Will A New Neutrino Change The Standard Model? (Patreon)

Since the discovery of the Higgs boson, physicists have searched and searched for any hint of new particles. That search has been fruitless. Until, perhaps, now. Today on Space Time Journal Club we’ll look at a paper that reports a compelling hint of a new particle outside the standard model: the sterile neutrino.

Regular neutrinos are a bit aloof. They don’t interact by the electromagnetic or strong nuclear forces – only by the weak nuclear force and gravity. They are so weakly interacting that they pass through matter like it isn’t there. To have a 50-50 chance of stopping any given neutrino you’d need a wall of lead … one light year thick! If regular neutrinos are aloof, then sterile neutrinos are the complete loners. They don’t even interact via the weak interaction. Even so, detection of sterile neutrinos would be incredibly important. Besides being the first expansion of the standard model family since the Higgs boson, sterile neutrinos are a candidate for dark matter, and their existence would have had a huge influence on the expansion of the early universe.

We can detect regular neutrinos by watching for the rare interaction between a neutrino and an atomic nucleus in some huge volume of matter – an entire glacier in the Ice Cube experiment, or a huge vat of oil in the experiment we’re about to discuss. Those regular neutrinos are spotted when they interact with matter via the weak nuclear force. So how on earth do you spot a sterile neutrino that doesn’t even undergo that interaction? By being extremely clever, obviously. This may have been achieved, as reported in the 2018 paper “Observation of a Significant Excess of Electron-Like Events in the MiniBooNE Short-Baseline Neutrino Experiment”

Before we get to that, we’re going to need to go a few inception layers deep to set up the knowledge. We’re going to drop through the standard model of particle physics, electric charge and antimatter, the bizarreness of quantum chirality and the Higgs mechanism, and finally to why all of this points to sterile neutrinos. Hold on to your butts.

First, the standard model family of particles. As we’ll see in upcoming episodes, these particles divided into the bosons which carry the forces and the fermions which comprise matter. The latter include the quarks – up, down which comprise protons and neutrons, and the more exotic top, bottom, strange and charm, as well as an antimatter version of each of these. There are the leptons – the ubiquitous electron and its heavier cousins, the muon and tauon, and again, each with antimatter counterpart. 

Neutrinos are also leptons, and each of the heavier lepton “flavours” has a neutrino version. These have far lower mass and unlike quarks and leptons they have no electric charge – hence “neutrino”, the “little neutral one”. Each flavour of neutrino also has an antimatter counterpart.

So let’s drop down into antimatter. An antimatter version of a particle has the same mass and the opposite electric charge.  So an electron has a charge of negative one and an anti-electron has a charge of +1.  Neutrinos don’t have charge, so what’s the difference between a neutrino and anti-neutrino? There’s actually another property that gets reversed in anti-particles – and that brings to our next level chirality. 

The physical interpretation of chirality is pretty abstract. To explain we need to start with helicity. Helicity is just the direction of a particle’s spin relative to its direction of motion. Helicity can be right-handed, which means clockwise rotation or left-handed, or anticlockwise.

Like helicity, chirality can be left- or right-handed, however the physical interpretation is more abstract. It’s related to the direction in which the particle’s phase shifts under rotations. Helicity depends on your own motion relative to the particle in question – it flips direction if you start moving faster than the particle. However chirality is fundamental to the particle and doesn’t depend on your own velocity. 

This is where we need to expand our picture of the particles of the standard model a little, and open up the possibility of the sterile neutrino’s existence. There are actually two versions of each fermion – one with right-handed chirality and one with left, and that’s on top of the matter-antimatter split. As we saw in our episode on the Higgs mechanism, real quarks and electrons are actually a combination of left- and right-chiral particles that oscillate back and forth between those particles through interactions with the Higgs field. That oscillation is what gives these particles their mass.

Still with me? Good. Like electric charge, chirality is also reversed in antimatter. For example, both the left- and right-chiral negatively-charged electrons have their own positive-charged antimatter particles, which are right- and left-chiral respectively. These different chiralities are thought of as completely separate particles. There’s good reason for this. Chirality determines whether a particle can interact with the weak nuclear force. The left-chiral electron feels this force and right-chiral electron does not. This interaction is flipped for antimatter. The right-chiral anti-electron feels the weak force while the left-chiral anti-electron does not.

OK, got all of that? We’re finally ready to bring it back to neutrinos. Every neutrino we’ve ever observed was spotted due to the weak interaction. That means we’ve only ever seen left-handed neutrinos or right-handed anti-neutrinos. The opposite chirality – right-handed neutrinos or left-handed anti-neutrinos should only interact gravitationally, so would be near-impossible to detect. These are the sterile neutrinos. They aren’t part of the standard model because, until now, we had no concrete evidence that they exist.

But there’s good reason to suspect their existence. If neutrinos gain their mass by the same mechanism as quarks and electrons, that means their chirality oscillates. That would require regular left-handed neutrinos to spend at least a bit of time as sterile right-handed neutrinos. Now we know that neutrinos have mass due to a completely different type of oscillation. We’ve observed that a neutrino’s flavour can change – electron neutrinos can become muon neutrinos can become tau neutrinos. In order to evolve that way neutrinos must experience the flow of time, which means they can’t move at the speed of light, which means they must have mass. That mass MAY indicate the existence of the sterile neutrino, but it could also come from a more exotic mechanism. For example the Majorana mechanism that would require the neutrino to be its own antiparticle, and would break the standard model even more than the existence of the sterile neutrino.

OK, let’s get to the experiment. MiniBooNe is an experiment at FermiLab in Illinois. Neutrinos are created by colliding protons together to create a beam of mostly muon neutrinos. These then travel to a 800 ton vat of mineral oil. Rare interactions with nuclei in the oil reveal the nature of the neutrinos. The evidence for sterile neutrinos is subtle, and they certainly weren’t directly detected. Instead, MiniBooNe detected way too many electron neutrinos that expected. I told you that neutrinos oscillate between type – electron, muon, tau. So miniBoone experiment starts with muon neutrinos and some of these transform into electron neutrinos by the time the hit the vat.

According to the standard model, that oscillation should be extremely rare over the very tiny distance of the neutrino beam. A lot more muon neutrinos made the transition to electron neutrino than was expected according to the basic standard model. But one way to speed up that transition is to introduce sterile neutrinos as an intermediate step in the oscillation. If muon neutrinos can flip their chirality and become sterile neutrinos, it’s then an easier transition from sterile neutrino to electron neutrino. That’s a proposed explanation of the MiniBooNe team.

The team finds an over-abundance in electron neutrinos at the 4.8-sigma level. That’s actually slightly below the critical 5-sigma level required for claiming a high-confidence detection. However MiniBooNe then combined their results with that of an older experiment that had also detected a hint of this excess. That was the Liquid Scintillator Neutrino Detector – LSND experiment at Los Alamos, which in 2001 published a 3.8-sigma excess in electron neutrinos. Combined with the 4.8-sigma MiniBooNe result, the authors claim a 6.1-sigma signal, which would be considered extremely significant.

If this is right, then it’s the first particle outside the standard model since, well, the beginning of the standard model. Even the Higgs boson was an official part of the standard model before its discovery due to its essential role in giving particles their mass. And if we really have seen the influence of the sterile neutrino we now know something about it. It would have a relatively low mass, at around one electron volt – forgive the particle physics energy units for mass. That’s heavier than regular neutrinos, but way too light for it to be a candidate for dark matter.

OK, I know you’re excited but don’t crack the champagne bottle yet. This result is in conflict with some other measurements. The Ice Cube neutrino detector in Antarctica has found no evidence of the existence of the sterile neutrino based on the transition of muon to electron neutrinos as they travel through the body of the Earth. And analysis of the cosmic microwave background radiation by the Planck satellite shows that the early rate of expansion of the universe is consistent with only three neutrino types. Add more neutrino types, like the sterile neutrinos, and the early universe would have expanded faster.

There’s an interesting conflict here. The MiniBooNe result looks compelling. Hopefully it ISN’T some sort of experimental error – which it might be. Remember those faster-than-light neutrinos? Yeah, we don’t talk about that anymore. If this one is real then something is missing in our understanding of physics – and glitches between experiment and theory are exactly how new physics gets discovered. The sterile neutrino may have been discovered, or we may have just spotted something even more interesting. Either way, we’ll have peered just a little deeper into the fundamental building blocks of space time.