When Wolfgang Pauli first predicted the neutrino in 1931, he considered that he had done a terrible thing. Radioactive beta decays were losing energy and momentum to what he believed to be new particles. Particles so abundant — and yet so elusive — that it would take almost three decades after their invention to confirm their existence. That is the great misfortune of neutrinos. They are spirits ambling through the colossal landscape of the cosmos. To them, atoms are nothing more than voids through which they pass with rare interaction. They have been, since their creation less than a second after the Big Bang, fulfilling quite lonely expeditions into and out of galactic arms, or into and out of our planet and everyone on it. Even now there are dizzying amounts of them traversing the inches of your skin. Unfelt, but hauntingly present, these characteristics are what’ve earned neutrinos the nickname “ghost particles”.
And while we’ve succeeded — with ambitious vats of oil and water, or by burying detectors a mile and a half (2,500 meters) under Antarctic ice — to observe the effects of neutrinos, they continue to present new mysteries to us, becoming the first particles nonconforming to the Standard Model. They are the second most populous particle in the universe after photons. Their name means “little neutron” because they have no electric charge and, similarly, have almost no mass. But therein lies one of the biggest mysteries surrounding these particles.
The Standard Model predicts that neutrinos should have no mass and yet experiments from 20 years earlier show that they do have, however slight, some mass that they acquire through unknown means. The other particles in the Standard Model get their mass according to how much resistance they experience from the Higgs field. The more a particle interacts with the Higgs field and is resisted by it, the greater their mass will be. But this is not true for neutrinos. To examine the origins of their mass requires new physics, a new way of thinking that hasn’t yet given us any definitive numbers. This is because a neutrino has no set mass and instead is made up of a combination of three possible masses. Scientists aren’t sure what the value of these three masses are but have named the two configurations. A normal hierarchy involves having two lightweight masses and one heavier one, and the inverted hierarchy involves two heavyweight masses and a lighter one.
Most incredible of all, a neutrino can go through what are called “oscillations”.
In the late 1990’s in Japan, 50,000 tons of water filled an underground detector in the river-cut city of Hida. The watery cavern was surrounded by phototube detectors which would alert researchers to blinks of light emanating from the collisions between neutrinos and water molecules. This is how we realize a neutrino has interacted with matter — they set into motion charged particles which can often leave trails of light in their wake. They can also change the composition of molecules, such as when they interact with chlorine to produce Argon. Neutrinos created 12 miles away (20 km) in the planet’s atmosphere were compared to those detected 8,000 miles (12,870 km) away. Researchers realized that the longer the particles were forced to travel, the more they underwent a strange and fascinating change.
There are three neutrino types: electron, tau, and muon. When a neutrino collides with an atom within a detector, it will create only its respective kind of particle. That is, electron neutrinos will only create electrons, muon neutrinos will only create muons, and so on. This was confirmed in an experiment from the 60’s where neutrinos created alongside taus only ever produced tau leptons, exhibiting something like a memory of where they were born. Regardless, neutrinos can change during travel. The values of their mass fluctuate, allowing them to morph from one of the three mass states (sometimes called “flavors”) into another. This change is the reason we know the particles must have mass. In order to switch identities, neutrinos have to experience time, and anything that experiences time is not traveling at the speed of light. Particles which do not travel the speed of light must have mass.
It is thought this change is able to take place because of special relativity’s claim that particle velocity at near light speeds is determined by mass. Each particle is made of a combination of three mass states. But the mass states travel at different rates through the universe and eventually this difference in rates leads to the neutrino morphing between flavors.
This coming decade, DUNE (the Deep Underground Neutrino Experiment) will become operational, constructed with the hopes to study just this phenomenon. It doesn’t occur anywhere else in our Standard Model. And as such a striking anomaly, it is the ghost particle which has the potential to redefine what we understand about physics.
It’s even a candidate for dark matter.
The internal motion of our galaxy’s glimmering stars, the galaxies themselves — colliding or remaining spinning from where they hang in the inky blackness, and certain fluctuations of the CMB all present to us the same mystery: there should be five times more normal matter in existence for all of these events (and more) to take place. Our answer is dark matter. And it, just like our neutrinos, does not absorb nor emit light and has little interaction with regular matter. It would seem, then, that neutrinos and antineutrinos are the perfect solution to the question of dark matter. But in order for that to be true — and some scientists do believe it could be — we must ask for one more thing.
The neutrinos we’re familiar with would give rise to hot dark matter, a type of matter that moves quickly and thus wouldn’t allow for the formation of certain structures in the early universe. So, while neutrinos may explain some of the dark matter in the world, it’s estimated they’d make up at most only 1.5% of it. The rest of the cold dark matter effects have to come from a new kind of particle — the sterile neutrino.
The sterile neutrino is named such because it doesn’t interact with matter at all, not even using the weak nuclear force. The Z boson, which is in part responsible for the weak nuclear force, will sometimes decay into normal neutrinos after it’s created in a particle collider. It would not, however, decay into sterile neutrinos. If one takes into account both normal neutrinos and their sterile counterparts, it would provide an explanation for all of the dark matter we’ve observed. Experiments like the Liquid Scintillator Neutrino Detector (LSND) or MiniBooNE conducted in 2018 point to anomalies in neutrino oscillations. MiniBooNE, for example, gathered data on muon neutrinos transforming into electron neutrinos as they traveled. It resulted in many more electron neutrinos than allowed by the Standard Model.
But capturing evidence for these particles is a tricky thing. For every experiment that gathers evidence of sterile neutrinos’ existence, there are detectors in other parts of the world at odds with this conclusion. The latest comes in the form of the MINOS+ experiment, an analysis which reveals nothing special in the oscillations which would confirm the heady, sterile particles.
It takes clever and careful experiments to study a ghost. Dark matter has, for a long time, been one of the greatest mysteries surrounding the construction of our cosmos. And we may have an answer — one which has so far evaded us, as is in its nature to do. But the coming decade is focused on examining the solitary and fleeting spirits which course through every bit of space. Resolving, perhaps, this scientific haunting.