Why there is not righthanded neutrino,but there are righthanded electrons?

1. Oct 12, 2011

ndung200790

Why there are not exist righthanded neutrinos,but exist righthanded electrons in the lepton family?
Thank you very much for your kind helping.

2. Oct 12, 2011

DrDu

I thought one of the possibilities to explain the mass of the neutrino would involve the existence of heavy right handed neutrinos?

3. Oct 12, 2011

Staff Emeritus
Suppose there were a right handed neutrino. How would you know?

4. Oct 12, 2011

ndung200790

So,the not existing of righthanded neutrino is based on experiment but not on theoretical theory?

5. Oct 12, 2011

phyzguy

Correct - it is an experimental fact. I think everyone was surprised that nature acts this way, which is why Lee and Yang got the Nobel Prize in physics for discovering that nature violated parity. As for why, I don't think anyone knows.

6. Oct 12, 2011

ndung200790

By the way,the C,P symmetry violations in weak interaction is based on experiment or on theoretical theory or the both?

7. Oct 12, 2011

ndung200790

Now I can understand the problem,thank you very much for your answer.

8. Oct 12, 2011

arivero

More precisely, there is not non-null coupling of the righthanded neutrino to known forces.

A right handed neutrino, even if not coupled, is nicer mathematically, as it allows for a lot of GUT theories, as well as models of neutrino mass oscilations. And the MSSM extended with right handed neutrinos has 128+128 helicities, which gives a lot of playroom for our local version of cranky circle squarers.

Last edited: Oct 12, 2011
9. Oct 12, 2011

phyzguy

There are strong theoretical reasons to believe that the product of CPT is a true symmetry of the universe. Once it was discovered that P - symmetry was violated, people started looking at the other discrete symmetries (C and T) to see if they were violated as well. They soon discovered that CP is also slightly violated, which means that T must be violated if CPT is to be preserved. You might start here:

http://en.wikipedia.org/wiki/CPT_symmetry

10. Oct 12, 2011

TrickyDicky

Well that is one way to preserve CPT in the presence of a CP slight violation, another theoretical option instead of a T violation is a Lorentz slight violation.

11. Oct 12, 2011

phyzguy

Wouldn't a Lorentz violation imply CPT violation?

12. Oct 12, 2011

TrickyDicky

Not exactly, only if you assume Lorentz symmetry like the CPT theorem does and then it goes the other way around, since the starting point is Lorentz invariance, if CPT violation is found it implies Lorentz violation.
But CPT symmetry by itself doesn't assume Lorentz symmetry.

13. Oct 12, 2011

TrickyDicky

Maybe this can be better understood in this way: mathematically you can build a CPT symmetry in various ways, adding the CP asymmetry constraint reduces the possibilities but still we can do it either by having a T asymmetry and Lorentz symmetry or alternatively keeping T symmetry and postulating Lorentz asymmetry, this last way has the advantage of suggesting a mechanism for the CP asymmetry(neutrino left-handedness).
When physical considerations enter, classically Lorentz invariance was assumed since no experiment was able to show violations.

14. Oct 12, 2011

Parlyne

Independent of Lorentz symmetry, if CP is violated and T is not CPT is violated. Breaking Lorentz symmetry does not allow you to preserve CPT in such a case. In fact, the Lorentz violation will follow from the CPT violation.

15. Oct 12, 2011

TrickyDicky

Independently of Lorentz symmetry you are right.
I was showing examples in wich CPT symmetry is made dependent on Lorentz symmetry.

Like I said in post #12, this is stated in CPT theorem wich assumes dependency on Lorentz symmetry

16. Oct 13, 2011

ndung200790

I think that because neutrino has nearly zero mass,so that the left and right vertor currents of Dirac field are conserved independently,then this permits the violation of parity happening.But if neutrino were massive,then there must be exist the righthanded neutrinos.Is that correct?

17. Oct 13, 2011

phyzguy

I'm a little fuzzy on this point. I thought this was the case, since if the neutrino has mass I can always boost until I am traveling faster than the neutrino, in which case what was a left-handed particle becomes a right-handed particle. However, we detect only left-handed neutrinos and right-handed anti-neutrinos. Many theorists believe in the possibility that the neutrino is a Majorana particle, in which case it is its own antiparticle. In this case, if you boost until you overtake a left-handed neutrino, you will see a right-handed anti-neutrino. This implies that lepton number is not conserved, since in one frame I see a particle and in one frame an anti-particle. Lepton number violation has never been seen, but there are experiments looking for it.

18. Oct 13, 2011

ndung200790

I think that in this frame you see particle and in other frame you see antiparticle but the lepton conservation were not violated,because neutrino and antineutrino is the same particle.

19. Oct 13, 2011

ndung200790

I think by your argument above,the left-right asymmetry means the Lorentz symmetry violation.

20. Oct 14, 2011

TrickyDicky

It follows from this argument that if we can't make that kind of boost (no detection of right handed neutrinos) we are always travelling faster than massive neutrinos (thus only left-handed neutrinos are detected), and that could only happen if neutrinos had a preferred rest frame implying a Lorentz violation, as ndung200790 noticed in the previous post.