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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 d
by SaberTail 5y ago
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.
- irjustin 5y agoThanks for this! Question - if the model-dependent 0.1eV neutrino gets knocked out because the direct observation (anti)neutrino is say "0.3eV confirmed". How does this mess with our current understanding of physics?
- SaberTail 5y agoFor double beta decay, it wouldn't change anything, since neutrinos are not Majorana particles in the standard model. For cosmology, it would suggest something about our understanding of the big bang or early universe is wrong. I'm less familiar with cosmology, however, so I'm not sure in what way things might change. We'd need a new way to explain why matter is less clumpy than it should be due to the massive neutrinos, or why there weren't as many neutrinos then as we think there should have been.
- Aperocky 5y ago> it emits an electron and an antineutrino. If the neutrino were massless, the electron could carry away the entire energy of the decay. Photon are also massless but they are still able to carry energy? Why would neutrinos be different here?
- teraflop 5y agoNobody's saying a massless neutrino would be incapable of carrying energy. Here's how I understand it: If a neutrino has no rest mass, there would be a variety of ways in which the decay energy could be distributed, with some of it going to the electron and some of it going to the neutrino. So over many decays, the electron would be observed to travel at a range of possible speeds. You might see almost all of the energy going into the electron and almost none into the neutrino, or vice versa. But if some of the decay energy goes into the creation of a massive neutrino, that decreases the upper bound on how much energy is left to go into the electron. So by carefully measuring the extent range of energies of the decay products, you can hope to distinguish the massive and massless cases.
- pja 5y agoIn the original theory of the neutrino they were thought to be massless just like the photon. However, experimental evidence suggests that observed neutrinos oscillate between the three neutrino flavours over time & the sane way to allow for that theoretically is for them to to have slightly different small masses. The Wikipedia pages on the Neutrino & Neutrino Oscillation give a good overview.
- xelxebar 5y ago> ... mass of the electron (anti)neutrino. Hrm. My understanding was that electron/muon/tau neutrinos (bizzarely) don't have a defined mass, since the flavor eigenstates are a mix of the mass eigenstates. Did you mean that they are trying to put bounds on one of the particular mass states?
- lnauta 5y agoThey measure what is called the 'effective mass', which is a combination of the masses of the mass-states.