Finding neutrinos is hard, and not all neutrinos are identical. We know there are neutrinos related to reactions with electrons, muons, and tau particles, and each of these sibling neutrinos has its own mass and its own slightly different way of being found. These three flavors of neutrino are all part of the Standard Model and have all been observed, and they have been observed to oscillate from one kind to another as neutrinos exchange their mass for energy or their energy for mass. 

In addition to these known particles, there may also be what are called sterile neutrinos or inert neutrinos that come in a wide range of masses that are waiting to be observed. As the theory goes, the known neutrinos have a particle spin that is called left-handed. Basically, if your thumb is in the direction of motion, the property called helicities, which is like a spin but not quite the same, is aligned like your fingers curving around the direction of motion. There is no reason, however, that the other orientation shouldn’t exist, that particles shouldn’t act like your right thumb is the velocity and your fingers are the helicities. 

This “why aren’t they also flipped” question has led to the idea, to the hope that these alternate neutrinos, these sterile neutrinos, could be out there, refusing to interact with anything other than gravity and having masses anywhere from 1 eV to 10^15 GeV. And there could be many kinds of them with many different masses that together add up to explain dark matter. Trying to discover particles that don’t want to interact except via gravity, however, is a challenge. 

Luckily the KATRIN experiment has accepted this challenge, and in a new paper in Physical Review Letters, they describe how they are looking for sterile neutrinos alongside known neutrinos. Basically, in heavy water, protons will periodically be converted into neutrons, and give off one electron and one neutrino in the process. The entire system has a decay energy of 18.6 keV that can spread between their motions, and the neutrino’s energy can get measured as the flashes created when it hits something, and thanks to probabilities, we’ll see a range of energies in those flashes that reflect how often those neutrinos get how much of that 18.6 keV. If sterile neutrinos are formed, we should see a different family of flashes that reflect the energy of the sterile neutrinos. 

So far, the KATRIN experiment has been able to rule out masses between 3 and 30 eV for sterile neutrinos. If sterile neutrinos are out there, they are probably kinda big, and this is what we need to explain dark matter. The idea that dark matter is just a flipped version of something we already know about is one that really appeals to me. It doesn’t wreck the Standard Model but expands it in a sensible direction. Hopefully, KATRIN will stop ruling out masses for sterile neutrinos and start finding sterile neutrinos.

For now, though, we wait and wonder what is dark matter and what particles are out there, outside the incomplete Standard Model, waiting to be found.

More Information

Max Planck Institute for Physics press release

“Bound on 3+1 Active-Sterile Neutrino Mixing from the First Four-Week Science Run of KATRIN,” M. Aker et al, 2021 March 5, Physical Review Letters