In a paper posted this week to the arXiv pre-print server, physicists at the Antarctic IceCube experiment announced that in two years of data collection at the ice-bound particle detector no sterile neutrinos have been found. The absence is damning enough to call into question the very existence of the would-be fourth flavor of neutrino; none of the several other active neutrino detection experiments have registered sterile neutrinos either.
As hypothesized, sterile neutrinos should be far and away the most difficult variety of neutrino to observe, which is really saying something given the already ghostly nature of neutrinos in general. This is because, of the four fundamental forces of nature, they experience only gravity. Other neutrinos will interact very slightly with other matter—via the electromagnetic force—but the sterile flavor, not at all.
How then would we even be able to detect one? Not very easily.
The idea is to look at the rates incoming non-sterile neutrinos for conspicuous gaps. A characteristic of (non-sterile) neutrinos is that as they fly through dense matter and interact ever so slightly with the surrounding electrons and atomic nuclei, they tend to oscillate into other sorts of neutrinos. This would include sterile neutrinos, should such a thing exist.
So, for neutrinos inbound from some region featuring a lot of high-density matter, we should expect a gap in the number of muon neutrinos registered on Earth. That is, sterile neutrinos might be represented by an absence of muon neutrinos.
The IceCube experiment, which consists of a square-kilometer of clear ice riddled with thousands of dangling spherical photodetectors, offers astrophysicists two data sets representing muon neutrino detection events from 2009 to 2010 and from 2011 to 2012. In all, it winds up being around 42,000 detections. (If that sounds like a lot, consider that 100 trillion or so neutrinos pass through your body every second, with neither the neutrinos nor your body being much the wiser.)
"No evidence for anomalous [muon neutrino] disappearance is observed in either of two independently developed analyses, each using one year of atmospheric neutrino data," the IceCube paper concludes. There is still some slight hope that sterile neutrinos might exist at really unexpected masses, but the IceCube data covers an enormous range, leaving just a couple of slivers of potential mass-energy they might be hiding.
"We know that neutrino oscillations call for physics beyond the Standard Model," Francis Halzern, the IceCube experiment's lead investigator, told Physics World. "It looks now less likely that sterile neutrinos are part of it."
If "particles that interact with matter only through gravity" sounds a bit familiar, yes, that is the central high-level feature of dark matter as well. And, indeed, sterile neutrinos have been considered as such.
The catch, however, is that sterile neutrinos, should they exist, would still be neutrinos, which means that they would be prone to oscillating into other neutrino flavors, e.g. oscillating into non-dark matter. So, for things to work out, there would have to be many more sterile neutrinos in the universe than normal neutrinos. This raises all sorts of problems.
In any case, we may not have to worry about it too much with about a bazillion alternative dark matter candidates still on the table.