# Low energy Neutrinos

1. Jun 7, 2007

### Wallace

Hi guys, I have a basic question about neutrinos. Since it is now supposed from both cosmological and particle physics observations that neutrinos have a small rest mass, what are the lowest energy (and hence lowest speed) neutrinos we can/have observed?

Since they are so weakly interacting (and hence do not readily 'lose' energy) and most that we observe come from the Sun (nearby), I'm assuming that we only observe relativistic neutrinos.

Would it be possible (or make any sense) to study non-relativistic neutrinos? Or is the rest mass so small that even if a neutrino could someone be slowed to rest, the slightest force on them would cause them to be accelerated back to relativistic speeds.

Do my questions even make sense in this context?

2. Jun 7, 2007

### malawi_glenn

The cross section for neutrinos decrases with decreasing energy.
I do not remember what the lowest detected neutrinos is for the moment.

The neutrinos only interact weakly (gravity is neglectable).

3. Jun 7, 2007

### Wallace

Interesting, I was wondering about cross-section vs energy. I should know this, but I've clearly forgotten, but why does this decrease for neutrinos but for say X-rays the cross-section increases as energy is decreased? Is this a massive particle vs photon difference or is it not so simple? (I suspect the later)

4. Jun 7, 2007

### malawi_glenn

It has to do with the fact that neutrinos only interact trough the weak force, but photons via the electromagnetic (with is also much stronger than weak force). So you have two totaly different forces and therefore different interaction behaviors and cross sections.

5. Jun 7, 2007

### Wallace

Ah yes of course

6. Apr 30, 2008

### malawi_glenn

Or, to make it clearer. The coupling constant of the weak force is infact as large as 0.58 times the coupling constant of the EM force, but the RANGE of the weak interatcions is incredible small! About 1/1000 of the radius of a proton!