Ask Us Anything about the new Muon g-2 results!

Thank you all for the phenomenal work that you do. I have a few questions for you as someone without an extensive background in physics.

1) Is it right to interpret these results as further evidence that there are unknown particles or forces not currently accounted for in the Standard Model?

2) If that’s the case, what is your best guess as to possible new particles/forces that could explain these results?

3) With all this new info, where do you see particle physics heading in the next couple of decades?

Thanks again for pushing the boundaries of science and our understanding of the universe!

1. In a nutshell, the difference between the measurements vs. the theoretical calculations indicates a patch we need to add to our current theory. While we are constantly improving our measurements, the theorists are also improving their calculations. While the difference between the recently released result and the theoretical prediction released in 2020 reached 5 sigma (which means extremely unlikely the difference is caused by a statistical error), some of the newer developments in the theoretical calculations yielded values closer to our measurements. As of now, we cannot speak for sure whether there are new particles.
2. A difference only indicates incompleteness in our current Standard Model, and shaves away some new theories that have very different predictions, but it does not speak for sure which one it is. We need to do further experiments to test different theories.
3. As for the future of particle physics, there are a wide range of experiments on different frontiers that help us understand more details. The muon g-2 experiment, as an intensity-frontier experiment, uses high precision measurements to seek discrepancy between reality and theory. You can find more information on intensity frontier experiments at https://www.fnal.gov/pub/science/experiments/intensity/experiments.html. Other two major frontiers are the energy frontier and the cosmic frontier. Feel free to read more at https://www.fnal.gov/pub/science/experiments/energy/ and https://www.fnal.gov/pub/science/experiments/cosmic/.

Total layman, mildly off-topic question: Obviously we're getting better at detecting muons. Because they're everywhere, do you see a future where we can "miniaturize" muon detectors to get better passive "x-rays" of things?

In fact, it’s possible to detect muons with quite small detectors, even battery powered, and muons have been used to image inside structures and even reconstruct the shape of a mountain from inside a tunnel.

This is called muon tomography, here’s a wikipedia entry: https://en.wikipedia.org/wiki/Muon\_tomography and a paper about using cosmic ray muons for the shape of a mountain (Matt Bressler’s own research from years ago!): https://pubs.aip.org/aapt/ajp/article/85/11/840/1057935

If this result is the product of an actual new force and new gauge bosons, rather than a problem inherent in our predictions using the standard model, is there any idea what the nature of this force would be in order to explain such results? Since the weak force mediates flavor changes and decays, the strong force bind quarks together, etc, what is is that this new force would be responsible for doing? If that makes any sense

Excellent question. The quantity we measure, the anomalous magnetic moment of the muon, receives contributions from all of physics, including physics within the Standard Model (SM) and “new physics” beyond the SM.

If this result is confirmed to be in disagreement with the SM, then the difference between our experimental result and the SM prediction would represent the size of contribution from new physics, and would constitute an “indirect” measurement/discovery of physics beyond the SM.

That is, it would amount to strong evidence that new physics exists, without telling us its precise nature (whether it be a new force, a new particle, or something else entirely).

Unraveling the mystery of what this new physics actually is would be the work of theoretical physicists and future experiments, who would use this result as a powerful indicator as to where to look!

Congratulations to the 25 graduate students earning their doctorate from this data set! I'm so impressed by the level of control you have over this system, what an extraordinary accomplishment.

Are there any indications that the charged leptons have quadrupole or higher multipole moments? What kind of cursed, diabolical experiment would allow one to test this?

The magnetic moment, or magnetic dipole moment, results from the particles intrinsic property of its spin (and electric charge). The g-factor links the spin of a particle to its magnetic (dipole) moment.

What we are measuring is this g-factor, or more precisely, the anomalous component of it which is the deviation from 2. Hence the name g-2 of our experiment.

There are no magnetic monopoles, so the dipole is the smallest “magnetic unit”. So for fundamental particles, with only one spin, there is only one degree of freedom.

But for atoms that consist of multiple fundamental particles and hence multiple spins, for example, there can be higher order multipoles. As an example, a magnetic quadrupole moment leads to some shifts in the hyper fine structure (see here: https://en.wikipedia.org/wiki/Hyperfine\_structure).

You can find a related example of an experiment at Argonne, but for an *electric* dipole moment search, in Radium that exploits the octupole deformation of this atom:

https://www.anl.gov/phy/electric-dipole-moments-of-radium225

Can you describe your work in the most boring way possible?

Muons are one kind of fundamental particle. It’s like an electron but heavier.

Like electrons, the muons have a quantity called the spin, you can picture it like a little magnet within the particle. We measure how this spin interacts with a magnetic field.

More precisely we are looking at how fast this little magnet rotates in the magnetic field, similar to spinning top. All particles we know in the theory contribute to this interaction / rotation. With this, measuring it allows us to test this theory of all particles, called the Standard Model.

Check out this video for more details: https://www.youtube.com/watch?v=ZjnK5exNhZ0

The latest result comes from 2 years of measurements, for more details on that check out this video: https://www.youtube.com/watch?v=hkHd\_wxMfrs&ab\_channel=Fermilab

If theoretically we moved the experiment and accomplished it in outer space or had a facility like Fermilab or LHC on the moon, how would the results be impacted?

This is a fun question! There would be several pros and cons to working on the moon.

1. Getting the magnetic storage ring to the moon would be difficult. The storage ring is 14 meters across, and it needs to create magnetic fields with incredible precision and stability. A few years ago, we moved the storage ring from its original location at Brookhaven National Laboratory in New York all the way to Fermilab in Chicago, in an incredible journey over land and sea! Taking such a massive and fragile machine all that distance without it breaking was a major achievement. After that, we very well might be able to take it to the moon!

2. The magnetic storage ring needs to be pumped down to vacuum while the g-2 experiment is running, to make sure that the muons don’t crash into air molecules while they travel. It takes a lot of energy to create and maintain the vacuum on Earth. Since the moon has no air, it would be much easier to keep the ring at vacuum there.

3. Cosmic rays on the moon would be a major problem. On Earth, the planet’s magnetic field and atmosphere deflect or obstruct many particles hitting the planet from outer space. Without that protection, cosmic rays are much more intense on the moon. The cosmic ray particles can interfere with the particle detectors that we need to measure our muons, so we’d need to build new devices to shield them.

Additionally, we would not expect the result itself to change. A central assumption of Einstein’s theory of relativity is that physics remains the same no matter where you are in the universe or how fast you are moving. So, muons on the moon are governed by the same underlying physics as they are here on Earth.

It sounds like a lot of news articles have been attached to the idea that this experiment shows strong evidence for a possible fifth fundamental force. Is this the result that this experiment shows?

Not exactly - while the purpose of the experiment is to ‘look’ for new physics (which could be new forces) by searching for a discrepancy between the theoretical prediction for how muons should behave and how we’ve measured them to behave, currently the theoretical side of the puzzle is not clear.

For something to count as evidence for something new, the distance between theory and experiment has to be above a ‘5 sigma’ threshold (essentially, a difference of 5x the error bars on the points), but it’s not clear at the moment what we should be comparing to! There is a nice summary by the g-2 theory initiative here: The Status of Muon $g-2$ Theory in the Standard Model | Muon g-2 Theory (illinois.edu) . The good news is that the theory disagreements may also hint at new physics, so either way, we learn something new!

With such high precision in this experiment, what are some ways you control for systematic error or random fluctuations in temperature and magnetic field and other parameters?

Great question, those are exactly some of the systematic errors we need to worry about to get to this level of precision.

The temperature control is actually something that was improved between our 2021 result and this result – if you notice in the old photos of the ring magnet it appears as a blue circle, now in the newer photos you will see that the magnet is covered with a white blanket.

That reduces the effect of temperature variations on the magnetic field (temperature variation -> thermal contraction and expansion -> magnetic field variation). The magnetic field itself is continuously monitored and the value is used in a feedback loop which maintains a stable average field around the ring.

Even after the direct control of the field, we use the actual measured values of the field in the end result, so the uncertainty is dominated in the end by how well we can track the real value.

Welcome to the fun! We'll begin at 1:00 pm Central.

We've got a great team here looking forward to all of your wonderful questions!

If the experiment does point to a 5th fundamental force, what are the implications for wider physics?

Because we can see the other 4 forces so easily in comparison, what do you think this 5th force is actually doing?

Also, if there is confirmation of a 5th force, does it open the door to there being many other hard-to-detect forces? Or do you think this force will mainly be contained to subatomic particles and it's then unlikely we'll find more?

Thanks for the interesting topic!

Please also see our answer to your similar question that our experiment is only sensitive to new physics but not really to its nature.

Should we really discover new physics in the future (since we did not yet!), we would need follow up experiments that will tell us more about if this new physics is a new particle or an additional new force. Therefore it is difficult at this point to really speculate about the nature of such new physics – should it be discovered in the future.

Given that we have not yet discovered a fifth force or any new particles, it is probably reasonable to assume that it will be more and more difficult to find new physics but scientists around the globe keep on to this challenge with ever better experiments and theoretical calculations.

As experimentalists, our focus is on measuring as precisely as we can.

What developments helped make these measurements possible?

Maybe you would break things down in a different way (or there is too much overlap to separate), but I'm thinking in terms of:

• Technology/hardware

• Software/computer models/computing power

• Other experimental results, or developments in theory

That’s a great question.

The same magnet was used in a predecessor experiment at Brookhaven and was moved to Fermilab in 2013 to Fermilab.

That was actually pretty cool, check out this video: https://www.youtube.com/watch?v=rGLpMigWIIs&ab_channel=NationalScienceFoundationNews.

One of the main differences is that at Fermilab we get much more muons per time which is one of the crucial ingredients that help us to measure more precisely than it has even been done before.

Many detector systems were upgraded and GPUs are used for the data acquisition.

In addition, it’s a lot of very dedicated work from students and postdocs digging into the smallest details allowing us to understand our detectors on the level we do that is required for this kind of precision measurement

How are muons formed or generated, both in nature and the specific ones that you test in the experiment?

The muons we create in the experiment come from a beam of protons smashing into a target in the Fermilab particle accelerator complex.

When the protons hit the target they create a bunch of subatomic debris, some of which are pions. We capture pions of a specific energy and send them along a beamline where they decay into muons, which we then inject into our storage ring.

The muons which are most prevalent in nature (at least on earth) are created through a similar process when high energy cosmic rays impact nuclei in earth's upper atmosphere.

What do these findings mean for the average person?

This result as many others in particle physics are aimed to bring us a better understanding of the universe and how it formed.

While the specific result here will not necessarily directly have implications on you or me, it helps us in understanding the beauty of the universe. Haven’t many of us wondered how this amazing universe came along?

Who hasn’t looked at the sky on a cloud free night and wondered where all the millions of stars came from and how the galaxies formed?

Understanding the universe at both the largest scales (the universe and galaxies) and the tiniest dimension (like with elementary particles) hence satisfies one of the longest outstanding curiosities of humankind.

If you could rename muons, what would you call them?

Many describe them as “chubby electrons”, “wibbly wobbly timey wimies”, “the best particle of all time” (a.k.a POAT). Although, some of us think “muon” is just fine. Tau’s are the ones we need to change.

What do you suggest we should rename them?

Did the new theory results ruin the party? I feel like they made the excitement of the 5 sigmas go away a little bit. What's left for the collaboration to do considering that the new theory results are so close to the experimental results?

This is a nice question! The situation on the theory side is currently quite interesting, with tensions among some of the various methods for calculating the muon’s anomalous magnetic moment.

That said, the newest experimental measurement from Muon g-2 only increases the interest and excitement in resolving these tensions. There is a steady flow of new calculations, and our expectation is that there will be significant improvements in theoretical accuracy for both main approaches — the ‘data driven’ approach (involving dispersion relations, for the experts in the room) and lattice-gauge theory — over the next couple of years.

In the meantime, more and more precise measurements from Muon g-2 and other experiments can motivate this theoretical work. The next few years should therefore be very interesting depending on how theory calculations resolve and in which direction — either toward or away from the g-2 measurement.

Some more reading links on this topic, a nice summary is found here: https://news.fnal.gov/2023/07/what-does-the-standard-model-predict-for-the-magnetic-moment-of-the-muon/

A somewhat more technical, recent overview summarizes some of the main areas in which theory progress is expected: https://inspirehep.net/literature/2060022

How do you control the experiment? I imagine this is beyond the scope of running LabView on a single computer...

There’s a lot that goes into the data taking behind the scenes. The main interface for us to the experiment is our data acquisition system (DAQ) MIDAS (https://en.wikipedia.org/wiki/Maximum\_Integrated\_Data\_Acquisition\_System).

The main MIDAS program runs on a single machine, but we have dedicated computers for each one of the detector readouts which pre-process data (mostly to cut out periods of time where nothing hits our detectors) and feed that back to the main machine over a network. We have a bunch of control systems baked into MIDAS which give us the ability to monitor and change the settings of experiment on the fly.

We’ve also implemented a bunch of alarms which will alert us about any potential problems with the experiment, such as if temperatures are too high or if we’ve forgotten to switch on the file writing when we’re taking data (which, of course, never happens 😅).

MIDAS’ main function, though, is to take in the data from all of our systems — fast data streams from the main detectors (calorimeters, trackers, etc.) as well as slower readouts like magnetic field readings and temperature monitors — and package it into a single data file. After all the local preprocessing is done, when we take data we write about 2GB to disk every 6 seconds. This gets backed up to Fermilabs storage system for us to analyze later.

As with most other large particle physics experiments, we generally have at least one or two people watching the data collection live as it happens, 24/7, plus experts on call for various systems in case anything isn’t working as it should be.

The people running the experiment watch a few plots which update every few seconds and by eye can immediately tell whether the data collection is running well or if something needs to be changed.

How much of an impact has AI had and will have on work like this being done at the national labs?

Excellent question!

As you may have heard, AI and machine learning have a growing role in fundamental science at the National Labs as well as universities. While this most recent result of the Muon g-2 Collaboration did not strongly depend on AI methods, they are increasingly used in both experimental and theoretical particle physics.

In the former case, new AI tools play a role in experiments at the LHC, for example, to identify novel signatures in detectors and in particle identification.

In theoretical studies — including for physics beyond the Standard Model — AI can be useful to perform complicated numerical analyses of models with many parameters.

Going forward, results like the new g-2 measurement will likely stimulate further development in this area to improve the theoretical interpretation as well as increase experimental sensitivity.

When was the last time you binge-watched a TV show and emerged bleary-eyed from your cocoon?

No but one time I accidentally set my microwave to zero power and just spun my food around for 8 minutes, I didn’t know that was even possible until I found my food still frozen

Just last weekend to celebrate the release of our result! Good Omens S2.

As experimentalists, is there an accompanying theory for the proposed fifth fundamental force, or will that have to be filled in after potential confirmation?

From the experimental side, we would need follow up measurements to confirm the exact nature of any extra forces or particles which this result may hint at.

In Muon g-2 we are basically sensitive to the presence of unknown physics, but not directly sensitive to detecting most new forces or particles themselves. We will be looking for evidence of a few very specific forces or particles in our data over the next few years, such as the potential effect of dark matter on our measurement.

From what I understand, this experiment goes back many decades but only now did your team finally get the world average measurement precise enough such that the theoretical value is no longer within the 5 sigma range.

Whose idea was it originally to investigate (what looks like from the outside to be) such an obscure property? Was there some reason to expect an interesting result lurking under the surface here?

Measurements of Muon g-2 (and ‘g’ of the muon in general) go back to the 1950’s, and were proposed initially as an early check of the predictions of Quantum Electrodynamics (QED).

It is a relatively obscure property, but it has the somewhat unique characteristic that we can both measure it and calculate it very precisely. Many things in particle physics end up being just one or the other. Absent any effects from other particles in the vacuum, we would expect ‘g’ of the muon to be exactly equal to 2.

In 1948, however, the first order QED correction to g was calculated by Julian Schwinger. Since the prediction turned out to be a (relatively) large deviation from 2, it was ‘low hanging fruit’ to show that the relatively new theory could actually make solid predictions.

A measurement was very quickly done to confirm this prediction for the electron, but it wasn’t until 1959 that a measurement was done at the Columbia-Nevis synchro-cyclotron which confirmed Schwingers prediction for the muon. It also showed that the muon was basically a heavier version of an electron, which wasn’t at all obvious at the time.

From there, measurements at CERN continued to confirm the predictions of QED (and then, as the measurements got more and more precise, hadronic calculations as well) to increasing precision. It wasn’t until the 2004 result at BNL that we started to see real hints of a potential discrepancy between theory and experiment, which our new measurement has confirmed and built upon.

So in summary, what started as a ‘quick and easy’ check of the theory has evolved into a potential signal for new physics.

There is a somewhat technical article here written by one of our collaborators, which goes into more details of the history of these measurements: https://arxiv.org/abs/1811.06974

Any chance of a more efficient muon source for cold fusion?

While there is no direct link between the Muon g-2 experiment and “cold fusion”, also called muon-catalized fusion (https://en.wikipedia.org/wiki/Muon-catalyzed\_fusion), there are indeed efforts in this direction ongoing at Fermilab.

For more information, check out this talk (https://indico.fnal.gov/event/59506/contributions/269954/) in this year's New Perspectives conference at Fermilab. There is a recording with a great introduction.

Supposedly there is significantly more of dark matter and dark energy in the universe, than "normal" matter. However you seem to be accounting only for "normal" matter. Yet, even though you ignore significantly more of the dark stuff, you get very accurate results. Therefore, do your results indicate that the dark stuff doesn't exist?

Assuming your final conclusion after all the measurements are done is that there is really something new, how do you plan to figure out what it is that your measurements are pointing to? The fifth force, dark matter, dark energy, something completely new, some combination of those, ...?

This is a great question.

So far, the Standard Model contains no Dark Matter particles. We do not know what they are, or how they interact with “normal” particles already existing in the Standard Model. And this is precisely why we are doing muon g-2, as well as all the other precision measurement experiments and direct dark matter searches, so that we can see if there are unknowns out there.

While the discrepancy between the experiment and theory is not entirely understood due to new developments in theoretical calculations, as you can read from other answers in this AMA, an agreement between the two only indicates the dark matter particles do not interact with muons, or the interaction is so weak, that the impact on the muon g-2 is smaller than that the experiment uncertainties allow us to measure.

On the other hand, a confirmed discrepancy between the theory and the experiment will indicate interactions between muon and “something new”. This new particle, or new interaction will also leave traces in other physics processes. We can make new experiments to measure these interactions, then decide whether the behaviors fit some of the already proposed new particles / forces–or something entirely new after all.

As an experimentalist, I understand physics as a mathematical structure that explains the universe we live in. It’s the reality that determines the theory, so new theories will arise with new experimental discoveries

The level of detail in these responses has been phenomenal - kudos to you all!

What will be the next steps for this team? Another five-sigma improvement in precision? Tau g-2? As I understand, this experiment has been 20 years in the making and final results are still a few years away.

Response 1:

That is correct. This year's publication combines 3 years of data taking.

In the spring of this year, we finished our data taking after a total of 6 years. So we have more data, roughly 3X more than we already looked at, which will be our focus for the next few years.

As a rule of thumb, 4X more data leads to a roughly twice as precise measurement. So stay tuned, there is more to come from muons!

Unfortunately, taus decay so fast that it is not feasible to store them in a storage ring in the same way as we do it with the muons. There are proposals to measure it with different experiments (https://arxiv.org/abs/2205.12847 ).

Response 2:

Thank you, we appreciate it. We still have a set of data about 3 times bigger than what we’ve just published left to analyze, so we’re not shutting the book on Muon g-2 just yet!

Over the next couple years, we will complete the analysis on all of our data and we’re on track to meet our eventual goal of ≤ 140 parts-per-billion total uncertainty.

Tau g-2 is a bit trickier to do, since the tau is so short lived and requires a lot more energy to produce than muons.

There are a number of people who are interested in doing that measurement at the LHC though, so we might see some exciting results from them in the coming years: https://www.particlebites.com/?p=9145

How do you know that the result of the theoretical prediction is correct? These calculations can be tedious (if it's anything like with the electron), maybe someone skipped a diagram or something?

Like everything in science, we of course may be susceptible to basic human errors, so in both theory and experiment we are constantly checking our colleagues’ work.

The calculations are very tedious, so we write extensive documentation showing exactly what has been done so that it can be checked by other experts, and we are careful to benchmark calculations against each other to ensure their theoretical consistency. This is also why the theory calculations are now using two different techniques (lattice QCD vs the dispersive approach) for the hardest problems.

In the end, if we fully understand the underlying physics, the two completely independent approaches should agree.

The Muon g-2 theory initiative is currently working on understanding the comparison between the approaches in an effort to have an updated prediction to compare to the experiment in the next couple of years

What's your best guess for the new physics at play?

It is difficult to say but it is quite likely a new particle (or family of particles) that would be found by other experiments that can probe them directly (like the Large Hadron Collider in Switzerland) to explain if there was new physics to be found.

There are various theoretical ideas for the type of particles that could explain the current discrepancy.

The current deviation of the measured value and the theoretical prediction from 2020 also eliminates certain types of new particles but in the end our measurement would mainly show if new physics was out there. We could not really pin it down to the exact type of new particle (or new force).

The results obtained from Brookhaven, Fermi, and around the world regarding muon g-2 have uncertainties and thus your statistical error bars in presentations. Is there a particular minimal error threshold that is being sought where we can say, yes, that’s it. Or is it a case of keep running the measurements again and again, and we’re looking at measurement values compared to previous values?

The standard threshold for new discoveries in physics is called “five-sigma”.

This is when the uncertainty is at least five times smaller than the difference between a theoretical prediction and an experimental result.

In g-2, our goal has been to reduce the uncertainty in our experimental result by as much as possible, hoping to reach the 5-sigma threshold.

We’ve reduced the uncertainty significantly over these first years of data collection, and we’re hoping to reduce it even more with the rest of our data.

However, the experiment’s uncertainty isn’t the only factor in play. There’s also uncertainty in the theoretical prediction! Both of the uncertainties need to be combined to create the overall uncertainty in the comparison.

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The answers provided here are from the scientists listed also at the top which are all part of the Muon g-2 experiment plus one theoretical physicist joining us from Argonne:

Muon g-2 scientists:

• Peter Winter: Muon g-2 Co-Spokesperson and Physicist, Argonne National Laboratory

• Matthew Bressler: Postdoctoral Researcher, University of Massachusetts Amherst

• Simon Corrodi: Assistant Physicist, Argonne National Laboratory

• Sam Grant: Postdoctoral Appointee, Argonne National Laboratory

• David Kessler: Graduate Student, University of Massachusetts Amherst

• Josh LaBounty: Graduate Student, University of Washington

• Yuri Oksuzian: Assistant Physicist, Argonne National Laboratory

• Fatima Rodriguez: Engineering Physicist, Fermi National Accelerator Laboratory

• Dominika Vasilkova: Postdoctoral Appointee, University of Liverpool

• Yongyi Wu: Postdoctoral Appointee, Argonne National Laboratory

Featuring input from Argonne theoretical Assistant Physicist Tim Hobbs.

And two awesome communications professionals are here with us to help edit our responses!

awesome communications professionals

et al - nameless faceless drones.

My question is: Did you think to ask either of their names?

Yes. They are Savannah and Justin. (Justin is the one adding comments).

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Muons are subatomic particles, similar to electrons but with larger mass and very short lifetimes.

We measure muons by containing them in a magnetic storage ring and recording the energy of particles they emit as they decay. To get our result, the muon magnetic moment, we compare how the muons spin to the strength of the magnetic field in the ring.

The major announcement is that our result is the most precise measurement of the muon magnetic moment ever measured! This could potentially be the first clue towards physics beyond the Standard Model.

Science communication is a major area of interest. While many of us are working on scientific discoveries, our teammates work alongside us finding new ways to convey science news to general audiences.

We’re all very excited about these results, and we’re eager to share our excitement with the rest of the world!

Is it necessary to attach the level of schooling to people's title?

Well... Sometimes that is their title, so...