# Measuring neutrino spin

1. Feb 23, 2008

### kilokhan

If a neutrino is unaffected by EM and strong forces, and only interacts weakly, how is it possible to measure its spin? I don't mean measurement in a practical sense, as in looking at pion decay.. but how it could be measured in principle. I guess what I mean is are there any physical phenomena where it is a factor (apart from decays).

2. Feb 23, 2008

### pam

Why do you rule out decays? That is how most particle spins are determined in practice and in principle.

3. Feb 23, 2008

### kilokhan

what about the usual thought experiment used to explain spin, with magnetic fields? do we expect neutrinos to be deflected?

4. Feb 23, 2008

### humanino

Without thinking too much about it, I think they would not be deflected because they don't have magnetic moment (because they don't have charge...). The magnetic moment of neutrino is less than $$0.74 \times 10^{-10} \mu_{B}$$ (upper known limit in terms of the Bohr magneton, which is the natural unit)

Now maybe a higher order process I have not thought of right now could indeed deflect this neutrino...

Apart from that, it is possible to deduce the spin of the neutrino from more general principles, without actually measuring it.

5. Feb 23, 2008

### malawi_glenn

Well you said in your first post that they dont interact via the EM-force so have answered that one yourself :-)

6. Feb 24, 2008

### CarlB

We detect neutrinos by looking at how they interact with other particles. A good choice is electrons. Of course neutrinos don't leave a track but electrons do.

The target electrons all have spin 1/2 but they are oriented randomly. We can look at the characteristics of the electrons that are hit to learn stuff about the spin and orientation of the neutrinos.

For example, if the neutrinos had no spin, then the interaction would follow the symmetry of a spin-0 + spin-1/2 system. Since the spin-1/2 systems aren't polarized, the resulting collisions aren't polarized either.

Instead, the neutrinos turn out to have spin-1/2 and they ARE polarized, with their spin aligned in the direction of travel (or antiparallel). You should be able to detect this in the characteristics of the ejected electron.

7. Feb 25, 2008

### Barmecides

I would say it can be measured like for every other particle.
Indeed differencial cross-section depends of the spin of incoming and outgoing particles. So looking at the differencial cross-section of well chosen observable (like an angle) will answer your question.