227

Imagine you are an electron. You have decided you have lived long enough, and wish to decay. What are your options, here? Gell-Mann said that in particle physics, "whatever is not forbidden is mandatory," so if we can identify something you can decay to, you should do that. We'll go to your own rest frame--any decay you can do has to occur in all reference ...


68

Strictly speaking, it is indeed incorrect that neutrinos travel at "close to the speed of light". As you said, since they have mass they can be treated just like any other massive object, like billiard balls. And as such they are only traveling at nearly the speed of light relative to something. Relative to another co-moving neutrino it would be at rest. ...


51

The situation with supernova is not about speed of flight but about time of emergence. A type IIa supernova candidate is big, even with the vastly powerful explosion of the core it takes hours to blow the envelope off and expose the violence of the interior—and it is only after that happens that the star becomes brighter in the electromagnetic spectrum. But ...


45

The statement is true for decays, where lifetimes can be measured. It is not true for interactions though. A suicidal electron meeting a positron has a good probability to disappear, together with the positron, into two gamma rays, at low energies. Electron-positron annihilation It is intriguing that this is not true for neutrinos. If an electron ...


45

I thought the particle was hypothesised in order to maintain the conservation of momentum in a beta-decay. If it was massless, this would have no effect, right? This is where you are confused. Having no mass does not mean having no momentum. I think you are probably thinking of momentum as Newtonian mechanics would express it : $P=mv$ However Einstein came ...


42

No, you have a wrong understanding of the Wikipedia statement. Wikipedia says only "Gravity is extremely weak" on a subatomic scale. It does not say "neutrinos are not affected by gravity". They cannot pass through a black hole just as light cannot pass through a black hole. Photons are even lighter (no mass is as light as can be!) than neutrinos, and ...


42

The existence of the neutrinos was established using energy and momentum conservation in neutron decays. There have been experiments with neutrino and antineutrino beams both at Cern and Brookhaven and have established their interaction crossection with matter. To get one neutrino interacting in the detector it means that thousands have passed without ...


39

To maintain lepton number as a conserved quantity. Consider, in detail, what's going on in a beta decay (well, I'm going to ignore the nuclear context). The reaction is then $$ n \longrightarrow p^+ + e^- + \nu \,,$$ where you should take the symbol $\nu$ to mean some neutrino (without prejudice about matter-type or anti-matter-type for the moment). There ...


38

Yes, neutrinos "hit" electrons all the time inside the sun, on their way to getting out, which results into the resonant conversion of their flavor, predicated on the changing effective index of refraction. They interact with electrons, and protons, and neutrons, etc... through their favorite interaction, the weak , not electromagnetic interaction. (They can ...


34

This was a reference to the apparent measurement that neutrinos travel faster than light. FTL travel can be used to travel back in time (though the procedure for doing so is somewhat involved). Sadly the apparent superluminal speed turned out to be due to experimental errors: a fibre optic cable attached improperly, which caused the apparently faster-than-...


34

The neutrino mass, as you have defined it, will not be part of any calculation of the visible mass of the universe. There will indeed be a very large number of $>$MeV neutrinos whizzing about that have been emitted by stars during their nuclear burning lifetimes. These neutrinos would be a very small contributor to the dark matter in the present day ...


30

You have a few longer answers which were already updated, but here is a concise statement of the situation in mid-2014: An independent measurement by the ICARUS collaboration, also using neutrinos traveling from CERN to Gran Sasso but using independent detector and timing hardware, found detection times "compatible with the simultaneous arrival of all ...


30

The detector that took that image--Super Kamiokande (super-K for short)--is a water Cerenkov device. It detects neutrinos by imaging the Cerenkov cone produced by the reaction products of the neutrinos. Mostly elastic scattering off of electrons: $$ \nu + e \to \nu + e \,,$$ but also quasi-elastic reactions like $$ \nu + n \to l + p \,,$$ where the neutron ...


30

An interesting aspect not touched on by existing answers at the time of writing. What was observed about beta decay is that for the same change in the atomic nucleus, the emitted electron (which, being charged, is easily detected) had a wide range of energies. Some electrons come out with high energy, some with low, even though the same initial and final ...


28

Light rays are a combination of oscillating electric and magnetic fields, and both fields interact strongly with charged particles. That's why light rays are strongly scattered by matter. The relationship between light rays and photons is more complicated than you might think, but this also explains why photons scatter off charged matter even though the ...


28

Why did he figure it couldn't be detected? Because the neutrinos interact so weakly that it was for some time believed that it will not be possible to detect. E.g. in the 1934 paper by Bethe and Pierls, where the crossection for neutrino interacting with matter was computed, they concluded that "It is therefore absolutely impossible to observe ...


27

There are other neutral particles with antiparticles, such as the neutron and the $K^0$ meson. In those cases we have a microscopic theory that says those particles are made of quarks: for instance, the $K^0$ is made of a down quark and an anti-strange quark, while its antiparticle the $\bar K^0$ is made of a strange quark and an anti-down. The neutrino is ...


27

The experimental detection of slow neutrinos is indeed a big problem, but one that is very important. The cosmic neutrino background is at a temperature of around 2K and likely to consist of non-relativistic neutrinos for plausible neutrino rest masses - with a density of around 340 cm$^{-3}$ (all flavours). It is at this low temperature for precisely the ...


26

Can neutrinos “hit” electrons? Yes, e.g., Image credit


26

There are many different types of neutrino detectors, using various techniques to turn a neutrino interaction into an electrical signal. Detectors that use a large quantity of water or oil, like Super-Kamiokande in Japan and MiniBooNE at Fermilab, are Cherenkov detectors. Their basic principle of operation is as follows: when neutrinos pass through the ...


23

The cross-section for neutrino interactions is energy dependent. For solar neutrinos at $\sim 0.4$ MeV, which would likely dominate any neutrinos likely to interact (the cosmic background neutrinos have way low energies) , the cross-sections are $\sigma \sim 10^{-48}$ m$^2$, for both leptonic processes (elastic scattering from electrons) and neutrino-...


23

Light takes a long time to escape not because the sun is particularly large but because they run into a lot of electrons and protons on the way. So the light is taking a path that is more like a drunkard's walk than a straight line. With neutrinos, though, they interact very rarely. A typical neutrino can pass through more than a light-year of lead before it ...


23

This has been attempted, however the energy released in a neutron decay is a shade under a MeV and the neutrino masses are probably below $0.1$ eV. The energy of the neutron decay simply cannot be measured accurately enough to determine the neutrino mass. The closest estimate I kmow of is reported in Neutrino mass limit from tritium beta decay by E. W. ...


22

As commented by Knzhou, neutrinos come in three different types: electron-, muon-, and tau- neutrinos. Each is paired with the particle it is named for in the sense that it is involved in particle reactions involving only that type of neutrino. The most common type of neutrino is the electron neutrino, which is often just called a neutrino even though it ...


21

At tree level, a matter-antimatter annihilation reaction doesn't just produce gamma rays, nor can you exclude neutrinos in the final state. Even the simplest such reaction can—given enough energy—produce a variety of particle-pairs. However, those pairs are subject to two subsequent processes: If the particles are not stable, they will decay towards ...


21

Here is the KATRIN experiment: The KArlsruhe TRItium Neutrino (KATRIN) experiment, which is presently being assembled at Tritium Laboratory Karlsruhe on the KIT Campus North site, will investigate the most important open issue in neutrino physics: What is the absolute mass scale of neutrinos? ...... Predecessor experiments at Mainz and Troitsk were able ...


21

As others have noted, the neutrinos come from the sun. Given that, there are two broad ways of estimating the flux of neutrinos: one is theoretical, and the other is experimental. The theoretical way is based on the Standard Solar Model. This is a well understood model with solid experimental validation, and astronomers and astrophysicists therefore have ...


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