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

[ndung200790]

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

 

[DrDu]

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

 

[Vanadium 50]

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

 

[ndung200790]

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

 

[phyzguy]

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

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.

 

[ndung200790]

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

 

[ndung200790]

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

 

[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.

 

[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

 

[TrickyDicky]

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.

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.

 

[phyzguy]

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.

Wouldn't a Lorentz violation imply CPT violation?

 

[TrickyDicky]

Wouldn't a Lorentz violation imply CPT violation?

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.

 

[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.

 

[Parlyne]

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.

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.

 

[TrickyDicky]

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.

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

In fact, the Lorentz violation will follow from the CPT violation

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

 

[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?

 

[phyzguy]

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?

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.

 

[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.

 

[ndung200790]

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

 

[TrickyDicky]

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.

It follows from this argument that if we can't make that kind of boost (no detection of right handed neutrinos) we are always traveling 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.

 

[kaksmet]

Hello,

there is a bit of confusion in this thread originating from the difference between helicity and chirality.
Left/Right handed chirality is a property of a particle which is independent of reference frame (i.e. a lorentz invariant). Left/Right handed helicity is not. Helicity is the projection of the spin on the momentum direction of the particle and hence, if one makes a boost to a reference frame which has larger momentum than the particle the helicity changes.

For massless particles, helicity and chirality is the same. There does not exist any reference frame where the helicity is changed, but for a massive particle it is not.

In order to create a mass term for neutrinos (the "usual", i.e. Dirac type) one needs a right handed neutrino in the chiral sense. But a special feature of the neutrino, since it does not carry any electric charge nor any color, it has the possibility to be its own anti-particle and one could then form another type of mass term (called Majorana mass).

There is in the standard model, no problem with introducing a right handed neutrino and create a mass for the neutrino. However, this has the very unpleasant feature that we would postulate the existence of a particle which we essentially have zero possibility to detect experimentally.

There are experimental attempts to determine if the neutrino is majorana or dirac type, such as neutrino less double beta decay, but these are extremely rare.

Hope this could shine some light on things.

 

[TrickyDicky]

The helicity/chirality difference is perfectly understood and actually your comment just reinforces the logic from phyzguy arguments and the conclusions that can be inferred from them about LV.

 

[yoron]

So what do the LHC say about antiparticles?
Do they break CPT symmetry, or hasn't that been studied yet?

 

[yoron]

I tried to look but couldn't find anything more than they were in a preliminary stadium, able to trap anti atoms for a thousand seconds.

But I found this.

Interesting effect at the Tevatron hints at new physics

A question.

1. How serious should one take such a discovery (CPT symmetry breaking), some seem to consider it to be the end of the standard model. Or does it just mean that some parameters need to be redefined?