a bonobo humanity?

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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

Ask Astro: Are neutrinos dark matter?

https://www.nature.com/articles/d44151-022-00024-6

https://golem.ph.utexas.edu/category/2022/12/neutrino_dark_matter.html

https://en.wikipedia.org/wiki/Lepton#:~:text=Lepton%20was%20first%20used%20by,a%20particle%20of%20small%20mass

Why Neil Turok Believes Physics Is In Crisis

http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/neutrino3.html#:~:text=Left%2DHanded%20Neutrinos&text=For%20neutrinos%20the%20spin%20is,rule%20for%20vector%20angular%20momentum.

https://en.wikipedia.org/wiki/Helicity_(particle_physics)