Thread by @freyablekman

The hype for this experiment is slowly rising, as new results are expected to be presented at a Fermilab colloquium later this week. Now, why is this #gminus2 experiment so interesting and gets even jaded particle physicists like me excited? A thread:

https://twitter.com/Fermilab/status/1378765826929651716

While the standard model is awesome and can predict a huge number of different behaviours of the tiny particles that we are all built from, it is considered an effective theory, meaning physicists eventually expect it to stop working (pic: @symmetrymag )

Effective theories are not necessarily bad - they're great at predicting things, but only within certain limits

A familiar example is the theory of Gravity as developed by Newton, it works really well, but if you go extreme it goes bad and you need Einstein's general relativity

Now, back to the Standard Model. There are many different aspects to it that hint that it is an effective theory, and what physicists think is the most compelling of those depends on the physicist (and their backgrounds/specialisation).

Some (cherry-picked) highlights:

The standard model doesn't do gravity. Obviously a complete theory that describes everything also does gravity.

The standard model does not explain why the different forces (things like electromagnetism and the nuclear forces that are important for radioactivity) are different

Even worse: the standard model structure is not explained, so it does not justify why there are six quarks, three charged leptons, three neutrinos

Talking about neutrinos, the standard model is not fantastic at predicting anything neutrino (like their mass) precisely or at all

There are also astronomy-oriented puzzles, such as the fact that there are no particles or forces in the standard model that can explain dark matter, let alone dark energy! Not really my expertise, @Astropartigirl explains it much better than I ever could

https://twitter.com/Astropartigirl/status/1378877461027454977?s=20

But let's go back to the particles. Imagine you would have a way to look for particles that we don't know yet. That's a particle accelerator: a microscope to study tiny particles. And just like with other microscopes: the bigger the microscope, the tinier things you can see

But while the Large Hadron Collider is impressive and that, it is limited by how small it can go. Particles can just be too small/heavy to make (we would need a bigger accelerator with higher energy) or too rare (we would need more collisions with the current accelerator)

But there is a parallel way to check for proof that the standard model stops working, or as physicists call it, for "Physics Beyond the Standard Model", and that is by using the standard model's amazingly accurate predictions in a clever way and checking when the data disagrees

The trick is to find the combination of something that we test/measure really accurately, and that at the same time is something that the standard model can predict really accurately, and that would be different if there would be more particles/additions to the standard model

Now, this is where the g-2 #gminus2 experiment at @Fermilab comes in: it measures how one of the elementary particles spins in a magnetic field.

A bit like when you have a top, you can draw conclusions on where the material is inside the top, just from how the top spins

The muon g-2 experiment does exactly that, it measures, ri-di-cu-lous-ly accurately, how a muon 'wobbles' in a magnetic field. And the standard model can predict that magnetic moment with the same precision.

The amount of wobble should be exactly 2 if there is only the muon

But of course there are other particles that we know (from the standard model!), and quantum fluctuations (essentially: all those particles popping in and out of existence close to the muon) already change the magnetic moment by a tiny amount

So over the years, the consensus (and this took an extremely impressive amount of insanely complicated calculations) is that the value is 0.00116591804 away from 2, That comes with an uncertainty (because #science) and should be somewhere between 0.00116591855 and 0.00116591753

And now here comes the puzzle: an experiment at @BrookhavenLab in the early 2000s measured this value of the anomalous moment of the muon. And found it to be between 0.0011659203 and 0.0011659215. Those numbers are not consistent with 0.00116591804!

But the experiment was not precise enough to really say it was consistent or not using the statistics rules that particle physicists use.

So the muon-wobbling magnet was shipped from brookhaven to Fermilab, and a more precise experiment was built there

And the new results, which should be at least three times as precise, are coming out this week. And that is why I am excited, we want to see if the number is still not consistent with the standard model. Because the easiest way to explain that is with undiscovered particles.

Some more details about the new experiment here symmetrymagazine.org/article/the-my…

And a nice #online event for the general public is being organised by @Fermilab that will surely explain this measurement much better than I could do in this thread how wobbly muons can teach us about new particles: events.fnal.gov/arts-lecture-s…

Correction, one of my colleagues @Mark__Lancaster reminded me that the #gminus2 experiment will *eventually* be three times as precise. For this week, we only expect a result about as precise as the previous one.

What do you think, will it still disagree with the prediction?

and I think it is time to #unroll

@threadreaderapp
#unroll