# Helicity of neutrinos

1. Feb 20, 2009

### spidey

All neutrinos are left handed and all anti neutrinos are right handed.so,it should be lorentz invariant and travel at speed of light. if it travels at c,then it is massless. but, neutrino oscillation requires mass? why there is contradiction?

2. Feb 20, 2009

### clem

A massive neutrino will generally have a helicity h=-v/c. This is so close to 1 that there is as yet no experimental spin measurement accurate enough to measure the difference.

3. Feb 20, 2009

### humanino

You don't even really need helicity to get a "contradiction". Neutrino oscillation can happen only of neutrinos of different flavors have different masses. If a neutrino is created by the Sun in an electronic flavor, it has a given energy and a given momentum. How can it be measured on Earth with a different mass if energy and momentum are conserved ?

I mean to say that the issue of symmetries in neutrino oscillation can not be thought of academically without proper treatment of (elementary) quantum effects. The answer is indeed what Clem points out : the problem is academic because the symmetry violations are so tiny, they merely stem from the Heisenberg's inequalities. I don't mean to say it is trivial either, such a proper treatment is rather tedious.

Neutrino mass, mixing, and flavor change
C. Amsler et al., Physics Letters B667, 1 (2008)

4. Feb 20, 2009

### Sideways

Well, you hit the nail on the head. All neutrinos are left handed (and anti-neutrinos right handed) only in the approximation mass=0. Now that we know they have mass, this rule simply isn't true any more (that is, it's possible to define a reference frame where neutrinos are right-handed).

5. Feb 20, 2009

### spidey

thanks for the replies mates..so not all neutrinos are left handed..there is a reference frame where it will be right handed..

6. Feb 20, 2009

### spidey

Thanks Humanino..but i dont understand the technical paper, very difficult to follow with my limited knowledge..

7. Feb 21, 2009

### clem

The usual terminology as I understand it is to call particles that enter the weak interaction with V-A (The sign is different in some formalisms.) are called "left handed" so that all leptons are left handed, even if they do not have pure alignment.

8. Feb 21, 2009

Staff Emeritus
I think there is a confusion between helicity and chirality. The weak interaction couples to the left-handed chiral component of the neutrino. If the neutrino were massless, that would also be the left-handed polarization state of the neutrino.

Since the neutrino has mass, one can boost to a frame where the polarization becomes right-handed, but there is still a left-handed chiral component, so it still feels the weak force.