Tell Your Tale – The W Boson Mass · 9:13am Apr 15th, 2022
Last week turned out much more exciting than expected. We got the first dose of Tell Your Tale on Thursday, which was very nice, but not quite the same level of awesomeness as the announcement from Fermilab on Friday that they’ve measured the mass of the W boson and it doesn’t fit the theory.
Let’s tell this tale.
The W boson is a particle discovered at CERN in 1983. It’s a mediator of the weak nuclear force, which is the fundamental force that we understand best, in a technical-particle-theory kind of way, but we have the most trouble explaining what it actually does. The weak interaction is responsible for nuclear fission and fusion, so absolutely essential for the universe to exist as it does.
The first mystery about the mass of the W boson is why it has a mass at all when the theory says it shouldn’t, but it does, and quite a big one. This mystery was resolved by Peter Higgs with his theory that predicted the Higgs Boson. As soon as the CERN leaders had collected their Nobel prize for finding the W and Z, the hunt was on for the Higgs.
The W boson mass played a part in this hunt. As the masses of fundamental particle are influenced by the masses of other fundamental particles, by taking precision measurements of the W boson and top quark mass, you could predict the range where the mass of the Higgs was likely to be and thus know where to look.
This was done by the CDF, or Collider Detector at Fermilab, experiment. They studied proton-antiproton collisions from the Tevatron accelerator from 1985 to 2011. They discovered the top quark in 1995. Then by identifying collisions producing a W boson, and adding up the energy of all things it decayed into (it only lives for about 10−25 second), they could figure out how much it weighs. In 2012, the Higgs Boson was discovered at CERN, with a mass right in the range predicted by the W and top studies at Fermilab and CERN.
While the Tevatron and CDF were turned off over a decade ago, and the researchers moved to other projects, they have also been quietly reanalysing the data, working on their code, refining the calibration of the calorimeters, and have now published a new result for the W mass. The most precise to date. And it doesn’t match the prediction of the Standard Model. By quite a significant margin (7 sigma). This would suggest there is something else out there, besides the Higgs, that is shifting its mass. This is just the sort of weird anomaly that could be a sign of some new scientific phenomena and gets us all excited. The other weird thing is that it doesn’t match the last measurement of the W mass made at the ATLAS experiment at CERN. It seems to weigh a bit more on one side of the Atlantic.
What is going on?
Of course, it is possible that the CDF team have somehow messed up their calculation. But, they are a very smart group of people, they have a lot of experience at doing this sort of measurement, and they’ve been working on it for a long time. They unblinded their data and saw this result back in November 2020, and have spend all the time since then checking everything without finding a mistake. So, it doesn’t seem very likely that they have just screwed up.
Everyone is now waiting for the next W mass measurement from the CERN experiments. After a long break to install upgrades, the LHC is now gearing up for Run 3. Watch this space.
Really hope that the CERN experiments, using different particle interactions, get yet another result.
How do they add up the energies gained, lost through neutrinos, best guess from neutrino detector experiments and how many are expecte for each interaction? new particles are when missing energies or even excess energies are consistant when compared to integer steps?
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The energy taken away by a neutrino can't be measured directly. You can sometimes infer it from other information, but if you need an accurate measurement of the total energy then you use events which don't contain neutrinos.
ah, good :) I saw this mentioned on Twitter a few days ago, but the article they'd linked to was atrocious
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Same except for... okay actually pretty much completely different except I saw the headline somewhere.
I've in my systems studies yet to dive too deeply I to particle physics. It will probably be nostalgic when I do though: Precocious little me wrote a short illustrated report on particle accelerators when I was 6.
I heard someone say that in some way the LHC actually could have a harder time getting a nice measurement of the W boson than Fermilab, because of something about luminosity and protons.
I don't really understand what they meant, because it seems like if you had the bigger detector, you should have more collisions and more data to work with.
If the LHC can do more and bigger, I'm confused why it couldn't also "step down" to Tevatron's level if it needed to!
Do you know why they might have said that, and if there's any truth to it?
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I'm not an expert, but my guess would be that it's like the difference between a scalpel and a hatchet. Even if you're being super careful, it's hard to make a small, curved incision with a large, straight blade.
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The LHC detectors are much more sophisticated and more collisions means more data, so overall it is a much more powerful machine. However there are a few things that work in favour of the Tevatron for this measurement:
The LHC researchers will always have a harder time analysing the data, as when you go to higher energy and higher luminosity (rate of collisions) there is a lot more happening. With some fifty protons colliding at the same time, there will be a lot more background particles from other interactions, which make it harder to identify the thing you are interested in.
The extra energy at the LHC is not so important for making W bosons (mass 80GeV) as it would be for Higgs bosons (mass 125GeV). The Tevtatron wasn't powerful enough to discover the Higgs, but it is perfect for studying the W.
The Tevatron was a proton-antiproton collider, while the LHC is proton-proton. As you need a quark and antiquark to combine to make a W, the Tevatron is set up for this. At the LHC, you need a quark to radiate a gluon, which produces a quark-antiquark pair. This isn't such a big deal as it may sound as at the LHC energies this happens all the time, but it might make things simpler for CDF.
While the LHC could run at lower energy, it makes more sense to run and collect data at maximum energy, where they have a chance of seeing something new, than to just repeat what the older experiments did.
7 sigma? Holy carp!
If I had to bet money on it, I'd bet on an error of some sort, particularly as CDF got a mass right on the prediction. (It's shame it's no longer around, but we needed that money to bail out
casinosbanks and businesses that are "too big to fail.")But, as the CERN team was so careful to check their findings... I dunno. That makes for two intriguing questions: Why is the mass so far from the predicted value, and why did Fermilab get it so wrong?