Today’s issue of Science highlights a breakthrough in astroparticle physics many decades in the making. After tantalizing hints in the past year, the IceCube Collaboration now reports on a follow-up analysis, leading to a larger event sample and compelling evidence that the first high-energy neutrinos from outer space are starting to be seen.
The IceCube Neutrino Observatory is a strange kind of telescope, because it searches for neutrinos—rather than photons—from space. It’s situated at the South Pole, and it consists of thousands of detector modules buried up to 2.5 km deep in the glacial ice there. At that depth, the ice is very clear and very dark. What the modules see is the passage of subatomic particles zipping through the ice, emitting a well-known, faint blue light called Cherenkov light. When enough modules detect this light from a passing particle, the information is sent to the surface where a computer cluster starts crunching the `event’ to make a rough estimate of the direction and energy of the particle.
In the analysis reported today, 28 events are in the final sample, sifted out of over 10ˆ11 (one hundred thousand million) events in total, collected over two years. The overwhelming majority of these events are background particles from cosmic rays striking the atmosphere. Finding the handful of neutrinos that potentially originated in space is extraordinarily challenging. One of the innovations of the new analysis is to use the outermost detector modules as a `veto’, to tag any particle observed coming into the detector through the outer layer as a background event. All that remains are events which first appear, mysteriously, in the middle of the detector. This is the tell-tale signature of a neutrino event. In fact, famous for being “ghost” particles, almost all the neutrinos which pass through the detector never leave a trace at all. But fortunately a tiny fraction do happen to bump into an atom in the ice. When that happens, many other particles (like electrons, and their heavier cousins, muons) are created, and these particles create the Cherenkov light that IceCube detects.
It turns out that this veto design, selecting only those events where the light emission begins inside the detector, is extremely efficient for separating high energy neutrinos from background. Applying this technique and selecting only very energetic events, the two years of data would be expected to boil down to only about ten surviving background events (including some background neutrinos called atmospheric neutrinos). The fact that 28 events survive is thus strongly inconsistent with an explanation purely in terms of background. There are, moreover, many additional details such as their energy distribution and their distribution over the sky that are consistent with an origin in space but not with any known backgrounds.
The two highest energy events are remarkable in their own right. Their energy weighs in at about 1 PeV each (that’s roughly 100 times more energetic than the particles in the world’s most powerful accelerator, the LHC at CERN). They are the highest energy neutrinos ever observed, (or rather, were, as the news has already leaked that another, even higher energy neutrino, has been found in a newer analysis not yet published.)
With the results reported in Science today, the picture that is starting to emerge is that the universe is filled with a flux of high-energy neutrinos extending up to a few PeV in energy. The origin of this flux is not yet known. One clue is that the energy distribution seems to drop off not far above the highest energy events. IceCube has sensitivity for similar events at much higher energy, but none have been detected so far.
Another clue about the neutrinos’ origins is that they appear to be coming from many different directions. With the current data there is not yet significant evidence of any preferred directions in the sky. Such evidence would of course greatly narrow down the possible sources. Because of the way the veto technique works, however, it sacrifices many potential neutrino events that would have much more precise directional information. New analyses using other techniques will try to identify more of these neutrinos with good directional reconstruction, and hopefully a sharper picture will start to emerge. It is widely believed that finding the sources of high-energy neutrinos will also reveal the origins of cosmic rays, a question that is now over a century old.
We thus arrive at a wonderful moment where a goal has been reached, yet upon reaching it we see new and more exciting challenges ahead. In Sweden, this dream to open the field of neutrino astronomy has a long and distinguished history. Swedish research groups were involved from the beginning twenty years ago with the first neutrino detector at the South Pole, AMANDA. Over the next decade that transitioned to the much larger IceCube detector, of which Per Olof Hulth from Stockholm University was the first spokesperson. Today the IceCube collaboration consists of about 250 scientists in a dozen countries, with Francis Halzen of University of Wisconsin as Principal Investigator, and Olga Botner of Uppsala University as spokesperson.
– Chad Finley (researcher at the Oskar Klein Centre) – email@example.com