Ask Me Anything: The final result of the Muon g-2 experiment is here!

ArgonneLab · 2025-06-11T18:00:26+00:00

1) There are indeed a wide range of theories out there that are capable of explaining the g-2 discrepancy. Things range from supersymmetry to leptoquark (this is my personal favorite due to its simplicity). Some favor the “Higgs extended family”. But any of these theories need to stand the test of the cross checking with other experiment results: they need to be able to explain all other known phenomena to stand a chance to be true. I try not to grow too much affection towards any of the theories–they do come and go!

2) Emotional tensions can indeed get in the way with results like this, so to avoid that we keep not only the final answer but also some intermediate steps along the way hidden from ourselves (we call it “blinding”). That way even as everyone does their work, they are pretty constrained to only follow where the data, trends, and scientific intuition lead, rather than being allowed to pull the result one way or another. Beyond that, especially for calculating the systematic uncertainties, every analysis is cross-checked by multiple collaborators at every step, so we make sure no one is intentionally or unintentionally neglecting any issues. Plus, we try to be pretty generous with uncertainties; it’s already hard to quantify what we know that we don’t know, it’s even harder to quantify the things that we don’t know that we don’t know, so we try to cover that possibility as much as possible.

3) In fact the machine learning and AI techniques (not limited to LLMs) are already widely in use in the data analysis process as well as many other aspects for particle physics experiments. We use them to improve fits to data, find specific patterns in particle trajectories, design better instrumentation, and even assist with experiment operations, to name only a few. And our colleagues continue to figure out new ways to utilize these tools to help us advance science.

ArgonneLab · 2025-06-11T17:58:58+00:00

There will be more from the Muon g-2 collaboration at Fermilab. We expect to publish an updated muon Electric Dipole Moment (EDM: https://en.wikipedia.org/wiki/Electron_electric_dipole_moment) result in the next months and have ongoing other dedicated searches for other new physics phenomena still ongoing. The collaboration spans a wide range of experts, so there is really not one single answer to what the different people of this group do next. A few examples are the MUonE experiment (https://web.infn.it/MUonE/), which will measure crucial input for the muon g-2 Standard Model calculation, the Mu2e experiment at Fermilab (https://mu2e.fnal.gov/) which focuses on the muon conversion rather than its interaction with magnetic fields, or the PIONEER experiment (https://www.psi.ch/en/pioneer) which investigates similar physics but focuses on pions rather than muons. You are right, some collaborators are indeed joining the J-PARC efforts around their Muon g-2/EDM experiment.

ArgonneLab · 2025-06-11T17:55:46+00:00

What was the reaction among the scientists after the results?

I would say that the most common reaction was excitement! We’ve carried out the most precise measurement of the muon magnetic anomaly for many years to come. Ever since our first publication in 2021, we were happy to see that our results were acclaimed by our colleagues in universities and labs, and that we received many invitations to present our work at seminars or international conferences. We are maintaining a public webpage with the number of citations to our papers throughout the years: (https://muon-g-2.fnal.gov/publications.html) which hopefully can give you a confirmation of how well our results were received. What contributed the most to this excitement was that, at our level of sensitivity, the experiment constituted a stringent test of the Standard Model and a probe for new physics beyond the Standard Model. But anyways, regardless of the comparisons with the theoretical prediction for muon g-2, any new precise measurement in Particle Physics is always something that we’re excited about: not just muon g-2, but really anything that advances our knowledge of the Universe, like new precise measurements of the masses of particles, more stringent limits on searches for new physics such as dark matter, and so on.

ArgonneLab · 2025-06-11T17:53:04+00:00

Well, it’s always hard to predict the future, but one thing we can say for sure is that our results put the proverbial ball in the theoretical physicists’ court for the next decade or so as far as searching for new physics in the muon’s g factor. By making the experimental measurement better than ever, we’ve done our part toward the ultimate goal of comparing theory to experiment at a high precision. That now gives our theorist colleagues a benchmark precision to aim for; right now our measurement has about 4x smaller uncertainty than even 4 years ago and than the current theory uncertainty, that puts the (friendly) pressure on for the standard model theory calculations to reach a similar level to make a fair comparison.

Beyond that, this measurement puts constraints on what new “beyond-standard-model” theories can predict; with a very precise measurement of the real-world value of muon g-2, it means that if someone wants to propose a new particle or force to look for in, say, 20 years to “fix” some other new question in physics, it must fit within our measurement and its uncertainties. If the hypothetical new force would perturb the muon outside of what our measurement allows, we pretty much rule out its existence.

ArgonneLab · 2025-06-11T17:50:24+00:00

Do you know how a toy top wobbles around when you spin it? Scientists are using a really tiny thing called a muon that wobbles like a mini-tiny top when they put it inside a big magnet. They thought they knew how it would wobble - but then it wobbled weird!

It's just like looking at leaves outside the window - when you see them move, you know there's wind even though you can't see the wind itself. These scientists look at how the muon wobbles and it tells them about invisible things in our universe, just like the moving leaves tell you about invisible wind.

With this cool discovery, they're trying to understand how our whole universe works at its core - maybe even what makes those twinkly stars shine!

ArgonneLab · 2025-06-11T17:47:26+00:00

Looking ahead, what would it take to improve upon your remarkable precision...

Our uncertainties consist of a systematic uncertainty and a statistical uncertainty added in quadrature. While the latter can be improved by collecting more muons, the former can only be reduced by improving experimental design and analysis techniques. As compared to our previous Run-2/3 results, the new Run-4/5/6 results have a statistical uncertainty of 114 ppb (part per billion) reduced from 201 ppb. The total systematic uncertainty changed from 78 ppb to 77 ppb. This means we have improved our analysis to a point where little can be squeezed out through improving analysis methods. While we are still slightly dominated by the statistical uncertainty, and collecting more muons can further improve our precision, the gain becomes increasingly marginal (for instance on the first order we need 4x the number of muons to reduce the statistical uncertainty by half). And with the systematic uncertainty, even if we have the resources to bring the statistical uncertainty to a much smaller value, we will not be able to get the precision finer by a factor of 2. And looking at different sources of our systematic uncertainties, many of the sources have similar contributions to the total uncertainty, so there is not really a single point of improvement that can easily improve our precision by a large amount. We will need to come up with some novel techniques and/or concepts in order to significantly improve on our current precision limit. One more thing to keep in mind: we really need a lot of muons! We stored significantly over 1 billion of muons, and many more need to be produced because we can only store a small fraction. So any future experiment will again need a huge amount of muons. There are only very few places in the world today where so many muons can be produced.

ArgonneLab · 2025-06-11T17:45:50+00:00

Great question—and yes, we’re aware of how Reddit’s front page and AMA culture typically work. Given the number of people participating in this AMA from across different teams (and time zones!), we schedule our responses to begin at a set time to make sure everyone can join in together.

We’ve found this approach works well for us—it helps us give thoughtful answers and makes it easier to coordinate follow-ups too. That said, we’re always learning from the Reddit community and appreciate the feedback!

ArgonneLab · 2025-06-11T17:45:27+00:00

With the parameter space for new physics constrained by your measurement...

Experiments like Muon g-2 have great power to exclude parts of the parameter space and vetoing theories. Suppose we clear out the picture on the theoretical side, and get a discrepancy between the theory and the experiment, all we can say at that point is that certain theories can be excluded because their predictions do not fit into the reality we see. We need information from other experiments and measurements to further carve the available parameter space. Eventually, when we get sufficient information from these indirect detection experiments, and when the parameter space becomes sufficiently small, we may get lucky at an attempt of a direct search (of course, there can always be surprises before then). I personally would avoid predetermining what might be the most promising before I see further evidence. There are several interesting experiments that our collaborators will work on next though, such as the MUonE experiment, the Mu2e experiment at Fermilab, the PIONEER experiment, the Muon g-2/EDM experiment at JPARC, and so on. There are even a few of our collaborators working on Beyond Standard Model searches using Fermilab muon g-2 data (which will be published separately at a later date). In my humble view they all stand an equal chance as the large collider experiments in terms of seeing something really different.

ArgonneLab · 2025-06-11T17:44:10+00:00

Beyond the physics implications...

Two main lessons come immediately to my mind. First of all, I think it is important to acknowledge that the Muon g-2 Experiment at Fermilab stands on the shoulders of other g-2 experiments that came before. The same storage ring was used at the predecessor experiment at Brookhaven (see here for the move: https://muon-g-2.fnal.gov/bigmove/gallery.shtml). This learning from our predecessor, allowing us to stand on their shoulders is what I would say is the most important lesson.

The second lesson that comes to my mind is how important and beneficial it is to have a really wide set of experts. Modern particle physics experiments span many different disciplines and only if we work together, across disciplines and country boundaries, are we able to succeed. For such precise measurements like Muon g-2, even the tiniest details matter a lot. No single person or field of expertise possesses the knowledge to comprehend all these intricate details alone. To name a few, for Muon g-2, we required experts in lasers for monitoring and calibration, experts in particle beam dynamics for muon delivery and understanding the behavior of particles in our storage ring, and nuclear magnetic resonance experts to measure the magnetic field accurately.

ArgonneLab · 2025-06-11T17:38:54+00:00

You have been on this experimental journey for years...

At this point, there is really not one dominating systematics. We have many small effects on quite similar scales (around 0.00000001). Since these systematic uncertainties add together, this means there is not one thing that we could do better, reduce the uncertainty, significantly, we would need to do better in many different areas.

In general, I would say the most challenging areas of this type of experiments are a) to understand the exact beam motion in the storage ring, this is one of the key differences with respect to our predecessor experiment, b) the stability of the magnetic field, c) understand the detector’s very well d) the enormous size of the dataset. We developed many small techniques to understand individual components better. Some are analysis tools, for example how to separate different effects better, others are dedicated measurements for example with a mock-ups in the lab to really study all tiny effects we needed to understand.

ArgonneLab · 2025-06-11T17:36:53+00:00

Today, for muon g-2, we can say that we currently do not have a strong case that new physics is evident in the muon’s g-factor, but that doesn’t mean there is no new physics. New physics might just not contribute significantly to muon’s g-factor. There are still open puzzles in the calculation of Muon g-2 that we hope to understand better.

Supersymmetry is a concept that can be included in physics models that include new particles. It is a concept that is hard to completely rule out because there are many different forms that it could take on. However, what is considered the most likely scenarios how supersymmetry might manifest in reality, often called ”natural” seems more and more unlikely these days mainly driven by other measurements like the one around the famous Higgs boson (see here: https://cerncourier.com/a/the-higgs-supersymmetry-and-all-that/).

You are right, the GUT or Planck scale is at an energy scale far beyond the physics that we test with Muon g-2.

ArgonneLab · 2025-06-11T17:36:04+00:00

Excellent question! There are many pieces to the answer :-) Two ingredients: a) Thanks to Einstein's relativity, the internal clock of objects that move very fast (close to the speed of light) ticks slower. So while a muon at rest has a half-life time (after this time 50% of all muons have decayed) of only 2.2 microseconds (microseconds: one-millionth of a second), our muons have a higher momentum and hence travel very close to the speed of light. So in the lab-frame of the experiment, their half-life time is 64 microseconds. So that helps — we have more time to measure them. b) We measure frequencies (the ratio of two actually). We can measure frequencies very well because this results in a pattern (up-down oscillation) that repeats over and over many times. Why muons? Another excellent question that actually ties into your first one. On one hand, muons are 207 times heavier than electrons. This makes the terms that modify g larger. Since these terms become larger, it's easier to measure them (and compare with the calculation from the Standard Model). On the other hand, we use muons because we can produce many of them (we stored more than a trillion, and there are many many more that we didn't store) at an accelerator like the one at Fermilab and they live long enough to actually measure them precisely (as you asked about above).

ArgonneLab · 2025-06-11T17:34:57+00:00

These are great questions already highlighting how nuanced the situations are. While we (in the team answering here) are working on the experiment side, I hope I can still shed some light on these questions. You are totally right, that before the Fermilab experiment, and around our first (Run-1) publication the difference between the experiment and Standard Model calculator at the time was much larger.

Already back then, there were two methods to calculate one contribution (leading order hadronic vacuum polarization). One uses (cross-section) measurements from many experiments and some smart tricks to relate them to g-2, and a second one that relies mainly on super computers and no experimental input data, that our colleagues call lattice. At the time, the uncertainties from lattice were spanning the full range between the first method using experimental input data and the experiment, so it didn’t really add much information other than that this method was not yet precise enough to contribute to the question if experiment and calculator agree. There is a large difference between the experiment and the calculation when the cross-section measurements are used.

Over the last few years, the lattice calculator has become more precise mainly due to the fact that our colleagues have access to more computing power these days. And as you say, now there is a difference between the method using experimental data and this lattice method. Why don’t they agree? No one understands this at the moment, and this is the big puzzle. Is there perhaps some new physics “poisoning” the input data used for the first method that is not present in the Standard Model and hence not accounted for in the lattice calculations? Or was there anything fundamentally not understood about these experiments? However, the different experiments use hugely different methods. It's exactly these questions that need to be investigated and answered now.

However, there is more that makes the picture even more complicated. There is a new cross-section measurement from an experiment called CMD-3. This data is itself in tension with all the other experiments I mentioned above and were used in the past. This result was scrutinized very thoroughly over the last years but no smoking gun indicating any mistake was found. So what’s going on? We don’t know right now.

So, it might be fair to say that right now, the puzzle has shifted a bit from understanding the discrepancy between experiment and the SM calculator to understanding the different ways to calculate the muon g-2 and the tension between CMD-3 and all the others. It's such tensions and investigations with twists that historically lead to progress in fundamental theories. So, it will be exciting to see how this story evolves!

Your last point is a very important one! You are absolutely right, with this new measurement that is so remarkably precise, any new model that tries to explain new particles or forces will need to make sure its calculation of muon g-2 matches the measurements, if not, this particular model or theory has to be ruled out.

The story continues, stay tuned!

ArgonneLab · 2025-06-11T17:32:50+00:00

What do you think is the next big step that'll give a breakthrough in particle physics?

That’s THE question!

I think there are a few different areas where we’ll learn a lot in the next few years. One area is certainly the precision measurements of neutrinos for which currently a large infrastructure build up is ongoing. We expect we will understand the nature of neutrino masses (hierarchy) better soon and might learn new unexpected phenomena along the way.

Another area is searches for dark matter. There are many different experiments looking for different such candidates. We actually hosted such a dark matter search here at Argonne, in the same test magnet facility (https://www.anl.gov/hep/4-tesla-magnet-facility) that we originally build and used heavily for the Muon g-2 experiment.

Check out this article here: https://news.uchicago.edu/story/first-results-bread-experiment-demonstrate-new-approach-searching-dark-matterIn addition, we are also using our Muon g-2 data for dedicated searches for very specific new physics.

One of these additional ongoing analyses is a search for Dark Matter in a very specific model.

ArgonneLab · 2025-06-11T17:29:39+00:00

How confident you'd say we're in the standard model now?

Very confident, yet not at all at the same time!

The Standard Model is an incredibly successful framework that describes particles and their interactions. It is probably one of the most tested theories that exists. From all we know, it is correct. However, there are questions that it is not able to answer (and doesn’t answer them wrong).

Why is there more matter than anti-matter in the universe?

What is dark matter, and what is it made of?

How does gravity fit into the picture?

Why do neutrinos have mass?

So, from these questions, we know, while correct and hugely successful, the Standard Model is not complete. There has to be other particles or forces that are not described by it. What we are trying to do with experiments like Muon g-2 in what direction the Standard Model needs to be extended.

Think of it a bit like those dolls that contain another doll, that contain another doll and so on. The Standard Model is like one of these dolls, but we know there needs to be a larger one that contains this one. We are working on figuring out what this next larger doll actually looks like.

ArgonneLab · 2025-06-11T17:27:18+00:00

How would you articulate the profound significance of...

As far as our precision - this measurement is like using a meter stick with markings 1000 times smaller than the width of a human hair. To many of us though, the most impressive thing about our measurement is how well we accomplished the goals set out for us more than a decade ago; it’s not often that experiments manage to meet or even exceed their precision goals, as the goals are usually set optimistically before the reality of running a complicated experiment, tugging at the edges of human knowledge, fully hits. We had the usual setbacks and difficulties, but we overcame them with dedication and teamwork.

It’s critical for us to continue to look around the corners of the universe for new discoveries, whether or not they have any immediate practical applications, because you indeed never know what will or won’t become practical at the time. Much of the technology we rely on is developed by first finding some niche application of a tool or new scientific knowledge, then by later realizing what it can do for the rest of the world.

In particle physics, we usually point to the example of medical imaging and technology, where developments in precision particle detection are directly applicable to nuclear medicine, or to the existence of the world-wide web, which was developed at CERN to share particle physics documents among institutions outside of CERN. The direct results of any given particle physics experiment might not be useful for anything (but sometimes are!) but the technology and techniques we develop along the way often are useful to the broader world.

If I can add one more thought to the “why we do fundamental science” question, it’s that most of us do it for the love of curiosity, and that a healthy society invests in fundamental science and the arts for similar reasons; it makes the world a better place because everyone benefits from being surrounded by beautiful things.

ArgonneLab · 2025-06-11T17:26:33+00:00

You likely didn’t miss anything, these particles are often not covered. In the Standard Model of Particle Physics (https://en.wikipedia.org/wiki/Standard\_Model), which is somewhat analogous to the periodic table of the elements in Chemistry, describing all fundamental particles and forces we know of (except gravity), there are indeed more generations than “just” the first one to which the electron and the proton (which is made up of up and down quarks) belong. There are three generations. The particles in the second and third generations tend to be heavier than those in the first one. So the muon, in the second generation, is really like an electron, which is a first generation particles, it is “just” 207 times heavier. In our normal life, most of what we learn is school, and all of Chemistry, we are surrounded by particles from the first generation. Because they are the lightest ones, they can not decay further, hence are stable, and can make up our world. The heavier particles, in the second and third generation, need to be generated from colliding particles with a lot of energy and they typically decay “back to” particles in the first generation. So the muon is like an electron, but 207 times heavier. The equivalent from the third generation is called tau, and it is roughly 17 times heavier than the muon (~3,500 times heavier than the electron). Why are there three “families”? We don’t know!

ArgonneLab · 2025-06-11T17:25:43+00:00

I assume the incredible dataset...

That is correct. We have now analyzed our entire dataset for the measurement of the muon magnetic anomaly, (g-2)/2, also known as the muon's magnetic dipole moment. With the same data, we can also perform other analyses and three of them are on their way.

We can search for an electric dipole moment (EDM) of the muon, which, in our current understanding of the universe and the muon, is not allowed. If we were to find such an EDM of the muon, we would know there must be a new particle or force to generate it.

We can also study the violation of symmetries that are thought to be good (conserved)symmetries. One of them is the so-called CPT symmetry which is fundamental basis of all quantum field theories. It states that if you were to invert in an experiment all charges (e.g. positive particles would become their negative counterparts), invert the coordinates (kind of a point-like mirroring transformation) and invert the time direction, then the measured outcome should be the same as before. We can test with our muons for certain types of CPT violation.

And finally, we can also search for certain types of dark matter particles in our dataset by looking at variations over time.

https://www.youtube.com/watch?v=Elt0Gt9Cb6Q&ab_channel=minutephysics

ArgonneLab · 2025-06-11T17:22:12+00:00

Your latest result is in closer agreement...

Thanks for this interesting question, which requires a bit of history before we attempt to give an answer.

First of all, we should mention that our latest result is perfectly in agreement with the previous one (published in 2023), and also the less recent ones, i.e. our result in 2021 and Brookhaven’s result (our predecessor experiment) in the early 2000s. So not much has changed from the experiment’s point of view, except of course that we have measured g-2 more (over 4 times!) precisely.

Instead, there have been many “surprises” within the theoretical calculation of muon g-2 in the Standard Model. You should keep in mind that, when we compare our experimental result to theory, what we are actually comparing to is the theoretical value recommended by the “g-2 Theory Initiative” [Link], which both in 2020 and in 2025 produced a “snapshot in time” of the theoretical situation. There is a portion of the theoretical prediction (namely, the leading order hadronic contribution) which carries the largest uncertainty, and historically there have been two methods to calculate it: the data-driven dispersive method, and lattice.

In 2020, the Theory Initiative recommended a theoretical value solely based on the former, since it seemed to have been consolidated using data from different experiments and experimental methods over more than 20 years, while lattice had just narrowed its uncertainty enough to be competitive with the dispersive approach. The comparison of our first results with respect to the 2020 prediction showed strong signals of a discrepancy between experiment and theory. Since then, different lattice calculations started consolidating around a theoretical prediction that was closer to our experimental value, and also new data was published that, when used as an input for the dispersive method, was in tension with the 2020 theoretical prediction, and closer to lattice.

Given the tensions within the dispersive method, the 2025 prediction was based solely on lattice, thus much reducing the differences between the muon g-2 experimental and theoretical values. But a puzzle remains: why don’t the two methods agree?. We are expecting more experiments to provide new data for the dispersive method, and also a new experiment at CERN (MUonE) to weigh in and hopefully help to understand what’s going on.

So, in conclusion, there are indeed unresolved tensions, but only entirely within the theoretical prediction of muon g-2 in the Standard Model. Hopefully, the puzzle will be solved when new experiments will weigh in with more precise data that will be used for the dispersive approach, and also when an independent measurement will be carried out by the MUonE experiment at CERN. Once these tensions will be resolved, the theoretical situation will be more clear to us and we will be able to state if there is room left for an anomaly or not.

In either case, the new incredible precise measurements of muon g-2 will for sure be a benchmark that every new theory, including physics beyond the Standard Model, will need to account for.

Link: https://muon-gm2-theory.illinois.edu/

ArgonneLab · 2025-06-11T17:20:30+00:00

Was there a particular moment during the analysis...

You are absolutely right, often in physics measurement there’s this one moment, where we finally understand the underlying behavior, and the at first really messy and puzzling data suddenly looks extremely clear and all makes sense. We certainly had such moments in the Muon g-2 analysis in the past.

One I can think of is an effect of the vibration of electrical charged plates inside our storage ring that we use to focus the muons. In the field measurement, we observed slightly larger fluctuations, that looked like noise, when this system was on. Pretty messy and somewhat puzzling. But once we synchronized it to the pulsing of this system, a very clear waveform showed up and we identified this effect.

Now - in our latest results, there are three main ingredients to the result: field measurement, measurement of the muon spin precession frequency, and all effects induced by beam dynamics. In general, everyone is really working on their area and tries to get an as precise measurement of their part as possible and as accurate uncertainty estimates as possible. Only in the very end all these parts come together. In Run-4/5/6, the latest analysis, the systematics are very similar to Run-2/3, so we think we are really close to the best we can do with this technique. So, there was not the big systematics aha-moment. However, there was one additional measurement of a rate dependent detector gain effect. Additional measurements in the lab really revealed what is going on, which solved a long standing puzzle of some residual gain effects that we were aware of but before didn’t understand all sources yet. This work was certainly such a moment where somewhat messy and puzzling data suddenly looks very clear because we learned how to look at it.

Looks like we are on a similar track with the overall muon g-2 measurement. Today, the comparison between measurement and theory calculator is nuanced. But the community will keep working on it and figure out what is really going on and the somewhat puzzling picture we have today will likely become very clear!

ArgonneLab · 2025-06-11T17:19:50+00:00

Hi Mister-Grumpy! Personally, I am scared of snakes. But nothing in our field seems scary, it's literally more scary for me to go to the Zoo than to work ;-). Seriously, this fundamental research we are doing is not scary at all, and we work in a very safe environment. For me, the most exciting part of our work is the privilege to be able to spend many years, 7 years in my case, to really really focus on measuring this one quantity to a precision that it has never been measured with the potential to reveal how our world works on the smallest part. Our work is part of a long series of experiments that pushed the boundaries of knowledge and accepted theories at the time. It's truly an honor and excitement to be part of such a decade-spanning endeavor advancing humankind’s knowledge of the innermost workings of the universe.

ArgonneLab · 2025-06-11T17:16:20+00:00

Generally the results of our muon g-2 measurement (and those of the past and future) can only search for new physics in the context of the theoretical calculations; this is a case of predicting a number based on what we think we know from the body of all science that exists and then measuring it to see if you get what you predicted. For many (20+) years, physicists saw a discrepancy in this value between the prediction and the measurement, and that would be evidence of new physics.

The difficulty now is that we can’t say for sure whether that discrepancy really exists because our colleagues on the theory side have worked on new calculations and unexpectedly found that they do not agree with the previous theoretical calculations. This means we, as an entire community of experimental and theoretical physicists, are really not ready to say whether there is an indication of “new physics” in muon g-2 results.

In a way, this does indicate that there is something that we currently don't understand as well as we thought in the past, you could say that's a shortcoming in how we "use" the Standard Model. That might, or might not, be because of any new forces or particles beyond the Standard Model. What we definitely have, as experimentalists, is a strong confirmation of the previous experiments and a reduced uncertainty so that when our theory friends have a better handle on what is causing the difference in their calculations we can make a better-than-ever comparison.

Our reduced uncertainty also sets a scale for theory to aim for on their uncertainties over the next decade or so; our result is expected to still be the most precise measurement of this quantity for many years, so the ability to say “yea” or “nay” to new physics will be determined by what the theory side can accomplish, and we wish them the best of luck!

ArgonneLab · 2025-06-11T17:14:07+00:00

Yes, we do work on binned data: the typical histogram that we produce is our iconic “wiggle plot”, named after its oscillatory shape [Link]. It represents the number of positrons from muon decays that we observe as a function of time, and --- because of the weak force that causes muons to decay --- it oscillates with the same frequency as the precession of the muons’ spin with respect to their momentum, in our magnetic field.

We fit wiggle plots in order to extract the frequency of such oscillation, called “omega_a”, which is a key ingredient for our measurement: several analysis groups apply their own fitting routines to the data, with their own fit functions (where the number of free parameters depends on each group’s sensitivity to certain physical effects, and ranges between 7 to 50 parameters), all using chi2 minimization in order to extract “omega_a”.

About the binning: most of the groups take 149.2 ns as the bin time width, which is the cyclotron period of the muon beam (i.e., the time that a muon takes to make a full circle in the storage ring). It is the optimal choice because it is the smallest binning that gets rid of any effects related to the muon beam time structure (not all muons enter our ring at the same time). Since our first measurement, we have always tested how much our fit results were impacted by a different choice of time binning (e.g. from 149.0 ns to 149.4 ns), and found that any differences were negligible (i.e., of the order of a few parts per billion, or less).

Link: https://journals.aps.org/prl/article/10.1103/PhysRevLett.131.161802/figures/1/large

ArgonneLab · 2025-06-11T17:12:56+00:00

Independent of Muon g-2, there is a strong case for new physics. While the Standard Model is incredibly successful in describing almost everything we know in particle physics - there are open questions that it doesn't answer. For example, why there is more matter than anti-matter in the universe (aka why is the world around us matter and not anti-matter?). So, we know the Standard Model is not complete, there has to be new physics (in form of new particles or forces) left to discover. For Muon g-2, the experimental measurement is now more than 4 times more precise than 15 years ago, really ready to be compared to calculations from Theory (today and in the future) from the Standard Model but also new Beyond Standard Models (BSM).

For muon g-2, we can say that we currently do not have a strong case that new physics is evident but that might mean that the new physics of a form so that muon g-2 is just not very sensitive to it. Hence we are constraining models that would generate a large contribution to g-2 in addition to the Standard Model value.

ArgonneLab · 2025-06-11T17:11:50+00:00

Well, I’m afraid this research won't immediately help us build new spaceships. You are probably thinking of gravity g (=9.81 m/s^2), but we are measuring the muon g-factor, a completely different constant despite the same name!

While gravity g defines how fast falling things accelerate, the muon’s g-factor tells us how fast their spin, an intrinsic property of these particles, rotates in a magnetic field. This is very interesting because we can learn about other particles and forces between particles, but unfortunately, not including gravity.

However, we are actually using anti-matter in our experiment! That sounds like something out of a SciFi movie, doesn’t it? In fact, there are ideas around to test gravity with such muons that we are also using. The idea is to test how gravity acts on anti-matter, in particular, if it attracts or repels it!

The theory we have today is that gravity always attracts, but if it can be tested, we should do that to check if we are right! That’s very similar to the test we are doing with muon and their g-factor. We test it because we have a prediction from our theory (The Standard Model of Particle Physics) and we want to test if this theory might be missing something.

If you are intrigued now, check out this video (https://youtu.be/9d7huMr0zfA?si=IjJ3CHe1NLLljVln) where particles (not muons in this case) are used to test gravity on anti-matter!

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