The Trouble-Makers of Particle Physics

Postcards from the Energy Frontier by Prof Jon Butterworth

The chances are you have heard quite a bit about the Higgs boson. The goody-two-shoes of particle physics, it may have been hard to find, but when it was discovered it was just as the theory – the Standard Model – said it should be. It has followed all the rules, so far. Neutrinos, on the other hand, are trouble.

Where the neutrinos are. Illustration by Chris Wormell, from “A Map of the Invisible”.

Neutrinos are unique amongst the fundamental matter particles in that they carry no electromagnetic charge. In fact the only force of the Standard Model that neutrinos deign to notice is the weak nuclear force. This makes them very hard to detect, one reason they get away with so much.

Physicists go to enormous lengths to spot neutrinos. Cutting-edge neutrino detectors have included a giant bubble full of deuterium under Canada, an enormous underground lake surrounded by photomultipliers in Japan, tonnes of steel under the US, and the entire Antarctic ice-pack. A few weeks ago I stood in a huge golden box in CERN, which is now full of 725 tonnes of liquid Argon. This is protoDUNE, a one-twentieth-volume prototype of the neutrino-detector-which-is-to come, DUNE, to be sighted in the Sanford Underground Research Facility in South Dakota, USA. The DUNE detector will measure a beam of neutrinos produced at the Fermilab accelerator complex, hundreds of kilometres away near Chicago. ProtoDUNE recently saw its first particle tracks.

The Deep Underground Neutrino Experiment. Pic by CERN

The main reason for all this effort is the fact that neutrinos are the only particle known to disobey the rules of the Standard Model.

In the Standard Model, as originally conceived, neutrinos were supposed to be massless. This was another way they had found to be “special”, since all the other fundamental particles have mass. This specialness, however, fitted quite well with the fact that the only Standard Model force they interact with is the weak force, as follows.

Matter particles carry angular momentum – spin. This means they have a “handedness”, like a corkscrew. As they travel, their spin can either be clockwise with the motion, like a right-handed screw, or anti-clockwise – left-handed.

For reasons we don’t understand, the weak force plays favourites with this handedness. It only affects left-handed particles (and only right-handed anti-particles). This means a right-handed neutrino would not be affected by any of the forces in the Standard Model. A rather useless particle. In fact, in the Standard Model, by application of Occam’s razor, there were no right-handed neutrinos, and no left-handed anti-neutrinos. Why postulate them if they don’t do anything?

Now for particles with mass, this doesn’t work. If a particle has mass, it must travel slower than the speed of light. So in principle at least, it is possible to overtake it, and that would flip the handedness, turning a right-handed particle into a left handed one. But it doesn’t make sense for the weak force to suddenly switch off or on depending how fast the observer is travelling. So we end up with one definition of handedness based on the spin (which can flip if you overtake it) and one based on the weak force (which can’t). We call these two types of handedness “helicity” and “chirality” respectively.

For a massless neutrino, these two things coincide. A massless particle travels at the speed of light, so can never be overtaken. So both the helicity and the chirality are fixed. A left-handed particle is always left-handed. This is why we could have a Standard Model with no right-handed neutrinos.

But neutrinos have mass.

It is very small, but it is there and it messes things up. I told you they were trouble.

Neutrino masses had to be added to the Standard Model – the only such indignity it has suffered since its inception. This means that we must have right-handed (helicity) neutrinos because we could simply make one by overtaking a left-handed neutrino. And this means in turn that right-handed (chirality) neutrinos have to exist. (You can see why: if the right-handed neutrinos travel very fast, the chirality and helicity almost coincide. So if you have one, you need both.)

This is unpleasant. We have required the existence of a particle that experiences none of the forces of the Standard Model.

The Standard Model of Particle Physics which neutrinos are far too blasé about

The neutrino mass is also anomalously small – many orders of magnitude below the masses of the other particles. This really just emphasises that even though the theory behind our well-behaved friend the Higgs boson allows these masses to be there, it doesn’t help us understand why they have the values they have. 

And there is a final piece of potential trouble. Antimatter differs from matter by the fact that all its “charges” are opposite. Not just the electric charge, but the equivalent of “charge” for the strong force, and for the weak force too. But for the right-handed neutrino, all these are zero. So, applying Occam’s razor again, maybe there is no difference. Maybe, unlike the other matter particles, the neutrino doesn’t have an antiparticle. Or rather, maybe it is its own antiparticle.

With behaviour like this, it is no wonder that we are keen to learn more. And perhaps it is no surprise that (even if they do respect the speed of light limit after all) neutrinos seem to be continually throwing up anomalies. Maybe there are extra “sterile” neutrinos that, like the right-handed chirality neutrino, don’t interact at all except by mixing with the others. Maybe some bizarre heavy neutrino is a significant component of Dark Matter. Where do the neutrinos seen by the ICECUBE experiment come from? What about these two weird events seen by the ANITA balloon experiment over Antarctica?

Having changed the Standard Model once already, perhaps neutrinos are poised to do it again. To be honest we’d probably prefer it if they did. The Standard Model leaves quite a few important physics questions unanswered, and from that point of view the “model behaviour” of the Higgs boson is not what we need.

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Professor Jon Butterworth is a physics professor at University College London and a researcher on the ATLAS experiment at CERN involved with, amongst other things, the discovery of the Higgs Boson. He is the author of two popular science books Smashing Physics and A Map of the Invisible. Postcards From the Energy Frontier is the successor to Jon’s hugely successful blog for The Guardian, Life and Physics. He is @jonmbutterworth on Twitter.

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