# Why doesn't the Higgs give mass to neutrino? [duplicate]

We learn that elementary particles acquire mass through the weak interactions with the Higgs field. The photon and gluons do not interact weakly, do not coupe with the Higgs field and for this reason remain massles. Left neutrinos do interact weakly, but according to the diagram from Wikipedia do not interact with the Higgss.

Why doesn't the Higgs interact with neutrinos and how do neutrinos acquire mass?

## marked as duplicate by John Rennie, stafusa, Emilio Pisanty, SchrodingersCat, Daniel GriscomOct 18 '17 at 0:35

• – Andrei Geanta Oct 12 '17 at 6:50
• @Arthur The link is several years old while this is a quickly developing area of physics. Perhaps today there may be newer opinions. – safesphere Oct 12 '17 at 6:53
• While this is still under current research, there has been no advancement regarding neutrino mass, other than the fact that we now have confirmed that it is not zero. No model beyond the SM has been confirmed. – Omry Oct 12 '17 at 8:02
• For the OP, note that the answer @demosthene gave below could have been given for that previous question which has just been cited: not much new has been found, it's just demosthene's answer goes deeper into the technical details than John's answer to that previous question. – user154997 Oct 12 '17 at 10:44
• As alluded to in an answer, fermion mass is not due to the weak interaction but, rather, a Yukawa interaction – Alfred Centauri Oct 12 '17 at 12:17

First, let me point out that, up until recently when neutrino flavour oscillations were established experimentally, neutrinos masses were apparently consistent with $0$, removing the need for a neutrino mass term in the SM Lagrangian altogether. Now, they are believed to be $\sim <0.2$ eV.

There are essentially three reasons why the neutrino mass is zero in the SM:

1. The SM is renormalizable.
2. The SM Higgs appears in an $SU(2)_L$ doublet.
3. There are no right-handed neutrinos.

So it should be clear at this stage that, in order to make your neutrinos massive, you need to go beyond the SM, by breaking one or several of the above points. For instance, you could 1) introduce higher-order operators (non-renormalizable), 2) add more Higgses (no longer constrained to living in an $SU(2)_L$ doublet), or 3) postulate a right-handed neutrino.

Let's go with 3).

It's not my intention to give a complete derivation of the Higgs mechanism and the generation of masses in the SM, but suffices to say that you need to couple the Higgs both to a left- and a right-handed fermions in order to get the desired mass term. For example, the left-handed electron and electron neutrino live together in a $SU(2)_L$ doublet, which couples through the Higgs to a singlet - the right-handed electron. Because of that experimentally-motivated imbalance (remember that the SM is pretty much built from phenomenology - we can add by hand a right-handed neutrino the day it is discovered), the electron neutrino is left out of the resulting mass term.

Now you see that if we break condition 3) above and manually add in that right-handed neutrino, we move from a right-handed singlet to a doublet, and the mass term follows.

Here's a complication. The mass term we're building for the neutrino is a so-called "Dirac" mass term, that couples left- and right-handed components. This is assuming the neutrino and the anti-neutrino are distinct particles, much like the electron and positron are. But since the neutrino is electrically neutral, there's always the possibility that it might be Majorana; that is, it might be its own antiparticle.

So, for a Dirac neutrino, we need to include its right-handed version in the mix. But for a Majorana one, it's technically possible to build a mass term out of only the left-handed component. Why haven't we done it? Because it breaks lepton number conservation (bad!).

Again, the solution to that Majorana problem is either to introduce extra Higgses (2) or higher-dimensional coupling operators (1), or more straightforwardly, a right-handed neutrino (3). In that third case, you'll have a RR mass term, a LL one, but also a Dirac LR! the latter restoring total lepton number conservation.

One final point. If there is such need for a right-handed neutrino, how haven't we seen it yet? A popular model (or class of models) is the so-called "see-saw mechanism", whereby the mass scale of the RR term is very large (which is nice, because you can then associate it to a possible GUT or string scale), which then leads the inversely proportional LL term to be very small. This solves both the question of why we've only seen one handedness of neutrinos, and saves us from adding in an apparently not otherwise motivated extremely small mass term by hand (that would be a naturalness problem).

The answer is: we do not know.

The problem with neutrino masses is that we only see left-handed neutrinos.

The standard way for fermions to obtain a mass through interaction with the Higgs field is through a three point interaction involving a left-handed fermion, a right-handed fermion, and the Higgs field.

With no right handed neutrinos, there is no way to form such interactions, which lead to a mass term after symmetry breaking. This leaves the exact mechanism by which neutrinos acquire mass a mystery. The solution of which may or may not involve interaction with the Higgs field.

Note: It is simply wrong to say that elementary particles "acquire mass through the weak interactions with the Higgs field". The interactions of elementary fermions have nothing to do with the Weak interaction.