Why does the neutrino have a magnetic moment?

[Fastman99]

I've read that the neutron has a magnetic moment because it is made of composite particles, namely 1 up and 2 down quarks.

But why does the neutrino, which is electrically neutral and a fundamental particle, have a nonzero (albeit very small) magnetic moment? How is that even possible? Does this have anything to do with electro-weak unification?

 

[tom.stoer]

in the SM the neutrino's magnetic moment should be zero theoretically, but experimentally you can't prove that it's exactly zero, you can only determine a very small upper bound, mainly due to experimental and statistical uncertainties

 

[geoduck]

I've read that the neutron has a magnetic moment because it is made of composite particles, namely 1 up and 2 down quarks.

But why does the neutrino, which is electrically neutral and a fundamental particle, have a nonzero (albeit very small) magnetic moment? How is that even possible? Does this have anything to do with electro-weak unification?

Quantum fluctuations give the neutrino a non-zero magnetic moment.

Loosely speaking, the neutrino can be a mixture of W+ and e-, and the magnetic field couples to the W+ and e-, and afterwards the W+ and e- combine back to neutrino.

It is like the electron self energy, where instead of photon, you have a W+, and instead of the internal line being the same type of line as the external lines, it's a different fermion.

 

[Vanadium 50]

Well, there's non-zero and there's non-zero. At one loop, the neutrino magnetic moment will be of order:

$$\mu = \frac{1}{16 \pi^2}\frac{m_e m_\nu}{M^2_W} \mu_B = 10^{-19} \mu_B$$

That's a very, very, very small number. I'm also not 100% sure that in the SM this doesn't (at least approximately) cancel at one loop. So it could be a lot smaller.

 

[tom.stoer]

in the SM the neutrino mass is exactly zero! what you are talking about is a minimally extenended standard model with some additional mechanism for neutrino mass generation; so a non-zero magnetic moment is an indicator for physics beyond the SM

 

[Vanadium 50]

Well, we know the neutrino mass is not exactly zero. (And the statement "in the SM the neutrino mass is exactly zero" is somewhat debatable - it's a statement that was repeated a lot more often after the discovery of neutrino oscillations than before. The SM permits (but does not require) a nonzero neutrino Dirac mass.)

Nevertheless, this is still really, really small. If I magnetized a pound of neutrinos, it would have a smaller magnetic moment than about picogram of iron.

 

[tom.stoer]

I agree that b/c of neutrino oscillations we can deduce that the neutrino mass is non-zero.

But a standard Dirac mass term is forbidden in the SM.

There is some additional effect required, e.g.
a) a Higgs-coupling which involves both left- and right-handed neutrinos; but we have neither seen right handed neutrinos, nor does the SM Lagrangian contain a neutrino-Higgs-coupling
b) a see-saw mechanism which generates a Majorana mass; but that requires additional heavy neutrino fields again beyond the SM
c) ...

In any case this requires an extenmsion of the standard model.

 

[Fastman99]

Thank you for all your replies, especially you geoduck I liked your explanation of how at smallest enough time and length scales, the neutrino will spontaneously split apart into a W+ and e-, because the uncertainty in energy becomes very large at very small time and space scales.

From what I gather here, it seems that the neutrino magnetic moment is not fully understood in terms of the SM, but rather a slightly "extended" version of it. I don't understand the SM at all yet, I'm still an undergrad physics major whose just starting to learn about the Dirac equation and QFT, but I think it's a very cool theory so far. It's very pleasing to me haha.

Also, while we are discussing weirdness of neutrinos, can anyone explain why all neutrinos have left-handed spin and all anti-neutrinos have right handed spin?

 

[AdrianTheRock]

Because neutrinos only 'feel' the weak interaction, and the weak interaction only touches left-handed particles. To be precise, left-handed as used here refers to chirality, not helicity.

 

[ParticleGrl]

But a standard Dirac mass term is forbidden in the SM.

But you can (and we do!) add a higher dimension operator/a majorana mass for the neutrinos.

The standard model is a collection of symmetries and matter fields- if an operator isn't forbidden by the symmetry, there is no reason not to include it. Sure, its non-renormalizable, but ultimately don't we expect the standard model to be effective?

 

[tom.stoer]

But you can (and we do!) add a higher dimension operator/a majorana mass for the neutrinos.

Of course you can do that - or many other things - it's not forbidden - but you shouldn't call it standard model ;-)

 

[ParticleGrl]

Of course you can do that - or many other things - it's not forbidden - but you shouldn't call it standard model ;-)

As I said, the standard model is the collection of symmetries and matter fields. If you don't add a new symmetry group or a new matter field, why isn't it the standard model?

 

[tom.stoer]

for me the standard model is a very specific Lagrangian; afaik the mass term for neutrinos is set to zero; I don't know a 'standard neutrino mass term', only some proposals

 

[AdrianTheRock]

Isn't the simplest modification to add right-handed, sterile neutrinos and then incorporate these into additional Dirac mass terms in the Lagrangian, just as is done for the quarks and charged leptons?

 

[geoduck]

From what I gather here, it seems that the neutrino magnetic moment is not fully understood in terms of the SM, but rather a slightly "extended" version of it. I don't understand the SM at all yet, I'm still an undergrad physics major whose just starting to learn about the Dirac equation and QFT, but I think it's a very cool theory so far. It's very pleasing to me haha.

You can calculate the neutrino magnetic moment and show that it's proportional to the neutrino mass, so if the neutrino mass is zero, then the neutrino has no magnetic moment. I can't really explain in words why the neutrino must have mass to have a non-zero magnetic moment. My best attempt is to say that EM interactions flip the chirality of a particle, so you need right-chiral neutrinos which enter in through mass terms. If the neutrino has mass, a left-handed neutrino has some amplitude to be right-chiral.

That's all that is meant by extension of the standard model, to give the neutrino a mass term (the neutrino really does have mass, since it has been discovered that they oscillate, and they only oscillate if they have mass) . There are several ways you can give the neutrino mass, with see-saw mechanisms, or just using the standard way leptons are given mass, but also there are questions on whether the neutrino is its own antiparticle (that is, is it a Majorana particle). I think it can be shown that a Majorana neutrino cannot have magnetic moment, even if it has mass. This all assumes the vacuum theory though. If you include temperature/chemical potential effects I don't know how that changes.