I’ve largely given up writing stories about new dark-matter candidates. Theoretical physicists keep coming up with more elaborate scenarios to make dark matter more interesting and less inert. It all seems a bit forced. About the only thing that dark matter has to do is provide mass. A particle that doesn’t interact with electromagnetism at all fits the bill almost perfectly (and does practically nothing else).

Still, when there is experimental data to support it, I get interested in dark-matter candidates again. My cynicism aside, there are actually a few results hanging around that seem hard to explain. For instance, the hydrogen in the early Universe seems to have absorbed less light than expected. The center of the galaxy emits an unexpected amount of gamma rays (though they might be due to ordinary matter). And the neutrinos observed by the IceCube Neutrino Observatory in the Antarctic seem to be a bit weird, too.

Neutrinos on ice

Out of all of these, a recent explanation for the IceCube data has caught my attention because it is reasonably simple. This is in contrast to a recent proposal for a Bose-Einstein condensate of dark matter to explain the lack of hydrogen absorption, which seems hideously complex.

IceCube is an enormous neutrino detector in the Antarctic. For a while now, it has been looking at the high-energy neutrinos arriving from outside of the galaxy. The neutrinos are detected in two different ways: by the track they leave in the detector and via the cascade of light-emitting particles that they generate when they collide with the ice.

These two measurements don’t seem to agree at the very highest energies. Neutrinos come in three flavors—electron neutrinos, muon neutrinos, and tau neutrinos—and a single neutrino will oscillate among these identities as they travel. Models predict that thanks to the vast distances the neutrinos have gone, there should be equal numbers of the different neutrinos. But IceCube sees far more electron neutrinos than expected.

More electron

To explain this discrepancy, a pair of theoreticians have proposed an invisible neutrino decay pathway. In this case, one or two of the flavors of neutrino decay into a particle called a majoron. The majoron is one of a zoo of proposed dark-matter particles. This one now has two roles: it provides the neutrino with mass, and it is created by tau and muon neutrinos when they decay. This is a bit of a double win because the evidence that neutrinos have mass is quite strong now—like from the flavor oscillations from atmospheric neutrinos, for instance.

The nice thing about this model is that it actually explains an experimental discrepancy that is becoming rather solid—the signal excess of electron neutrinos has about a one-in-10,000 chance of being an accident at the moment. It is also nice that the data is explained by a rather minimal modification: one dark matter particle of a sort that seems reasonably well motivated by experimental data.

Now, neutrino decay into an invisible particle is also very close to not agreeing with other experimental data (like supernova neutrinos). But the point is that the data doesn’t rule out neutrino decay yet. Or rather, the data allows two of the three neutrinos to decay, but one—the electron neutrino—has to be stable.

The researchers also discuss the requirement for more data. The IceCube data set will need to get larger so that we can be certain that the excess of electron neutrinos is real. (Beyond that, another data source would be good.) The problem there is that there are no other sources of high-energy neutrinos or large detectors to catch them, so it may be a while before we can confirm that it's something unusual about the sources or detectors.

Physical Review Letters, 2018, DOI: 10.1103/PhysRevLett.121.121802