Missing the Mark: Muon g−2 update · 8:41am Sep 17th, 2023
Somehow, I have managed to write a physics-themed blog post for every Make Your Mark Chapter 4 episode. This has been an interesting science writing exercise, if nothing else. And now, on the eve of the Chapter 5 release, we get to the final part. The bit you’ve all been waiting for. It’s time for an update on the Muon g−2 result.
This story has been going on for a while. I told about the role I played back in 2021. But it goes back to the 1960s when the first experiments were done at CERN to measure the muon g-factor—a measurement of how this favourite particle interacts with a magnetic field. The g-factor is close to, but not exactly, 2. Things got more interesting in the 1990s, when experiments at the Brookhaven laboratory in New York showed that the deviation from 2 was deviating from the theoretical predicted value, which could be a sign of some new phenomenon. A new force or particle. Seeking confirmation of this anomaly, the team shipped their big magnet to Illinois, rebuilt it with better instruments and repeated the measurement with the higher intensity Fermilab muon beam. They have now released new results after analysing the latest data, confirming the earlier results. We now have a 5-sigma deviation from the theoretical prediction. A sign of New Physics! It’s all we’ve ever wanted. Or is it?
How is this measurement done? You need a big lair with an accelerator system to produce a 3.1-giga-electron-volt muon beam, connected to a 14-metre storage ring, in a 1.45-tesla magnetic field, precisely adjusted to 1-part-per-million uniformity all around. Then a suite of sensors to monitor what’s going on, and an array of detectors to pick up the positrons that swerve into the ring after the muons decay. This is the muon g-2 experiment. The team behind it are very smart instrument scientists and engineers, and they’ve got great style.
Why go to all this trouble? Because this measurement could be the key to discovering new scientific phenomena. Picture the muon as a particle surrounded by an ethereal cloud of virtual particles and antiparticles, and pop into existence and disappear in the tiniest fraction of a second. This is what every particle looks like at this level, and the precise value of the g-factor, and other properties, is determined by what happens in this aura, involving every particle and interaction known and unknown. This means by measuring it and comparing the value with that calculated from theory, we have a way to search for a sign of New Physics.
We have a parameter that can measured to parts-per-billion precision, and calculated just as precisely. It’s time for a showdown between theory and experiment. A showdown that we all hope will show the theory is incomplete and give us a sign of something new. And, it would seem we have now reached that point.
But it’s never that simple. It has now emerged that there is a disagreement between different theorists about the calculated value. Computing the g-value means doing the Quantum Field Theory calculations for all the those virtual particles. You need to take into account everything that can interact with the muon (first order); and then everything that can interact with everything that can interact with the muon (second order); and then everything that can interact with everything that can interact with everything that can interact with the muon (third order)… It gets messy. It requires theoretical physicists with the patience to focus on calculating a known theory to this precision, without getting distracted by some novel new theory.
The electromagnetic part involving virtual photons and electrons and positrons and other charged particles is well understood. As is the weak nuclear force. However, the strong nuclear component, where virtual photons emitted by the muon produce quark antiquark pairs, is the tricky bit. This Hadron Vacuum Polarization (HVP) term has usually been calculated from data collected at electron-positron colliders studying this process. These include the CLEO experiment (in Cornell, New York); the BES III experiment in Beijing; KLOE in Frascati Italy; and the SND and CMD detectors at a collider in Novosibirsk, Siberia. An international effort.
Meanwhile a new result for the HVP value has come from a new technique: Lattice Gauge Theory, which calculates it without the need for any experimental data using formidable supercomputers. This new technique gives a different result to the data-driven one. And one that matches the experimental measurement of muon g−2. Different groups have now repeated the lattice gauge theory calculations and they seem to agree with each other. In a final twist, the CMD experiment released a new result that lines up with this, but disagrees with other collider measurements, including the SND detector at the same collider.
So we now have our super-precise measurement of the muon g−2 factor. But whether this is a sign of New Science is still an open question. What happens next? Will the LGT teams find an error in their calculations? Maybe they don’t quite understand their fancy new method as well as they would like? Or will it emerge that they are right and the collider experiments recording the data used in the first approach somehow screwed something up.
Perhaps the most interesting outcome would be if all teams are correct, but there is something else that we don’t understand about the particle interactions. Some other type of new scientific phenomenon.
On to Chapter 5.
Strong nuclear force diagrams look weird if youve just been reading cosmology books. The force increasing the further apart you pull teh quarks untill they split apart look like theyre trying to rip off wormholes.
Maybe the big discovery will be found by working out what is the difference between the two values, mreasurement verses calculation, and what happens if its just due to enviroment?
Chrenkov radiation is due to a mismatch in local and abbsolutel speed of light, but how do you measure changeing distance with only one ruler?
I just got stuck on VSEL because it can solve, or at least propose solutions to a whole bunch of problems, the big problem is getting it to remain consistant with everything else.
Its been a centuary with little advancement, because the fundamentals of Relativity havent been approached. Einstein didnt replace Newton, he expanded on him, why cant Einstein be expanded on without replacing him?
Id swear Hawkings last equation looks like it should have Heisenburgs Limit in it instead of the numbers, but Im just going off pretty pattens and shineis rather than calculable derivations, so Ill just stick with being confused about if at the escape velocity of a black hole is speed of light means it cant be escaped, but orbital velocity is 70% c, then at 85% c in orbit why cant you climb to a point where the escape velocity is lower and so escape? Something something climb a ladder verses space fish and gravity falls?
Why should they be vitual particles instead of particles that are there but cant be detected unless interacted with? Like a boat in water, you dont see the microplastic until you run into it, and al your measurements can only occur on the surface?
If it were just LGT vs. collider experiments, I'd know which way I'd bet, but with the CMD weighing in on the other side, it could be something very interesting and unexpected!
Neat!
…Somehow, I totally missed Chapter 5 is dropping tomorrow. I think off it being late September like last year, my brain mentally filed it away as the same "last Monday of the month" as last year.
Not much of a complaint, mind, as it means I can
struggleget through the six episodes before UKPC without rushing.Oh, and the science here is as interesting as ever, but I'm too tired this morning to formulate much of a response to it. Sorry, bud!
I'm starting to think particle physics is about the only honest brand of science left. Everybody else when they have an experiment that doesn't produce a result in line with existing
dogmatheory will quietly mess with the data points and get out the excuses. Physics says "Oy, that looks a bit odd. Somebody poke it again, only harder."5746817
That's what a lot of people are saying. The CMD result is very curious.
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I don't think particle physics is any more honest than other areas of science. Maybe it just looks that way.
I don't have anything useful to add but I did want to thank you for writing this up. It's always cool seeing how people push at the edges of our understanding.