# B The neutrino spin

1. Jun 4, 2016

### alba

Is there any experimental evidence that neutrino spin is equal to the electron spin ( I think h/4pi, right?)

If it is just an assumed intrinsic property,can you explain on what grounds it is assumed as such, is it juas a matter of parity?

2. Jun 5, 2016

3. Jun 5, 2016

### alba

I read those (an many more) articles, do they contain experimental data? I must have missed them.I am asking if there is any concrete experimental data that confirms that it has indeed some spin and that its magnitude is h/4pi

Last edited: Jun 5, 2016
4. Jun 5, 2016

### davenn

there probably is, but I haven't found any info in my looking

that link I gave ( and many others I read) identifies the type of particle the neutrino is ... that info was gained by experimentation
it is identified in being part of the same group as the electron ie the lepton, ergo it has the same spin as the electron ...
The neutrino has half-integer spin (ħ⁄2)

not sure what else you want ?

maybe some of our particle physicists can take it further ?
@ZapperZ

Dave

5. Jun 5, 2016

### alba

Thanks , Dave, I was just looking for some experimental evidence, or at least some indirect confirmation. I know the Standard Model assumes all fermions, leptons have spin 1/2. But suppose (ad absurdum) neutrino has no spin, what happens, what is the problem?

6. Jun 5, 2016

### ChrisVer

It is by conservation of angular momentum.
Also the helicity of the neutrinos has been measured (eg the Goldhaber et al experiment)

7. Jun 5, 2016

### my2cts

Accurate measurements are available of neutron decay.
My guess, as I have not analysed the paper, is that these would have shown up - Nobel prize winning - deviations from the present electroweak theory if the neutrino would not be (nearly) massless with spin 1/2.
http://neutron.physics.ncsu.edu/UCNA/protected/55_2_0119.pdf

8. Jun 5, 2016

### Staff: Mentor

A different spin would lead to different angular distributions of the decaying particles, something measured in the experiments of Wu and Goldhaber, and later much more precisely with particle accelerators.

9. Jun 5, 2016

Staff Emeritus
If the neutrino were spin-0, it would mean angular momentum is not conserved.

If the neutrino were spin-3/2, $\frac{B(\pi \rightarrow \mu \nu \overline{\nu})}{B(\pi \rightarrow e \nu \overline{\nu})}$ would be some number other than the measured ~12,000.

10. Jun 6, 2016

### alba

Why can't spin be added during the process?

11. Jun 6, 2016

### alba

What I meant is that the spin musn't necesseraly be an intrinsic property. Like in a billiard ball any spin can be generated by the cue or by a collision, the value of the spin +1/2 or -1/2 (or any other value) could be determined only by the process. This would also eliminate the akward assumption of an antineutrino antiparticle of itself. Does the linked experiment rule out this possibility? Ihave not the expertise to reach any conclusion.

12. Jun 6, 2016

### davenn

ahhh but it is

13. Jun 6, 2016

### Staff: Mentor

@Vanadium 50: I think you just want one neutrino in the decay.
What do you mean by "added"?
Angular momentum is conserved.

Assumption of what? It is possible that neutrinos are their own antiparticles, but they don't have to be - measurements are not precise enough yet to distinguish between those possibilities.

14. Jun 6, 2016

Staff Emeritus
You're right. The one I want is $\frac{B(\pi \rightarrow \mu \nu)}{B(\pi \rightarrow e \nu)}$

(Bonus points for guessing which reaction I was starting to write down when I thought better of it - and then blew it)

15. Jun 6, 2016

Staff Emeritus
But that's how it's defined. You can talk about angular momentum that is not intrinsic, but that is by definition, not spin.

16. Jun 6, 2016

### ChrisVer

By the way, have you ever seen a billiard ball "spinning" at half-integer angular momentum?