I Am Muon g-2 group at Argonne. We are part of the Muon g-2 collaboration and our experiment recently announced the first result which strongly hints at new physics which challenges the Standard Model. Ask Me Anything!

Can you give an explanation of your research and discoveries for an idiot like me?

In particle physics, we have one theory that describes all our understanding of all particles. We call it The Standard Model.

This theory allows the very precise calculation of how the muon interacts with a magnetic field. It turns out that all particles that exist contribute to this.

The (Fermilab Muon g-2) experiment measures this interaction very precisely. Our measurement, as did the predecessor, found that the measurement and the calculation from the theory probably don't agree. This means that the Standard Model is missing some particles or forces.

Okay, an ELI5. I think this is accurate, but there are a couple of pieces that might be beyond my understanding - please correct if I’m misleading.

When we fly muons and electrons in a circle at really high speeds using magnets we get to observe the quantum “soup” - all the little things that happen too fast for us to normally see.

This is because electrons and muons are kind of like bar magnets. When we spin them around a circle using powerful magnets we can measure how quickly the bar turns to adjust to the magnetic field.

We did this experiment with the electron back in the day, and the measured value didn’t match the prediction from the maths.

Well, we realized the reason for this is because the electron’s magnet doesn’t actually stay the same the entire time, for very short periods it changes into a “different vegetable in the quantum soup” that is not magnetic. interacts with the other "vegetables in the quantum soup" in a way that changes the strength of the electron’s magnet. When we did all the maths for the different “vegetables” it could possibly turn in to interact with we got the same result as experiments. How cool is that?

So we did the same thing to the muon, and when we had taken the muon and all the known vegetables from the soup, we’re still left with something behind. An unknown vegetable!

If this experimental difference proves to be true, it means there is a standard model particle that we have neither detected nor theorized before, and that’s really exciting.

Edited to try and incorporate the correction below.

You kind of describe it illustrative in a pretty good way. The main correction we would have is that the electron and muon stay the same but they interact with the "vegetables" in that "quantum soup" for short periods of time and that changes their magnetic moment.

I have heard some speculate that the inaccurate prediction of the muon's g-factor is more due to a flawed theory prediction, rather than a flaw in the Standard Model itself. Lattice QCD evaluations seem to have predicted a g-factor value that was much closer to what was recently measured in Fermilab than predictions made using perturbative methods

http://resonaances.blogspot.com/2021/04/why-is-it-when-something-happens-it-is.html?_sm_au_=ivsQ7kP6NtW646BPVVMWvK37M00q2

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Do you really think the Muon g-2 experiment actually might imply new physics that aren't yet incorporated into the standard model? It seems to me that if lattice evaluations were better able to predict the value, then those should perhaps be used going forward, despite being much more computationally difficult compared to perturbative methods, and wouldn't necessitate the need for new physics.

There is a lot of work to be done on the theory side, too.

The new Lattice QCD calculation done by the BMW collaboration is quite new, and it will take time for the High Energy Physics community to investigate and evaluate their Muon g-2 value and uncertainty.

Just looking at the uncertainties of the experiment, the BMW calculation and those calculations using more conventional methods, no conclusions can be drawn yet.

Stay tuned! The theory community is working hard on it, and we are eager to see what they come up with, too.

How long will it take for your results to be independently replicated? Is anyone already working on this?

There is an independent experiment in Japan at J-PARC under construction. If everything goes according to their plan, first results are expected ~2025. (link: https://g-2.kek.jp/portal/index.html)

In addition we (Fermilab Muon g-2) took and are still taking more data. This first publication is only roughly 6% of the expected full dataset.

What sigma would you get when you go trough the whole data?

The goal is to reduce the uncertainty of the experimental measurement by roughly a factor of 4.

How many sigmas that will correspond to depends on what the value will exactly be and on future work on the theoretical prediction/calculation. We hope to achieve 5 sigmas. We consider 5 sigmas a discovery.

I previously heard about your result but have difficulties classifying the scale of how groundbreaking the change is.

When you say "new physics", how much exactly is the Standard Model challenged by it? Would it "just" need to be appended by some additional type of spin (if that's the right phrasing, I never fully grasped that concept)? Or does it suggest we would need to take a step like Theory of Gravity to General Relativity?

As you pointed out, the theory behind the Standard Model bases on symmetries. Adding additional symmetry (type of spin) is already kind of a big deal. One of the more prominent such extensions is SUSY (Supersymmetry).

The "new theory" doesn't only need to describe for example the g-2 discrepancy, at the same time it needs to agree with all the other measurements the Standard Model describes already so well.

How exactly does one go about adding up Feynman diagrams to get the result that diverges from your observed one?

This is a good question but not that easy to answer.

The Feynman diagrams are a descriptive way to illustrate the many possibilities that particles interact with each other. The diagrams are then linked to a set of fixed rules on how to interpret each part of the diagrams in terms of mathematical formulas.

Then there is also a fixed set of rules on how to sum up all the diagrams and integrate over all possible momenta, sum over various spin states etc. It is a quite elaborate technique and for example, for the contributions from quantum electrodynamics alone, more than 10,000 diagrams had to be evaluated and summed.

Thanks for doing this! Fascinating stuff. 2 questions!

Have physics measured the magnetic moment of all other charged particles(such as the Tau)? Was the muon the only particle to display this apparent discrepancy from the predicted value?

With regard to the statistical significance of the detection, what would it take to reach a higher degree of confidence and, dare I say, certainty? Conversely, is it possible that any new force is so weakly interactive that even a larger scale experiment would notice the same results?

Magnetic moments of many particles (not only charged ones) have been measured. At the moment, we mainly see the discrepancy in the muon sector. The muon is 207 times heavier than the electron. This makes it potentially more sensitive to new physics. The tau would be even more sensitive as it is heavier compared to the muon but the measurement is very difficult due to the short lifetime of the tau.

The current Muon g-2 experiment only analyzed 6% of its total data we are planning to collect. So our next results will improve our statistical precision which should help in increasing the confidence that there may be new physics.

Hey! This is exciting!

I was curious about the strength of the possible new force. How would it compare to the other forces? I know gravity is "weak" and I wondered how this new possible force compares, if that comparison would make sense, if at all, at the moment

This is an excellent question and there is not a unique answer.

Our experiment confirmed a former experimental result and there is strong indication that there is a difference between the predicted value from theory and the experimental measurement.

It will now need further information to really understand the origin and type of the possible new physics. The contribution to the anomalous magnetic moment of the muon from new particles typically is proportional to the coupling of this new physics to the muon divided by the squared mass of the new particles.

So the "strength" of this possible new physics could be large if the mass of the new particle was also large, or it could be "weaker" if the new masses are lighter.

That's fascinating, thanks for the answer!.
It's too early to tell if that proportion is symmetrical, right? I imagine the data comes from only a specific particle interaction?

Can you explain what you mean by 'symmetrical?'

Am I understanding this in layman's terms correctly? The new force is related to the spin of Muon's, and that spin is currently unaccounted for in the standard model?

The spin is accounted for in the Standard Model. The new force, if it exists, will affect how a muon interacts with a magnetic field, which can be perceived through the motion of the muon's spin.

In this experiment, we are watching how the muon's spin moves. More precisely we actually measure how fast the spin precesses, similar to the precession of a gyroscope.

To be accurate, we have not yet confirmed that this new force exists.

Learning about how we measured the g factor of electrons and how closely it followed theory was one of the coolest things. I remember it wasn't exactly the same but very close.

This difference with muons seems so small. It's so many decimal places down and seems so hard to do all this. Is this the route we will have to take to find new physics? Will we need years of experimentation and data with all our experiments just to find these tiny differences? Just seems so daunting.

Indeed you are right that these precision measurements, like the magnetic moment of the electron or muon, are hard experiments.

They offer one route to detect new physics. Another way is the direct production at high energy colliders. But for that one would need enough energy to produce the new, possibly heavy, particles. The precision measurements see indirect effects from those new particles without the need to directly produce them in colliders.

So it is a very good approach even if it takes a long time to get the precise measurement

Why is the muon affected and not the electron? Would the tau be affected even more?

The muon is 207 times heavier than the electron. This makes it potentially more sensitive to new physics. You are right, the tau is even heavier. From an experimental point of view, the issue is that the tau's lifetime is so short that it's essentially impossible to use them in this kind of experiment.

From my understanding, the results of the experiment hone in on the difference between our theoretical value of g and experimental value. What formula is the one that couples g to the spin of the muon, and how can we be sure that the difference in spin isn't coming from a different constant being used in that formula (ie. the mass or charge of the muon)?

The g factor relates the spin of the muon to its magnetic moment. This experiment does not measure the spin of the muon, but its magnetic moment.

You can see this Wikipedia page for the formula (how the muon magnetic moment is related to g): https://en.wikipedia.org/wiki/G-factor_(physics)

There are indeed other numbers in it, but other numbers are all measured in other experiments with a very small uncertainties.

Is your research a better fit for explaining expansion other than tossing words like dark matter, or dark energy around?

This research is not directly related to the expansion of the universe. Potentially, the "new particle " might be dark matter particles, but to confirm this we need inputs from more other measurements.

How does muon g-2 unify all of physics?

RoDeltaR is correct.

It does not unify all of physics but it further informs us that there may be some new physics beyond our current understanding of particles and forces. A grand unified theory would be the ultimate goal but g-2 does not by itself lead to that.

According to this new physics, is gravity a force?

The Muon g-2 experiment is not probing the gravitational force.

However, gravity is a force conceptually. What matters is how to formulate this force. Newtonian physics and General relativity formulate gravity differently. The new result of the Muon g-2 experiment is not trying to re-formulate gravity.

This sounds really interesting, but I feel like you are incorrectly assuming that the average Joe of Reddit has some (ok, any) knowledge of who or what you are talking about.

I'm really not so sure this statement is as true as you think it is: "Our group at Argonne is famous."

Maybe you could lay some basic groundwork to get us up to speed?

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I Am Muon g-2 group

What is this?

at Argonne

What is this?

We are part of the Muon g-2 collaboration

What is this?

the Standard Model.

What is this?

the Muon g-2 experiment.

What is this?

Hey! We totally get that.

Argonne National Laboratory is a multi-disciplinary research lab in the Chicago area. Our group is a small part within the lab and we work on a particle physics experiment, called Muon g-2, which is located at another national lab called Fermilab.

Here's our recent press release that can answer most, if not all, of these questions - https://www.anl.gov/article/field-guides-argonne-scientists-bolster-evidence-of-undiscovered-particles-or-forces-in-muon-g2