[HN Gopher] How light is a neutrino? The answer is closer than ever
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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)
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