forces in the universe. The minute differences in the masses of each particle affect the way that the universe expanded and evolved after the Big Bang. In turn, that affects everything from how galaxies are held together down to the nature of matter itself. That’s why scientists want to precisely measure how the butterfly flapped its wings.

The likely suspects

So far, there are three main possible explanations for the Muon g-2 results—if it is indeed new physics and not an error.

One is a theory known as “supersymmetry,” which was very fashionable in the early 2000s, Hooper said. Supersymmetry suggests that that each subatomic particle has a partner particle. It’s attractive to physicists because it’s an overarching theory that explains several discrepancies, including dark matter; but the Large Hadron Collider hasn’t seen any evidence for these extra particles. Yet.

Another possibility is that some undiscovered, relatively heavy form of matter interacts strongly with muons.

Finally, there could also exist some other kinds of exotic light particles, as yet undiscovered, that interact weakly with muons and cause the wobble. Krnjaic and Hooper wrote a paper laying out what such a light particle, which they called “Z prime,” could mean for the universe.

“These particles would have to have existed since the Big Bang, and that would mean other implications—for example, they could have an impact on how fast the universe was expanding in its first few moments,” Krnjaic said.

That could dovetail with another mystery that scientists are pondering, called the Hubble constant. That number is supposed to indicate how fast the universe is expanding, but it varies slightly according to which way you measure it—a discrepancy which could indicate a missing piece in our knowledge.

Almost any explanation for a new wrinkle in particle physics would have profound implications for the history of the universe.

There are other, further-out possibilities, such as that the muons are being bumped by particles winking in and out of existence from other dimensions. (“One thing particle physicists are rarely accused of is a lack of creativity,” said Hooper.)

But the scientists said it’s important not to dismiss theories out of hand, no matter how wild they may sound.

“We don’t want to overlook something just because it sounded weird,” said Hooper. “We’re constantly trying to shake the trees to get every idea we can out there. We want to hunt this down everywhere it could be hiding.”

Sigma steps

The first step, however, is to confirm that the Muon g-2 result holds true. Scientists have a system to tell whether the results of an experiment are real and not just a blip in the data. The result announced in April reached 4.2 sigma; the benchmark that means it’s almost certainly true is 5 sigma.

“If it’s really new physics, we’ll be much closer to knowing in a year or two,” said Hooper. The Muon g-2 experiment has much more data to sift through. Meanwhile, the results of some very complicated theoretical calculations—so complex that even the most powerful supercomputers in the world need to chew on them for months to years—should be coming down the pike.

Those results, if they get to a 5 sigma confidence level, will point scientists where to go next. For example, Krnjaic helped propose a Fermilab program called M3 that could narrow the possibilities by firing a beam of muons at a metal target—measuring the energy before and after the muons hit. Those results could indicate the presence of a new particle.

Meanwhile, at the French-Swiss border, the Large Hadron Collider is scheduled to upgrade to a higher luminosity that will produce more collisions. New evidence for particles or other phenomena could pop up in their data.

All this excitement over a wobble might seem like an overreaction. But tiny discrepancies can, and have, led to massive shakeups. Back in the 1850s, astronomers making measurements of Mercury’s orbit noticed it was off a little from what Newton’s theory of gravity would predict. “That anomaly, along with other evidence, eventually led us to the theory of general relativity,” said Hooper.

“No one knew what it was about, but it got people thinking and experimenting. My hope is that one day we’ll look back at this muon result the same way.”


“Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm” by B. Abi et al. (Muon g-2 Collaboration), 7 April 2021, Physical Review Letters.
DOI: 10.1103/PhysRevLett.126.141801

“Magnetic-field measurement and analysis for the Muon g – 2 Experiment at Fermilab” by T. Albahri et al. (The Muon g-2 Collaboration), 7 April 2021, Physical Review A.
DOI: 10.1103/PhysRevA.103.042208

19 Comments on "Muon g-2 Experiment Results – Profound Implications for the History of the Universe"

  1. “Finally, there could also exist some other kinds of exotic light particles, as yet undiscovered, that interact weakly with muons and cause the wobble. Krnjaic and Hooper wrote a paper laying out what such a light particle, which they called “Z prime,” could mean for the universe.”

    If these scientists understood, “The Halflec Model”, they would have no question of what was the source of the wobble.

    Just as if the scientists at the LHC understood the Halflec Model, they would know why there aren’t enough Muons in the Beauty Quark decay and why Muons are 8 times as large but have only an equal charge to an Electron.

    And if any of them understood the Halflec Model they would also understand what a Black Hole is and what the universe looked like the instant BEFFORE the Big Bang and why Electrons pair up in a superconducting superconductors and what magnetism is and the list of mysteries to modern science, that are self-evident in terms of the Halflec Model, goes on and on….

    • Torbjörn Larsson | July 11, 2021 at 5:20 am | Reply

      Scientists, and those interested in science, are not interested in unpublished and likely pseudoscientific models. And models that claims to explain anything you point them at trivially explain nothing at all, c.f. religion superstition. All of that is presumably why you write in science sites comment sections instead of trying to publish the model, and such behavior fools no one to search out your futile name dropping.

      We do know how the universe looked like at the end of inflation just before the hot big bang.

      “Inflation came first, and its end heralded the arrival of the Big Bang. There are still those who disagree, but they’re now nearly a full 40 years out of date. When they assert that “the Big Bang was the beginning,” you’ll know why cosmic inflation actually came first. As far as what came before the final fraction-of-a-second of inflation? Your hypothesis is just as good as anyone’s.”

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  2. BibhutibhusanPatel | July 11, 2021 at 3:13 am | Reply

    The value calculated for g_2 is accurate.Bùt thìngs are too critìcal tò ŕesolve for à good conclùsion.Hence, withoùt going to mòŕe detaiĺ,thìs can simpĺy be taken as ìntŕinsic pŕopòrty òf muon.This knòwĺedge can be applied to astrophysics proporly.

  3. BibhutibhusanPatel | July 11, 2021 at 3:30 am | Reply

    There ìs no error present ìn the measured value òf ģ_2 for muon.So this can be taken as its internàl proporty.Astrophysìcs can apply thìs experimental knowledge.

  4. Torbjörn Larsson | July 11, 2021 at 4:50 am | Reply

    I could overlook the overly enthusiastic article – because the result quite appropriately leads to excitement – but while it notes that the result is not yet at particle physics quality bar for discoveries of 5 sigma its title suggest otherwise.

    It also haste over the most likely cause of an error. The main error culprit would be on the theoretical side, as a peer reviewed paper by coincidence pointed out the very same day of the experiment result release.

    “Why You Should Doubt ‘New Physics’ From The Latest Muon g-2 Results”

    “The technique of Lattice QCD, then, represents an independent way to calculate the theoretical value of “g-2” for the muon. Lattice QCD relies on high-performance computing, and has recently become a rival to the R-ratio for how we could potentially compute theoretical estimates for what the Standard Model predicts. What El-Khadra highlighted was a recent calculation showing that certain Lattice QCD contributions do not explain the observed discrepancy.

    The elephant in the room: lattice QCD. But another group — which calculated what’s known to be the dominant strong-force contribution to the muon’s magnetic moment — found a significant discrepancy. As the above graph shows, the R-ratio method and the Lattice QCD methods disagree, and they disagree at levels that are significantly greater than the uncertainties between them. The advantage of Lattice QCD is that it’s a purely theory-and-simulation-driven approach to the problem, rather than leveraging experimental inputs to derive a secondary theoretical prediction; the disadvantage is that the errors are still quite large.

    What’s remarkable, compelling, and troubling, however, is that the latest Lattice QCD results favor the experimentally measured value and not the theoretical R-ratio value. As Zoltan Fodor, professor of physics at Penn State and leader of the team that did the latest Lattice QCD research, put it, “the prospect of new physics is always enticing, it’s also exciting to see theory and experiment align. It demonstrates the depth of our understanding and opens up new opportunities for exploration.”

    While the Muon g-2 team is justifiably celebrating this momentous result, this discrepancy between two different methods of predicting the Standard Model’s expected value — one of which agrees with experiment and one of which does not — needs to be resolved before any conclusions about “new physics” can responsibly be drawn.”

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  5. Torbjörn Larsson | July 11, 2021 at 5:12 am | Reply

    I also wanted to highlight the claimed cosmological connection between two sets of difficult to measure parameters. We can turn that around and note that non-robust tensions would disconnect that handwaving.

    As it happens, there is a new paper out that strengthens the tension removing method of measuring the Hubble constant, and points out that the earlier apparent clustering in low and high values goes away if you add all observations. Observations seems slowly converging on a reasonable value – with no new physics – which situation reminds of the theoretical side of the muon g-2 situation.

    “In a new paper, a major player in this dilemma takes a look at the available information and concludes that the best observations might be pointing to a triumph for our standard picture of how the universe has grown over time.”

    “”I think it’s a really interesting question: ‘Is there new physics beyond the standard cosmological model?'” Wendy Freedman, a cosmologist at the University of Chicago, told Live Science.

    Freedman has spent much of her career observing what are known as Cepheid variable stars. These stars, which pulsate regularly, have a relationship between the period of the fluctuations in their light and their intrinsic brightness, meaning how bright they would be if we were standing right next to them. By knowing this intrinsic brightness and a Cepheid’s luminosity as seen from Earth, astronomers can calculate its distance from us and then measure the speed at which the universe is expanding at that point in space.

    Cepheid data is one of the linchpins of the higher value of the Hubble constant, but Freedman and her collaborators have always wondered if perhaps they were making systematic errors in their observations. They have long searched for independent methods to corroborate or contest their results.

    A few years ago, she and her colleagues found one method in the light of giant red stars. These objects, which represent a later life stage for stars with a mass similar to our sun, reach a specific peak brightness at a certain point in their evolution. Much like with the Cepheids, astronomers can look at how dim they appear from Earth to get a good estimate of their distance.

    In 2019, Freedman and her team provided a number for the Hubble constant that sat just between the two other measurements: 47,300 mph per million light-years (69.8 km/s/Mpc). That result was calibrated using giant red stars in the Large Magellanic Cloud, a dwarf galaxy that orbits the Milky Way whose distance from us is relatively well determined.

    Since then, the researchers have added more data points, calibrating the distance to giant red stars in three other galaxies and regions of space, which ups the precision of their Hubble constant measurements. These findings, which found essentially the same middle-ground estimate, appeared in a paper that was published to the preprint database arXiv on June 29, and which has been accepted for publication in the Astrophysical Journal.

    “It’s landing in the same place, just shy of 70 [km/s/Mpc] with an uncertainty of just over 2%,” Freedman said of the new Hubble constant estimate from the red giant stars. “If we compare those results to the CMB, we wouldn’t say there’s an issue.”

    These latest red giant measurements point to the possibility of systematic errors in the Cepheid observations, Freedman said. Obscuring dust and background light from the universe are some possible culprits, she added, though it will take time to actually discover if that is the case.”

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    The Cepheid luminosity data underlies the supernova method that is the method that almost always comes to larger values. In the paper it is pointed out that Cepheid luminosity is a proxy based on observations of constancy, while the red giant tip luminosity is truly fixed – as far as we know – by a physical mechanism.

  6. Howard Jeffrey Bender | July 11, 2021 at 5:27 am | Reply

    Regarding supersymmetry, one possibility from a view of String Theory is that strings are formed as pairs, one open string matched with one closed string. This may explain some of the curious features we see, such as light being both a particle and a wave. Specifics can be found in this YouTube.

  7. The Muon g-2 experiment and the LHC, Beauty Quark decay anomaly are indisputable proof of the Halflec Model.

  8. SadScienceMan | July 13, 2021 at 8:27 am | Reply

    No meaningful input here, just wanted to mention that exchanges like the one between Rod and Torbjorn above are the source of my depression. My hope for humanity is lower than it was before I got here, and it was already pretty bottomed out. That’s all. Oh, well, while I’m at it I might as well insist that this article is totally wrong and that anyone who opposes my viewpoint is clearly an uneducated fool! Everyone knows the entire universe is just a mirror inside a wormhole being held in place by all of the religious figures of human history. DUH! You fools! Go back to Yahoo High School or whatever.

    • They would have to go back to conventional classes. Real science can not be leaned online. Only a fool would believe that millions of people spend billions of dollars to get from educational institutions, what they could just as easily get online. How idiotic and naive.

  9. Fantastic news if true. Keep me informed.

  10. I just wonder why they used a stack of steel rods as a thumbnail

  11. Wait!!what!!huh!!now I know what that hot Kelly c felt like talking to Sheldon on the big bang theory oh sorry were you dweebs trying to be serious its not muons

  12. Virginia L. Tyree | July 18, 2021 at 8:51 am | Reply

    7 18 21 Hello SadScienceMan, Appreciate your wit. Somehow I don’t observe myself to be a fool; low insight at times. I have no intention in returning to Yahoo High. Thank you, very kindly, for the suggestion though. Maybe, you may wish to conside, being in the present moment to help with that low mood/depression, maybe… In the meantime, stay safe, keep calm, & be well. v

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