[HN Gopher] How light is a neutrino? The answer is closer than ever ___________________________________________________________________ How light is a neutrino? The answer is closer than ever Author : _Microft Score : 29 points Date : 2022-02-15 20:47 UTC (2 hours ago) (HTM) web link (www.nature.com) (TXT) w3m dump (www.nature.com) | gus_massa wrote: | > _These data imply an upper bound of 0.9 eV, which goes down to | 0.8 eV when combined with the earlier results._ | | For comparison, the mass of an electron is approximately | 510,998.950 eV and the mass of a proton is 938,272,088. eV. | Koshkin wrote: | Too many digits... TL;DR: .5 MeV and .9 GeV, respectively. | gus_massa wrote: | I prefer to use the same unit for all numbers because it | makes the comparison easier. The bound of mass of the | neutrino is really low. Using M and G hides it. | | I was going to write: | | > _For comparison, the mass of an electron is approximately | 500,000 eV and the mass of a proton is 900,000,000 eV._ | | But all the numbers I wrote are measured experimentally. Both | values have a lot of experimentally measured digits! I only | removed the part that overlaps with the uncertainty, because | the notation with parenthesis is somewhat confusing. | _Microft wrote: | 511 keV and 938 MeV would have been _much_ clearer than a | distinction via comma /point like in 510,998 -> . <- 950 eV | and 938,272 -> , <- 088 eV. | gus_massa wrote: | I agree that the coma/point distinction is confusing. | Also, I'd like to cut all the numbers in the same digit, | but the next digit of the proton is too dubious. | | What about this version: | | > _[an upper bound of 0.9 eV] For comparison, the mass of | an electron is approximately 510998.9 eV and the mass of | a proton is 938272088.0 eV._ | codeflo wrote: | In contrast to some sibling comments, I think using the | same unit is a great idea. It's just that long numbers with | commas are a bit hard to read. At least I think 511 keV and | 938 000 keV would have been another clear way to present | those numbers. | fknorangesite wrote: | > I prefer to use the same unit for all numbers because it | makes the comparison easier. | | Thank you; this is a style guideline I wish more writers | would adopt. | throwhauser wrote: | Writing out the digits gives a more visceral impression of | the difference in weight, from roughly one, to a six-digit | number, to a nine-digit number. | pdonis wrote: | That's what the GP wrote. Their electron value has a decimal | point before the last three digits. The GP values are | approximately 510 keV and 938 MeV. | SaberTail wrote: | With the caveat that I'm a few years out from my PhD in the | field, this is both really interesting and also maybe not | surprising. | | This is the best attempt to date to directly measure the mass of | the electron (anti)neutrino. When a nucleus beta decays, it emits | an electron and an antineutrino. If the neutrino were massless, | the electron could carry away the entire energy of the decay. | What they've tried to observe here is the highest energy | electrons, to see if they can see the effect of some of that | energy going to the neutrino mass. It's an amazingly difficult | measurement, since only a very small fraction of those electrons | have the highest energies. | | Measuring the neutrino mass this way would be relatively | unambiguous, which is why it's exciting to see progress. | | That said, there have been other experiments that have put | stronger constraints on neutrino masses, but only for certain | models. Double beta decay experiments look for decays in which | the two neutrinos annihilate each other, allowing the full energy | to be emitted by the electrons. However, this requires the | neutrino to be its own antiparticle. This is allowed, because | neutrinos are neutrally charged, and wed call such a particle a | Majorana particle. The rate of these neutrinoless double beta | decays would tell about the neutrino masses, but only if | neutrinos are Majorana particles. The exact measurement would | also depend on how well we understand the energy levels of the | nuclei involved. So far, the upper limits for neutrino masses | from these experiments are on the order of 0.1 eV. | | And there are cosmological constraints on the neutrino mass from | experiments that look at the cosmic microwave background. Early | in the universe's history (like the first second), the mass of | neutrinos would have influenced how much matter clumped up due to | gravity, which would lead to fluctuations in the microwave | background. Modeling this requires us to understand particle | physics very well at those early high energies, so there's some | uncertainty due to modeling. But again, the limits from these | observations are on the order of 0.1 eV. | | So we now have one direct observation that's consistent with | other, model-dependent observations. The most interesting | scenario would be that KATRIN actually observes neutrino mass as | it gets more data, implying that our models might be wrong. But | even if the experiment doesn't, it's still great to have the | extra constraints. | treeman79 wrote: | https://what-if.xkcd.com/73/ | | Death by Neutrino | dgb23 wrote: | That's a very cool book and a nice gift idea. | [deleted] | 6gvONxR4sf7o wrote: | > The data still do not rule out the possibility that the mass is | zero, says KATRIN member Magnus Schlosser, a particle physicist | at the Karlsruhe Institute of Technology. But other lines of | evidence, in particular from cosmological observations, show that | the neutrino cannot be massless. | | I would love if neutrinos were massless, just because it would be | so _interesting._ The only way they would interact with gravity | would be through the shape of spacetime itself, which for some | reason is a fascinating to me. | pdonis wrote: | _> The only way they would interact with gravity would be | through the shape of spacetime itself_ | | I'm not sure what you mean by this. In General Relativity, | gravity _is_ "the shape of spacetime", so _any_ gravitational | interaction involves the shape of spacetime. | not2b wrote: | The reason we know that neutrinos aren't massless is that they | oscillate between neutrino types. A massless particle must | always travel at c, so it doesn't experience time, so it can't | decay or change into another particle. | kmm wrote: | Only two of the three neutrinos need to be massless, though | that would be quite a curious asymmetry, and everyone expects | all three to have mass. | | A massless particle might not have a restframe or experience | proper time, but it still propagates through spacetime, and | can definitely decay to other massless particles, at least in | theory. After all, moving at the speed of light doesn't | preclude it from interacting with ordinary matter either. | "Luckily", in our universe there are theoretical reasons for | photons to be completely stable (e.g. see | https://arxiv.org/abs/hep-th/9508018 ), but there's no such | general rule. | pdonis wrote: | _> A massless particle must always travel at c, so it doesn | 't experience time, so it can't decay or change into another | particle._ | | This is not correct, although it's a common pop science | misconception. For example, photons are massless, but they | can undergo interactions that, for example, produce particle- | antiparticle pairs. If your statement here were true, photons | would be unable to undergo any interaction at all. | | A correct statement would be, heuristically, that if all | three neutrino flavors were massless, they would all have the | same mass, namely zero, so they would all oscillate exactly | the same way, so any neutrino state that started out as one | particular mixture of flavors would stay the same mixture | forever. For example, neutrinos that were produced in an | interaction like those in the Sun, which only produces | electron neutrinos, would stay electron neutrinos forever. | But this would also be true if the different neutrino flavors | all had nonzero mass, but all the _same_ nonzero mass. The | only way for the mixture of neutrino flavors to change as the | neutrinos travel is for the different flavors to have | _different_ masses. One of those masses could in principle be | zero, but only one, not all three. | SaberTail wrote: | Based on what we've observed, and our current standard model of | particle physics, only one type of neutrino can be massless. | We've observed neutrinos oscillating flavors (for example, an | electron neutrino later interacting as a muon neutrino), and | the rate of those oscillations suggest nonzero mass differences | between the three different types. So even if one of them is | zero, the other two cannot be. | dilippkumar wrote: | Not a physicist, but I have questions. | | > Based on what we've observed, and our current standard | model of particle physics, only one type of neutrino can be | massless. | | Is the standard model complete to the point where we can | predict how many types of neutrinos exist and what their | properties should be? | | I always thought that the standard model as a set of | equations (a model) that fits observed data, without | venturing far into "why this model is the governing principle | for our universe". That is, it is not able to explain things | like "why an electron comes with two heavier varieties". | | Are neutrinos somehow different in a way that we can | understand them to the point where we know things like "only | one type of neutrino can be massless"? | SaberTail wrote: | The LEP experiment at CERN (the LHC now inhabits the same | tunnels) collided a lot of electrons and positrons to | create a lot of Z bosons. The standard model describes | these interactions really precisely. And we can observe how | often the Z boson decays "invisibly" to particles we can't | detect. The rate it does so tells us there are three | neutrinos with masses less than the Z boson. So that's | established. Could there be more, heavier ones? Possibly. | | We observe neutrino oscillations through a variety of | channels. We first observed fewer (electron) neutrinos from | the sun than expected, suggesting they were oscillating to | other flavors. And this has been further observed in | neutrinos produced in the atmosphere by cosmic rays, | neutrinos produced by decays of particles in beams, and | neutrinos from nuclear reactors. | | The best explanation, and the one that fits the standard | model, is that the pure "flavor" (electron, mu, tau) | neutrino states are mixtures of pure "mass" states. And | from those different channels, which look at different | energies and flavors of neutrinos, we can work out what | those mixtures are. | | When you go through all the math, it turns out the | oscillations depend on the differences of the squares of | the masses of the pure mass states. And we observe | oscillations that tell us that two of these differences are | nonzero. That is, if there are mass states 1, 2, and 3, | then we know that (mass 1)^2 - (mass 2)^2 is nonzero, and | (mass 3)^2 - (mass 2)^2 is also nonzero. So this implies | that at least two of them must have nonzero masses. | whatshisface wrote: | Adding new particles would change existing particles, | unless the new particles were very particularly set up so | that they either explained why the existing particles were | they way they were (more common) or didn't change them | (less common). | | That's because every quantum field is coupled with every | other field, all the time, and everywhere. | ephimetheus wrote: | The oscillation mechanism that we've come up with (and that | fits data from reactor experiments to astronomical ones | pretty well) only works if the mass eigen states of the | neutrinos are different from their flavor eigen states, | otherwise there is no mixing. | | We can measure the mass differences between the neutrinos | pretty well through these oscillation experiments, but this | also doesn't tell us which the mass hierarchy. It could be | bottom up or the other way round. | | In principle, one neutrino could be massless and the mass | differences we've measured so far would still be correct. | | Aside from this, pretty much anything is on the table. | Neutrinos being their own anti particles? Maybe. Fourth | generation of neutrinos? Could be. | whatshisface wrote: | > _The only way they would interact with gravity would be | through the shape of spacetime itself, which for some reason is | a fascinating to me._ | | Photons are massless particles, but alas, they still gravitate | because it's mass-energy that gravitates, not rest mass. | 6gvONxR4sf7o wrote: | > it's mass-energy that gravitates, not rest mass. | | TIL! That's a very fun fact. I never learned that, and I have | a physics undergrad! Or I forgot it, which is just as likely | this far out. | Koshkin wrote: | So... Just like the neutrinos? | whatshisface wrote: | Neutrinos aren't massless, but quite like the neutrinos | because they're light. Err, unheavy. | _Microft wrote: | You can read more about the experiment here: | | https://www.katrin.kit.edu/68.php | | https://en.wikipedia.org/wiki/KATRIN (the German WP entry is | better though if you don't mind using an online translator) ___________________________________________________________________ (page generated 2022-02-15 23:00 UTC)