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Why can't neutrinos be brought to rest? 
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#37
Sep2613, 06:52 PM

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P: 11,631

Neutrinos are fermions. They have wellpredicted (by the standard model) interactions with matter, and if they are slow their interaction crosssection is low.



#38
Sep2613, 06:58 PM

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#39
Sep2613, 07:04 PM

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P: 11,631

There are upper limits on the mass. Sure, the crosssection at low energies will depend on the (still unknown) mass, but as far as I know it will be small for all possible masses.



#40
Oct713, 08:40 AM

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Easier said than done, of course. 


#41
Oct713, 11:43 PM

Sci Advisor
PF Gold
P: 9,389

We would need an experimental setup capable of isolating a single neutrino  which would be an impressive accomplishment. It's relatively easy to isolate an electron.



#42
Oct813, 12:47 PM

P: 172

OK, I think I understand now thanks to your help. Let's see if I got it right.
The original question could have been phrased as a paradox. If neutrinos are massive, why don't we see a spectrum of nonrelativistic velocities for them as we do for other massive particles? Wikipedia says that the lower limit for neutrino velocities is 0.999976 c. The apparent answer requires two logical steps. First, when neutrinos are emitted: (mfb put numbers on it, and that helped me to understand.) Second, after emission: Because neutrinos interact so little with other particles, they do not become thermalized. They tend to conserve whatever energy they were emitted with. Put those two things together and we can see that it is possible to have neutrinos at any speed 0<v<c. However, nonrelativistic speeds are very improbable. The seeming paradox comes from confusing what's possible with what's probable. Secondary confusion comes from using the word possible in the ideal sense, contrasted with possible pragmatically in the laboratory. 


#43
Oct813, 03:08 PM

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#44
Oct913, 02:04 AM

P: 381

What is the shape of the tail of the beta decay?
If 100 % of the antineutrinos emitted by tritons have energy under 18 keV (because that is the total energy of the beta decay), what percentage have energy under 1800 eV? 180 eV? 18 eV et cetera? 


#45
Oct913, 09:43 AM

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P: 11,631

There is a formula for the electron energy spectrum. The neutrino energy is the difference between the total energy and the electron energy.
Close to the endpoint (and neglecting the neutrino mass), the probability is quadratic with the difference to that endpoint. The fraction of neutrinos below 1800 eV is roughly 1% (guessed, should be right up to a factor of ~5), the fraction below 180 eV is roughly 0.001% and so on  every factor of 10 reduces the fraction of neutrinos by a factor of 1000. 


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