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Since we now know that neutrinos have a rest-mass, we ought to be able to observe relatively slow-moving neutrinos. Have we seen any?

Billy Istiak
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j4nd3r53n
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    Related: https://physics.stackexchange.com/q/267035/226902 , https://physics.stackexchange.com/q/267492/226902 , https://physics.stackexchange.com/q/245963/226902 – Quillo Feb 10 '22 at 14:44

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The energy of neutrinos depends on the process that formed them (more energetic reactions create more energetic neutrinos). Since neutrinos have no charge, there is no way to use electric fields to accelerate them (we can do that with charged particles in colliders).

We may see neutrinos of the Cosmic neutrino background (CNB), which have so little energy to be non-relativistic (for a comparison of neutrino energies and their cross-section according to the process that created them, see this plot).

There are real proposals for detectors of neutrinos from the CNB, but we have detected none to date (as far as I know).

Quillo
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They haven't been seen and I doubt they will ever be. Since they were created they have traveled almost undisturbed. I find it difficult to believe most neutrinos from the Sun travel through the Earth. But it seems to be the case. Because they have mass and interact (though weak), they should be stoppable. Maybe in between the space between the two stars of a binary system, they could collide head on inelastically. If the stars emit the same number of neutrinos as the Sun, this should happen once in a while, though I'm not sure if this can happen inelastically.

We're not sure though. Of the zillions there might actually be some slow ones.

MatterGauge
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  • There are plenty of slow neutrinos, but they're invisible to our detectors. As I said here, the best detectors using current technology can only detect neutrinos with kinetic energy around 300,000 times their rest mass. Sure, some solar neutrinos get absorbed by Earth (otherwise we couldn't build neutrino detectors), but it's said that a light-year of lead would only stop ~50% of typical solar neutrinos passing through it. – PM 2Ring Feb 10 '22 at 15:24
  • @PM2Ring. What slowed the slow ones down? Or are the solar neutrinos created slow? Imagine, passing through a lightyear of led... – MatterGauge Feb 10 '22 at 15:28
  • In the processes that produce neutrinos (or antineutrinos), the kinetic energy is randomly shared by the neutrino & the electron or positron (with a tiny bit of KE going to the hadron or nucleus). This results in a continuous energy spectrum for the beta particle, so the energy spectrum of the neutrino must have a continuous spectrum too. – PM 2Ring Feb 10 '22 at 15:45
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    Also, neutrinos that have travelled a long distance have been red-shifted by cosmological expansion. That especially applies to the CNB neutrinos, which have been red-shifted a lot more than the CMB photons, since they were released about 1 second after the Big Bang. – PM 2Ring Feb 10 '22 at 15:46