Dear Dr B: Why are neutrinos evidence for physics beyond the standard model?

Dear Chris,

The standard model of particle physics contains two different types of particles. There are the fermions, which make up matter, and the gauge-bosons which mediate interactions between the fermions and, in some cases, among themselves. There is one additional particle – the Higgs-boson – which is needed to give masses to both bosons and fermions.

Neutrino event at the IceCube Observatory in Antarctica.
Image: IceCube Collaboration

The fermions come in left-handed and right-handed versions which are mirror-images of each other. In what I think is the most perplexing feature of the standard model, the left-handed and right-handed versions of fermions behave differently. We say the fermions are “chiral.” The difference between the left- and right-handed particles is most apparent if you look at neutrinos: Nobody has ever seen a right-handed neutrino.

You could say, well, no problem, let’s just get rid of the right-handed neutrinos. Who needs those anyway?

But it’s not that easy because we have known for 20 years or so that neutrinos have masses. We know this because we see them mix or “oscillate” into each other, and such an oscillation requires a non-vanishing mass-difference. This means not all the neutrino-masses can be zero.

Neutrino masses are a complication because the usual way to give masses to fermions is to couple the left-handed version with the right-handed version and with the Higgs. So what do you do if you have no right-handed neutrinos and yet neutrinos are massive?

The current status is therefore that either a) there are right-handed neutrinos but we haven’t yet seen them, or b) neutrinos are different from the other fermions and can get masses in a different way. In either case, the standard model is incomplete.

It is partly an issue of terminology though. Some physicists say right-handed neutrinos are part of the standard model. In this case they aren’t “beyond the standard model” but instead their discovery is pending.

I have a personal fascination with neutrinos because I believe they’ll be key to understanding the pattern of particle-masses. This is because the right-handed neutrino is the only particle in the standard model that doesn’t carry gauge-charges (or they are all zero, respectively). It seems to me that this should be the reason for it either being very heavy or not being there at all. But that’s speculation.

In any case, there many neutrino experiments presently under way to closer study neutrino-oscillations and also to look for “neutrinoless double-beta decay.” The relevance of the latter is that such a decay is possible only if neutrinos are different from the other fermions of the standard model, so that no additional particles are needed to create neutrino masses.

So, no, particle physics isn’t dead and over, it’s still full with discoveries waiting to happen!

Thanks for an interesting question.

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See also:

• What are the chances of the universe ending out of nowhere due to vacuum decay?
• Why do physicists worry so much about the black hole information paradox?
• What is emergent gravity?
• Where does dark energy come from and what’s it made of?
• What do physicists mean by “quantum gravity”?
• How come we never hear of a force that the Higgs boson carries?
• Why is Lorentz-invariance in conflict with discreteness?

or click here for all posts in this series.

The Quantum Quartet

I made some drawings recently. For no particular purpose, really, other than to distract myself.

And here is the joker:

Interna

I’m still writing on the book. After not much happened for almost a year, my publisher now rather suddenly asked for the final version of the manuscript. Until that’s done not much will be happening on this blog.

We do seem to have settled on a title though: “Lost in Math: How Beauty Leads Physics Astray.” The title is my doing, the subtitle isn’t. I just hope it won’t lead too many readers astray.

The book is supposed to be published in the USA/Canada by Basic Books next year in the Spring, and in Germany by Fischer half a year later. I’ll tell you more about the content at some point but right now I’m pretty sick of the whole book-thing.

In the meantime I have edited another book, this one on “Experimental Search for Quantum Gravity” which you can now preoder on amazon. It’s a, probably rather hard to digest, collection of essays about topics covered at a conference I organized last year. I merely wrote the preface.

Yesterday the twins had their first day in school. As is unfortunately still common in Germany, classes go only until noon. And so, we’re now trying a new arrangement to keep the kids occupied throughout the working day.

Away Note

I'm in Switzerland this week, for a conference on "Thinking about Space and Time: 100 Years of Applying and Interpreting General Relativity." I am also behind with several things and blogging will remain slow for the next weeks. If you miss my writing all too much, here is a new paper.

Interpretations of Quantum Mechanics: The Cat’s View

Something else I made for the book but later removed. (Click to enlarge.)