In 2018, using the balloon-borne Antarctic Impulsive Transient Antenna (ANITA), physicist Peter Gorham discovered unexpected tau-neutrinos. They’re going from the Earth toward outer space, rather than vice versa; and are right-handed rather than left-handed. (That linked article didn’t mention the neutrinos’ handedness: I saw that in more recent articles like this one. For some reason, the weird neutrinos are back in the news.) One explanation for the neutrino traveling away from the Earth is that it entered the Earth, changed into a low-energy neutrino — high-energy neutrinos would be absorbed by Earth — and then changed back into a high-energy neutrino when it re-emerged from the Earth. Does this sound far-fetched?
Gorham didn’t observe the neutrinos directly, of course. IIUC he observed radio waves emitted after the neutrinos’ interactions with leptons.
I wanted to post this in GQ, but I don’t have a single specific question. One question I have is: How in heck are these guys smart enough to be able to deduce neutrinos, and even the neutrinos’ handedness, from simple radio waves??? What the heck is neutrino handedness anyway? And how do you distinguish a right-handed neutrino from an “ordinary” anti-neutrino? (And what’s with Gorham? He works at the good-climate University of Hawaii but travels to Antarctica? )
All comments by the Board’s physicists are welcome.
Mandala - Thanks for the link! It will answer some of my questions.
Grrr! - Ordinary neutrinos pass through the Earth, undetected, all the time. This was a high-energy tau-neutrino which should have been hindered from that passage.
If you have some sort of neutrino process that results in radio waves, the waves will be circularly polarized, and the direction of polarization will tell you the handedness of the neutrino.
That said, if I saw results like this, my first assumption would be that the radio waves were getting reflected, not the neutrinos.
Gorham thinks the non-smoothness of the possible reflecting surface(s) would have degraded a radio-wave reflection image, but instead it was as pure as non-reflected waves. He mentions experiments to learn what reflected waves do look like.
As for the apparent right-handedness of the neutrino, does that imply it was an anti-neutrino? Is that so unusual? After all, anti-electrons are produced by some radioactive isotopes of ordinary matter.
That’s a much subtler question than you might realize. Let me first sum up the state of knowledge, in terms as neutral as possible.
There are charged leptons: The electron, muon, and tauon, and an antiparticle for each of these. These three are easily distinguished from each other, because they have very different masses. And the antiparticles are very easily distinguished from the particles, because they have opposite charges (+1 instead of -1).
There are also uncharged leptons, the neutrinos. These interact with the six charged leptons in various ways, and seem to correspond to them. You can have a neutrino that interacts in a particular way with an electron, but which doesn’t interact in that way with any of the other charged leptons. Or one that interacts in that way with anti-muons, and so on. Other than their interactions with the various charged leptons, however, the various neutrinos are very hard to distinguish from each other, because they all have zero charge and a mass that is nonzero but too small to measure with current technology (and in fact, appear not to even have specific masses at all, but that’s a different matter).
One fact that we’ve noticed about neutrinos is that all of the ones that correspond to negatively-charged leptons have a left-handed spin, while all of the ones that correspond to positively-charged leptons have a right-handed spin.
But the handedness of a particle’s spin depends on the direction of its velocity. And we now know that, small though it may be, neutrinos do have nonzero mass (at least, on average), and that they therefore do not travel at the speed of light. Which means that there must be some reference frame moving even faster than them, and hence, in that reference frame, the neutrino is traveling the opposite direction, and so has the opposite spin. And in principle, you could slow down, stop, and reverse the direction of motion of a neutrino, so that you’d see this even in practical reference frames like that of the Earth.
Well, what do you get when you reverse the handedness of a neutrino? There are two competing models. One model, the Dirac model, says that there’s an inherent difference between neutrinos and antineutrinos, and so if you took a left-handed neutrino (or a right-handed antineutrino) and reversed its handedness, you’d get a right-handed neutrino (or a left-handed antineutrino). Since the relevant reactions don’t allow such things, these particles would then not interact at all with other leptons, but they’d still exist.
The other model, the Majorana model, says that the only difference between what we call “neutrinos” and “antineutrinos” is their handedness. In this model, if you took a neutrino that corresponded to negative leptons, and reversed its handedness, it would now correspond to positive leptons. Which model is correct? The evidence is inconclusive. Well, some folks say that it’s conclusive one way or the other, but they disagree on which way it’s conclusive.
I am reluctant to mention the following, and hope that Dopers that address the far-fetched hypothesis treat it objectively, rather than just pontificating on its “impossibility!” But one reason I am intrigued by the ANITA discovery is the possibility mentioned in this article:
(I’ve edited the quote in the hope that Dopers will focus on the hypothesis rather than on the New Scientist writer’s over-confidence that the hypothesis is true!)
The normal progression of science is that there are boundaries to what we know. Someone comes up with a new experiment, out near those boundaries. Usually, those new experiments give the expected results, but sometimes, they don’t. When a new experiment gives an unexpected result, two things happen: First, one bunch of scientists (mostly theorists) start coming up with all sorts of wonderful and amazing models for what it might mean if the new results are true. And second, another bunch of scientists (mostly experimentalists) start coming up with all sorts of mundane things that might have gone wrong with the experiment to lead to those wrong results (of note, there is nonzero overlap between these two groups of scientists). Eventually (possibly requiring more experiments), one of those two groups of scientists is found to be right, and it’s usually some sort of mundane problem with the experiment. But occasionally, it is the wondrous new model that pushes back the boundary a little bit further, and increases the circumference of the unknown.
In other words, it probably isn’t a mirror universe. But hey, we’ll see.
The normal progression of science is that there are boundaries to what we know. Someone comes up with a new experiment, out near those boundaries. Usually, those new experiments give the expected results, but sometimes, they don’t. When a new experiment gives an unexpected result, two things happen: First, one bunch of scientists (mostly theorists) start coming up with all sorts of wonderful and amazing models for what it might mean if the new results are true. And second, another bunch of scientists (mostly experimentalists) start coming up with all sorts of mundane things that might have gone wrong with the experiment to lead to those wrong results (of note, there is nonzero overlap between these two groups of scientists). Eventually (possibly requiring more experiments), one of those two groups of scientists is found to be right, and it’s usually some sort of mundane problem with the experiment. But occasionally, it is the wondrous new model that pushes back the boundary a little bit further, and increases the circumference of the unknown.
In other words, it probably isn’t a mirror universe. But hey, we’ll see.
Seems similar to the OPERA experiment where they proposed a faster than light neutrino result. Though in this case I bet a slightly different experiment that relies on a different detection mechanism is going to be required.
It’s a real shame that folks interested in cutting edge science have to wade through garbage like this. The article is a confused mess with bizarre claims, misinterpretations, and misattributions. Just… just pretend you didn’t read it.
You’ve been bamboozled by that article. This isn’t the mechanism at play.
Neutrino interactions. A neutrino passing through stuff (e.g., the earth) zips through as if the stuff wasn’t there, up until the point when it suddenly has a (rare) interaction. The likelihood of having an interaction goes up as the energy of the neutrino goes up. The neutrinos relevant for the ANITA story are very, very high in energy. We have essentially no measurements of the interaction rate at such extreme energies, so we have to trust our extrapolation from lower energies (which is based on solid models, but it’s still a point of note). In any case, the expectation is that these neutrinos would interact after travelling only a fraction of the way through the earth, and thus they couldn’t make it all the way through.
But we have to go a step deeper.
When these neutrinos interact, they are smashing into an atomic nucleus. Out of that collision is a violent spray of particles, and each particle carries a portion of the inbound neutrino’s energy (and there’s a lot of energy to go around). One of the particles leaving the collision will directly correspond to the incoming neutrino. In other words, the neutrino “bullet” comes out (with less energy now), and all the other particles produced in the collision are shrapnel. The outgoing bullet will, about half the time, be just the neutrino with less energy. The other half of the time, it will be transformed by the interaction into its charged lepton partner. For the ANITA story, it’s the second case that’s relevant.
If we consider tau neutrinos, then the charged lepton partner is the tau. Taus will decay soon enough, and when they decay, they turn into a spray of particles that always includes one tau neutrino – the same particle we started with, now with even less energy.
This new tau neutrino can subsequently interact and produce (“turn into”) another tau, which can decay and produce a new tau neutrino, and so on. This sequence allows very high energy tau neutrinos to pop out the other side of the earth more often than you would expect if you ignored this “regeneration” process. They do lose substantial energy along the way, but again there’s a lot of energy available to start with. This process alone, though, isn’t enough to explain the signals without either some deviation from the extrapolation interaction rates or a transient (localized in space and time) astrophysical source.
The neutrino’s helicity (“handedness”) is not observable here. The radio waves are not produced by the neutrino but by all the post-collision particles (thousands upon thousands of them, each begetting more through their own collisions, until the so-called “shower” runs out of gas).
These neutrinos are at ludicrous energies and have to originate from extreme astrophysical environments. At much lower energies, yeah, you have to use other tricks to figure out where the neutrino might have originated.
This play on words (see: “mirror” universe) is subtle enough to maybe confuse folks. Note that there isn’t any hypothesis on the table about neutrinos getting reflected in any literal sense.
Hopefully the description above helps here, but I can expand on the process as desired. In particular, the tau neutrino here is colliding with an atomic nucleus (or the constituents thereof), and sometimes that collision produces a tau.
I don’t think anyone tries to say this is resolved. It’s fully wide open.
The article is behind a paywall, but the bit you quoted – “What’s left is shocking in its implications” – is bonkers. There are still plenty of mundane, and certainly plenty of less mundane but less crazy, options on the table. This single random idea isn’t anything close to the only thing left to restore sanity.
Having said that, we can look at the idea itself.
Every month, dozens of new ideas about the nature of dark matter or about possible new physics signals to look for or about novel explanations of experimental anomalies get published. This is a big part of the world of theorerical physics. The new ideas that get a lot of traction in the field tend to be ones that have some mix of these characteristics (with an Occam’s razor feel to the first three):
The new idea solves multiple problems at once.
The new idea doesn’t require “fine tuning” (needing to give multiple parameters carefully coordinated values or else it doesn’t work).
The new idea doesn’t require multiple unrelated changes to physics to accomplish its goal.
The new idea offers testable predictions.
The new ideas that get a lot of traction in the popular media tend to be the ones with some mix of these characteristics:
“Einstein” or “speed of light” can be mentioned.
Something weird and quantum is involved.
“Quantum computing” can be mentioned.
“Parallel universes” can be mentioned.
Etc.
While the idea here scores highly in the latter, it doesn’t score well at all in the former. The original scientific article is fine for what it is, but it has to jump through a lot of hoops (some only qualitatively treated) en route to explaining the ANITA signal. The full story leaves in its wake at least as many problems as it tries to solve, so the paper actually nets zero (or fewer than zero) solutions.
The short story of the paper: there’s a very heavy dark matter particle that decays with an extremely long lifetime (but not too extremely long); and it interacts with regular matter a little (and also with itself, to make things work out) so that it can scatter and be captured by the earth; but the spatial distribution of the captured particles within the earth wouldn’t be right so the earth maybe has had a recent history of passing through an overdense region of this dark matter; but getting that overdense region dense enough probably requires introducing a dark matter “disk”, which can happen in principle but requires more specific features of the dark matter. But if you do all that, the dark matter particles can decay to a Higgs and a neutrino. That neutrino would then only have to go a short way through the earth, where it could interact and produce an upward-going tau lepton that could produce an air shower visible to ANITA.
I mean… sure.
Quibble: it’s “tau”, not “tauon”. “Tauon” isn’t a thing. You only see that when someone thinks that “tau” mustn’t be right because of all the other “…ons”.
I was at a conference once giving a presentation on a neutrino-related topic, and one of the questions was why I was wasting my time including the calculations for the Dirac case, when it had absolutely definitively been proven to 7 sigma confidence that the neutrino was definitely Majorana.
I just said that the questioner should then feel free to use the Majorana calculations that I had also included.
I was at a conference once giving a presentation on a neutrino-related topic, and one of the questions was why I was wasting my time including the calculations for the Dirac case, when it had absolutely definitively been proven to 7 sigma confidence that the neutrino was definitely Majorana.
I just said that the questioner should then feel free to use the Majorana calculations that I had also included.
One prediction of the Majorana model is that it should sometimes be possible to get a double beta decay with no neutrinos released. Some groups claim to have observed this.
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