There has been a lot of discussion lately of the possible existence of sterile neutrinos in the Universe, due in large part to a few papers in the recent weeks making claims of possible signs of detection in one of the bigger neutrino observatories. And although these results are preliminary, with the formal announcements coming later this week, and may show no clear sign of anything other than a statistical fluctuation, the theory of sterile neutrinos is still quite interesting to the theoretical physics community, and worth exploring further.

Of course most people do not even know what a neutrino is, sterile or not, even though they have been known to the scientific community and studied extensively for nearly a century now. 

In 1930 Wolfgang Pauli first proposed the existence of a very light, electrically neutral particle dubbed the neutrino. Particle physics experiments had shown that in beta decay reactions (a nucleus emitting an electron or a positron and transforming into an isotope of a different nucleus), momentum and energy were not being conserved. All previous experiments in physics had shown that energy and momentum must always be conserved, and Noether's theorem in theoretical physics proved that this should always be true. What Pauli suggested was that energy and momentum were still being conserved, but that an invisible particle was removing energy and momentum from the experiment without being detected.

The neutrino remained a generally accepted by still theoretical particle until twenty-five years later when Cowan, Reines, Harrison, Kruse, and McGuire were able to capture neutrinos that had been produced in nuclear reactors and use them to convert protons into neutrons, which could then be detected. This experiment was considered definitive proof of the existence of neutrinos and would result in a Nobel Prize for the team that conducted it.

Over time the properties of neutrinos were further explored on both the experimental and theoretical sides. In the 1960s theorists showed that neutrinos are connected to the leptons (the general term for the electron and its heavier counterparts) and can interact through weak nuclear reactions. On the experimental side it was discovered that there were actually three different types of neutrino, known as the electron neutrino, muon neutrino, and tau neutrino, and that the sum of all of their masses was extremely small, with the possibility that they were actually massless.

Then in the 1990s new experiments revealed more interesting details of the neutrino. Scientists at the LEP collider made precise measurements of the decay of the Z-boson, which is known to decay into each flavour of neutrino at the same rate, and demonstrated that there could only be three flavours of neutrino in the Universe (with the possible exception of a fourth neutrino that was heavier than any other known particles at the time, or a fourth neutrino that lacked the same interactions as the other three). The other great discovery came at the end of the century with evidence that the three neutrino flavours could mix together, and in effect oscillate from one flavour to another. This discovery also proved definitively that the neutrino does have mass, since a massless particle cannot evolve in time in this manner.

And that brings us to the discussion of the sterile neutrino.

The precision measurements of the invisible decay of the Z-boson proved that there could only be three flavours of neutrinos, since a fourth neutrino with any sort of reasonable mass would have increased the decay rate and been easily detectable. But there was one loophole in the argument - what if there were a neutrino that did not interact through the weak nuclear force in the same way as the other three?

There are many theoretical models that include exactly such a particle. Some contain many such particles. A heavy particle, with no electromagnetic or nuclear interactions would not be detected in any of the existing experiments. They do not affect the invisible decay of the Z-boson because they do not interact with the Z-boson. They will interact through gravitational forces - which are too weak to detect in any particle physics experiments - and they may also interact through the Higgs mechanism or through more exotic new forces. It is possible that there are massive clouds of sterile neutrinos in space, and that they are the explanation for the mysterious dark matter that seems to dominate our Universe and allowed galaxies to form and evolve billions of years ago. It is possible that there are only a few of them in the Universe and we won't be able to detect them for several more centuries, if ever. We just don't know what they might be, or how they might interact, or even how massive they are. They are just an interesting theoretical construction for now.

But that might be changing later this week. On Thursday there will be a press conference featuring several experts from the IceCube neutrino observatory, speaking about a new discovery that they have made. Many astrophysicists believe that they might have a positive signal of a sterile neutrino, Most likely it will be only a preliminary hint of something interesting and that warrants increased study and experimental analysis, but IceCube does have the ability to make a fairly strong detection of some forms of sterile neutrinos that have been discussed by the theoretical physics community for many years. And that is especially true if the sterile neutrino has the properties of the long sought after dark matter particle.

If that is the case, and if IceCube has discovered the identity of dark matter or even just a new flavour of sterile neutrino, then it will be one of the biggest discoveries of particle physics and astrophysics in nearly twenty years!