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Particle physicists make wishlist. I'm underwhelmed.
🕑 Added 2023-12-19 17:00:07 +0000 UTC[This is a transcript with links to references]
Particle physicists in the United States have tried to come up with a strategy to revitalize their research area, and their ideas are… somewhat underwhelming. But let’s have a look.
Last week, a panel of scientists named the Particle Physics Project Prioritization Panel, or P5, signed on to a list of policy recommendations for the US Department of Energy and the National Science foundation. And since 5 is such a nice number, they’ve come up with 5 priorities.
Their highest priority, after continued funding for already running projects is, interestingly enough, not a particle physics project. It’s a radio telescope array called CMB-S4 whose detectors would be distributed in Chile and Antarctica and possibly other places.
CMB stands for Cosmic Microwave Background and S4 means it’s the 4th stage of an already existing project in stage 2. The major purpose of this experiment would be to measure the polarization of the cosmic microwave background.
The microwave background is the radiation that’s left over from the hot plasma that was was around in the early phases of the universe. The polarization of this radiation gives you an orientation for each patch of the sky. This orientation is partly caused by dust in the Milky Way but partly it comes from fluctuations of spacetime in the early universe. It’s this signal from the space-time fluctuations that physicists are after.
If you remember, this CMB polarization is what the BICEP experiment claimed to have found in 2014. But their detection turned out to be dust in the milky way, which prompted my friend and YouTube neighbour Brian Keating to write a book called “losing the Nobel prize”.
They want to measure the imprint of the space-time fluctuation because, you see, they are a prediction of the most popular, but unconfirmed, theory about the beginning of our universe, inflation. The idea of inflation is that our universe was born out of a quantum fluctuation of a field called the inflaton field. It supposedly caused a rapid expansion of the universe, so, the universe inflates, thus the name.
The inflaton later decays into normal matter, which is convenient because then it isn’t around today to detect it. But the quantum fluctuations of the inflaton field should also cause fluctuations of space time, and this should cause this polarization in the CMB that they’re looking for.
The devil is as usual in the details. The issue is that there are other ways to create these space-time fluctuations and the polarization that comes with them, it doesn’t have to be inflation. And the other way round, there are some models for inflation in which the effect would be so small one wouldn’t see it. So even if they can measure this signal, it’d neither confirm nor falsify inflation.
It's also somewhat surprising that particle physicists go for what’s a cosmology experiment, but rumours say it’s because the astro budget wouldn’t completely fund the experiment, so they’re helping out their friends basically. But friends don’t let friends look for inflatons!
Okay, that was the top priority, then let’s look at the second one. The second one is an upgrade of the DUNE experiment. DUNE is the Deep Underground Neutrino Experiment currently under construction at Fermilab. Its task is to shoot a beam of neutrinos through the Earth’s crust to Sanford Lab in South Dakota, about 1,300 kilometres away. With that, particle physicists want to learn more about neutrinos, especially their role in CP violation.
Some particle physicists think this CP violation is necessary to explain why there is more matter than antimatter I the universe. There’s no reason it should have been the same amount, other than that particle physicists think that’s what nature should have done. So I think there’s nothing to explain and it’s a pseudo problem. But then I wasn’t asked for an opinion. I wonder why.
But why would they give so high priority to upgrading a project that hasn’t even yet been completed?
It’s because DUNE has become a major headache for Fermilab. The management has been miserable, and the cost has spiralled out of control. DUNE was original proposed with a cost project ion about 1 billion dollars but kept getting more expensive. The department of energy put a cap on 3 point 1 billion dollars.
I have a lot of sympathy for that. If someone was spending my money, at 3 point 1 billion I’d also get a little antsy.
Fermilab got a new director, a bad grade from the DOE, and is about to get a new management. That budget cap however means that the project would have to be shrunk so much it wouldn’t be able to measure the CP violation before 2035 or so.
So what particle physicists say is that they want the project to be built as originally planned despite the ballooning cost because otherwise chances are that some other country manages to do it first, which from the point of view of international competitiveness would blow the entire money into the wind.
This makes me think that the likely reason for making this a priority is the sunk cost fallacy, also known as throwing good money after bad.
The third priority is to support the international efforts to build a huge particle collider that would produce a lot of Higgs bosons to further study them. They didn’t put a new US based collider among the top priorities, but they have them in a later recommendation. They want to build a muon collider in a project they called the muon-shot, a pun on “moon shot”, the title of a book about the American Apollo program that brought the first people to the moon.
Muons are elementary particles in the standard model of particle physics. That they are elementary particles give a muon collider a big advantage over hadron colliders like the LHC.
That’s because hadrons – like the protons that the LHC accelerates, are composite particles. If you collide them, you divide up the entire energy into collisions between the constituents. It’s not very efficient. In a muon collider you get more of the energy from the particles directly into the collision, so a muon collider of 10 TeV could do much more than the LHC already.
This is what the muon-shot is all about. But at the moment they’re just saying they need to make plans and the project itself is not a priority.
So let’s get back to the priority list. The fourth item is a dark matter direct detection experiment reaching the neutrino fog, and preferably sited in the US.
Dark matter is one of the explanations for some astrophysical observations, such as the too-fast rotations of galaxies. Particle physicists like the idea that these observations can be explained by a new type of particle called dark matter, if that exists, which it may not.
The trouble is, no experiment has found those particles. And each time an experiment comes back empty handed, they say the particle is a little more weekly interacting. Like this XenonNt experiment that was upgraded from Xenon10 to Xenon100 to Xenon1T and now to XenonNt without finding anything.
For a long time, particle physicists said there’d be an end to this building of ever larger detectors, because eventually the hypothetical particle would have to be more weakly interacting than neutrinos, so the neutrinos create so much noise you can’t find it. This is what’s called the neutrino fog.
But, some others pointed out, this just means you need to figure out how to subtract the neutrino noise, so you can find the supposed dark matter signal in the data. This new experiment which they want would be reaching this neutrino fog. Of course, there’s still no reason to think there’s actually a new particle to find there, so I continue to be unimpressed.
Let’s then look at the fifth and final priority that’s an upgrade of icecube, which is a detector at the South Pole. The detectors are actually sunk down into boreholes into the ice. It’s amazingly cool in any sort of interpretation of the word because there’s very little noise down there. IceCube can tell us more about neutrinos and dark matter particles, if they exist, which they may not.
In summary, this plan of particle physicist is basically “more of the same stuff that hasn’t worked in the past 4 decades.” Personally, I think what they should do is spend some money on serious theory development and then come up with some well-motivated experimental proposals rather than wasting money. But then again, it’s not my taxes which are being spend on it. Mine, apparently, subsidize coal. I might be warming up to those dark matter detectors…
Comments
Thank you for mentioning that "neutrino fog". Finally an explanation as to why dark matter experiments would be so tough, at least an explanation for me.
Unfortunately? Oh! I got it.
Rad Antonov
Indeed, P5 is just a wish list Oscar. My confusion is over the mixed messages from our creator. I thought she said the problem was too much theory. We need data to constrain the zoo. I know the party line is to diss any and all things that particle physicists do, but why drag the cosmologists into it? Simons is no dummy. He is betting on a good horse in the Atacama. CMB polarization might not prove inflation but it could rule out other cosmologies. As for the rest, I doubt they’ll get what they wish for. Unfortunately, other scientists are not going to see a bump in their funding when the DUNE upgrade is cancelled. That’s not how it worked with the SSC and it’s not gonna work like that now.
Rad Antonov
1. Yes. Sabine herself pointed out we still don’t understand exactly how neutrinos get their mass, though the oscillation experiments probably can’t tell us directly. The detectors will also be useful for multi messenger astronomy. I doubt DUNE will get funds for an upgrade after the initial botched job. 2. True and it’s not gonna happen. Even the community is itself divided on it.
1. Wouldn't the neutrino experiment be useful regardless of the CP violation thing? For research of neutrino oscillation for instance? 2. A muon collider? They are incredible unstable and what would be the source?
