Archive for the ‘neutrinos’ Category
Dummies on dark matter 3: all these neutrinos…
wateva
Canto: So, look up neutrinos and dark matter together on any bona fide sciency website, such as Astronomy magazine, or Nature, and you’ll get apparently contradictory claims – ‘neutrinos cannot constitute dark matter’ and ‘neutrinos may solve the mystery of dark matter’, so what’s a dummy to think?
Jacinta: It’s ongoing, and exciting, we must suppose. Dark matter is often given another adjective – cold dark matter – and neutrinos are too ‘hot’, which is to say they travel close to light speed. The clumpy nature dark matter is believed to have – remember they’re believed to clump around the outskirts of galaxies, explaining the observed higher velocity of outer galaxy stars – that clumpy nature isn’t consistent with zippy neutrinos.
Canto: Yes – neutrinos are kind of slight and speedy whereas dark matter is fat and lumpy?
Jacinta: Well that’s one way of putting it, but if it was fat it’d be visible, but it appears to be ‘transparent’ as Hossenfelder describes it.
Canto: That’s funny, a lot of fat people would prefer to be invisible, maybe dark matter has worked out a way… But if this matter is transparent or invisible, how can they detect it, or know that it’s clumpy? It seems to be just a placeholder to explain the gravitational behaviour of galaxies – doesn’t it?
Jacinta: Obviously I can’t answer that. Mathematics, however, may find a way…
Canto: I was hoping you wouldn’t mention that word.
Jacinta: Well it’s a return to neutrinos – sterile neutrinos. They only interact via gravity, but they are heavy, as needs to be the case. It’s all about missing mass after all.
Canto: Sounds like a similar profile to WIMPs but I think WIMPs, which are just postulates, I think, only interact through the weak nuclear force, an interaction that brings about nuclear radioactive decay. But I don’t think gravitational interactions have been ruled out for them.
Jacinta: WIMPs have gone off the boil recently, I think. It’s all such groping in the dark stuff at the moment, and if you have virtually no mathematics, it’s deadly. I’ve just been reading a dialogue between a physicist and a mathematician on neutrinos and dark matter, which after various increasingly heated exchanges of equations and talk of Minkowski spacetime, Lagrangians, anti-commuting spinor-valued fields, Weyl spinors and the like, it got to the point of pistols at dawn and aim for the heart. But the equations did look impressive.
Canto: Time to get back to basics. Remember we know about three types of neutrinos, also called flavours – tau, muon and electron. And remember they’re called leptons because they’re elementary particles and not very interactive….
Jacinta: That doesn’t explain why they’re called leptons, though, does it? Actually, when I try googling that very question, all I get is what leptons are, or what physicists thank they are.
Canto: You didn’t frame the question well enough:
Lepton was first used by physicist Léon Rosenfeld in 1948: ‘Following a suggestion of Prof. C. Møller, I adopt—as a pendant to “nucleon”—the denomination “lepton” (from λεπτός, small, thin, delicate) to denote a particle of small mass’.
Jacinta: Okay, all Greek to me. And by the way there are six lepton types, let’s get this clear – the three neutrinos and the particles they’re connected with, the electrons, muons and tauons. But I don’t know how or why they’re connected.
Canto: It seems that the three neutrino types are electrically neutral versions of, or sisters of, the negatively charged electrons, the also negatively charged muons – which have a half-spin, apparently – and the tauon or tau particle, which is also negatively charged with a half-spin. How can they tell them apart you ask? Well, according to the US Department of Energy, ‘Muons are similar to electrons but weigh more than 207 times as much’. Which is a bit like saying I’m similar to my neighbour but she weighs more than 15,000 kgs.
Jacinta: Ah yes, I’ve met her. A gentle giant, but a bit negative.
Canto: Well, multiply my neighbour’s mass – I mean a muon – by 17 and you have the mass of a tau particle. You’d think they’d be unmissable, but the first lepton to be discovered was by far the smallest, the electron. That was in 1897, and the rest are 20th century discoveries. And there are anti-leptons, of course.
Jacinta: Of course. So for completeness’ sake, and for our education, there are leptons, mesons and hadrons. Oh, and fermions. I’m just throwing those names out there. And gluons, and quarks, and bosons… and that might be it.
Canto: Well considering that we can account for only 4 or 5 percent of universal mass-energy – unless something’s very wrong with our accounting – we might be adding a few more possibly speculative particles in future. Is it really exciting or is it just a mess?
Jacinta: You want me to answer that?
Canto: Rhetorical, rhetorical. But it’s no wonder that respected physicists like Neil Turok is finding that we’ve complicated the field way too much. As he says, the LHC, the most touted experimental device in physics in the last 40 years, has discovered nothing but the Higgs boson, which of course was a really important discovery, but…
Jacinta: He says the dark matter is probably a right-handed neutrino, which, whatever it means, sounds simple enough. And that the universe is a kind of flat space, with nice and simple geometry…
Canto: Okay, a right-handed neutrino, let’s follow that up. The first thing I would think would be – it’d have to be heavy, and non-interactive, which means very difficult/impossible to detect. And then – if there are right-handed neutrinos there must surely be left-handed ones. These terms relate to spin, and the Standard Model, I think, gives neutrinos a left-handed spin, with a ‘helicity’ of -1, and these are paired with right-handed anti-neutrinos, with a helicity of +1.
Jacinta: So Turok is out on a limb here?
Canto: How would I know? It starts to get into mathematics and if-then speculations very quickly, and I get lost. But Turok feels that there must be more simple solutions to the big dark matter-dark energy conundrums without positing all these new particles. I know he seems to be positing one himself, but it’s just a variant of the neutrinos that’ve been proven to exist.
Jacinta: Helicity, by the way, is ‘the projection of the spin onto the direction of momentum’. Just another head-scratcher for dummies. Helicity, at any rate, is conserved. It doesn’t change over time. And here for, what it’s worth to the likes of me, is what one commentator says about Turok’s hypothesis:
For a heavy neutrino to serve as dark matter, it needs to be quite stable. Apparently this is tough if it interacts with the Higgs—how true is that, exactly? But a neutrino that’s its own antiparticle can have a mass without interacting with the Higgs: a so-called ‘Majorana mass’.
In Turok’s theory all the neutrinos have Majorana masses, described by a mass matrix. To make the heaviest right-handed neutrino stable, a bunch of matrix entries must vanish—and this makes the lightest left-handed neutrino massless!
Canto: Yeah, ain’t mathematics magical.
Jacinta: Hmmm, I’m wondering if we should leave all this dark stuff behind us for a while. Leave it to the Dark Lords to work out.
Canto: Haha, not very female supremacist of you…
References
https://www.nature.com/articles/d44151-022-00024-6
https://golem.ph.utexas.edu/category/2022/12/neutrino_dark_matter.html
on physics and the universe – what’s a neutrino?
more on the standard model later…
(Years ago, in the early 1980s, I bought the monthly magazine Scientific American regularly, to improve my education. A couple of books I read at the time brought this on – The Magic Mountain by Thomas Mann and The Selfish Gene by Richard Dawkins, probably in that order. I was then around the same age as Hans Castorp, Mann’s central character, which really helped me get into the novel. Call me Narcissus.
It wasn’t so much the whole (rather multifarious) novel that grabbed me, but a section in which the tubercular Hans, through his reading, reflects on the nature and origin of life, and then of matter itself. I have a romantic image of myself at the time jumping up from the book and pacing my bedroom, my mind abuzz with thoughts and wonderings. Science! Why is it so? How did it all begin? How did one become another?
Perhaps sadly, perhaps not, my reading of The Magic Mountain marked a fairly rapid switch in my reading habits, from fiction to non-fiction. And yet the big questions still elude me. I’m still very much an amateur, and I used to call this blog An autodidact meets a dilettante to mark my inexpertise. I changed the name to A bonobo humanity? because I hoped it would narrow my focus a bit, and of course because a female-dominated human world, a ‘world turned upside-down’, is a fantasy of mine, but one worth working towards. And yet, the even bigger issues stimulated by Hans Castorp’s reflections, like – why is there something rather than nothing? – still bug me. So, here goes…
What is a neutrino? I first read about them in a Scientific American magazine, which described experiments and facilities designed to detect them. They’re not so much rare as difficult to detect, and we don’t even know whether they have mass. But isn’t a massless particle a contradiction in terms? According to a Scientific American article from 1999, Wolfgang Pauli first postulated their existence in 1930, and they were first detected, as antineutrinos, in 1955. The article begins thus:
A neutrino is a subatomic particle that is very similar to an electron, but has no electrical charge and a very small mass, which might even be zero.
The weird idea that its mass might be zero is somewhat explained by this more recent intro to neutrinos from the US Department of Energy:
The neutrino is perhaps the best-named particle in the Standard Model of Particle Physics: it is tiny, neutral, and weighs so little that no one has been able to measure its mass [my emphasis]. Neutrinos are the most abundant particles that have mass in the universe. Every time atomic nuclei come together (like in the sun) or break apart (like in a nuclear reactor), they produce neutrinos. Even a banana emits neutrinos—they come from the natural radioactivity of the potassium in the fruit. Once produced, these ghostly particles almost never interact with other matter. Tens of trillions of neutrinos from the sun stream through your body every second, but you can’t feel them.
That last sentence is pretty mind-blowing! So, FWIW, neutrinos have 3 types, electron, muon and tau. They’ve been detected in human-constructed underground detectors such as the Sudbury Neutrino Observatory (SNO), a 1000 ton heavy water facility in Canada. And there’s still a lot to discover, apparently. As an amateur, and ‘knowing’ via Einstein that mass and energy are in a sense interchangeable, is it neutrinos as energy that are being detected, or neutrinos as mass? It seems that they’re being detected (and the neutrino type is relevant here) due to interactions with other matter particles more than anything else. There’s a sort of mathematical calculation called the Standard Solar Model (SSM), based on physicists’ understanding of stars in general, which predicts, inter alia, the outflow of solar neutrinos, and our inability to detect enough of these neutrinos early on became known as ‘the solar neutrino problem’. Virtually all the neutrinos detected in those early, pre-SNO days were electron neutrinos. Fuck knows why (but read on, as I learn…).
Neutrinos are fermions – elementary particles with a half-spin, like every other elementary particle – particles that aren’t composed of other particles (though not all fermions are elementary). There are bosons, hadrons and fermions, apparently. But particles also ‘exist’ as waves….
All of this has to do with the Standard Model, which recognises two types of elementary fermions – quarks and leptons. Neutrinos are a type of lepton. As mentioned, there are three types of neutrino, and another three particle types also known as leptons – electron, muon and tau. So each of these has a connected neutrino, making six lepton types in all. And then each has its antiparticle… As for the quarks, which combine to form hadrons, such as protons and neutrons, they come in types called flavours, of which there are six – up, down, strange, charm, top and bottom, which all sounds like Alice in Wonderland meets Willie Wanka and his Cocaine Factory, but no – all these particles, though often proposed through some kind of mathematical modelling (methinks), have been confirmed observationally.
I’m starting my explorations of particle physics and quantum mechanics and the magic of mass-energy with neutrinos only because I have to start somewhere, but what I’ve learned already poses questions. The zillions of neutrinos passing through our bodies all the time come from the sun, so they say. If we lived further from the sun, say on Mars, or even Jupiter, would we still be getting this flux of neutrinos? We amateurs tend to think of the space between planets, or ‘outer space’, as pretty vacuous.
My guess is that, assuming all the neutrinos in our solar system come from the sun, the only energy-generating source in our solar system, and since they radiate outward from that one source, they’ll be more thinly spread the further out they go, and that could be mathematically formulated, FWIW?
Apparently they’re all electron neutrinos. And their antiparticles, presumably. But neutrinos can change type, or ‘flavour’, as they travel (a more recently discovered fact which solved the Solar Neutrino Problem, as previous experiments could only detect electron neutrinos) – and this also indicates that they have some slight mass. But what’s most mind-boggling, to me, is that this thing called the Standard Model was formulated, back in the early 70s, from theoretical and experimental work done in the decades before, to explain all the matter in the universe, dividing it up into categories and subcategories – though presumably there’s still a big issue with the ‘missing’ dark matter.
So I suppose there’s no point in asking why neutrinos exist, or what ‘purpose’ they serve, we just have to accept they exist, in their three flavours and together with their anti-particles, as other leptons exist, and all fermions and baryons and bosons, which almost sounds as if I know what I’m talking about. But we know about them because of a lot of brilliant theorising and collective experimental activity which the vast majority of us would find very difficult to comprehend. But this is the universe that made us, for better or worse, and, while I don’t think it’s necessarily our duty to understand it, it helps to pass the time, and I can think of a lot more boring things to do. And so, dark matter…
References
Thomas Mann, The Magic Mountain, 1924
https://www.scientificamerican.com/article/what-is-a-neutrino/
https://en.wikipedia.org/wiki/Sudbury_Neutrino_Observatory
https://en.wikipedia.org/wiki/Standard_solar_model
https://en.wikipedia.org/wiki/Fermion
Solar neutrinos