An enormous “telescope” buried deep under the ice of Antarctica has made the first observation of cosmic neutrinos. The international collaboration operating the IceCube laboratory says that the detection of these chargeless, almost massless and very high-energy particles marks the beginning of a new era in astronomy in which electromagnetic radiation is no longer the only means we have for probing the distant universe.
Detecting neutrinos from space is not new. For decades physicists have been able to observe the neutrinos generated by nuclear reactions inside the Sun, as well as those produced by cosmic rays interacting with nuclei in the Earth’s atmosphere. But neutrinos from further afield have until now remained elusive, their extremely high energies making them rarer and much harder to detect than those from closer to home.
At the same time, cosmic neutrinos are particularly prized as information carriers because their inertness allows them to pass through clouds of gas and dust that would otherwise keep distant astrophysical objects hidden from view. In particular, they might be able to reveal the origin of cosmic rays. Cosmic rays are charged particles and the paths they take to Earth are bent by galactic and intergalactic magnetic fields, which obscure their origins.
Photomultipliers watch the ice
The $275m US-led IceCube telescope, located at the Amundsen–Scott research centre at the South Pole, comprises 86 cables, each up to 2.5 km long, suspended inside vertical holes in the ice. Attached to each cable are dozens of photomultiplier tubes. The photomultipliers record the Cerenkov radiation given off by the secondary particles created when incoming neutrinos collide with hydrogen or oxygen nuclei inside the ice.
The tubes and cables are spaced so as to create a total detector volume of 1 km3. Neutrinos interact with other matter only extremely weakly, which means that the detectors have to be as large as possible if they are to register a significant number of neutrinos in a reasonable timeframe. To maximize its chance of detection, the IceCube collaboration had originally focused its efforts almost exclusively on muon neutrinos, since these generate muons that continue to travel in a forward direction for several kilometres after the neutrino has collided, so allowing interactions from beyond the photomultiplier tubes to be included in the dataset and thereby effectively increasing the detector volume.
However, a twin discovery made using data collected between May 2010 and May 2012 persuaded the team to take a different approach. The data contained two collisions – nicknamed Bert and Ernie and each involving a whopping 1015 eV of kinetic energy – that were located inside the bounds of the detector. As a result, the researchers started a new analysis using only high-energy events originating inside the instrumented cubic kilometre of ice.
This move limited the amount of data the researchers had to work with but made it easier to filter out collisions involving the far more abundant atmospheric neutrinos. This is because high-energy events of cosmic origin tend to create showers of secondary particles in the detector, whereas lower-energy atmospheric neutrinos tend to produce single muon tracks (see figure “A shower called Ernie”).
The change of strategy paid off and the researchers have since found another 26 events with energies of at least 3 ×1013 eV. Furthermore, the team calculates that only about 11 of the total of 28 events were likely to be caused by atmospheric neutrinos or muons. These results, the researchers concluded, provide what is known as 4σ evidence for the detection of cosmic neutrinos; in other words, their statistics suggest only a one in 15,000 chance that all of their events can be explained via purely atmospheric phenomena. “This is the first evidence we have of neutrinos that are not of atmospheric origin,” says IceCube principal investigator Francis Halzen of the University of Wisconsin-Madison. “It opens up a new wavelength in astronomy, thanks to a different kind of particle.”
Emilio Migneco of Italy’s National Institute of Nuclear Physics in Catania, who is not a member of the IceCube collaboration, points out that 5σ evidence (less than one in a million chance of statistical fluke) is the standard usually required to claim a new discovery. Nevertheless, he says that “the signal seems to clearly emerge from the background”, adding that the results are “extremely exciting and will open a new window on the observation of the universe”.
Low angular resolution
Migneco cautions, however, that the window has not been opened just yet, pointing out that astronomical observations will require the neutrinos that are detected by IceCube to be correlated with specific objects in the sky. The latest results provide a hint that at least some of the cosmic neutrinos detected were generated in the centre of the Milky Way, but the angular resolution of the measurements is not high enough to prove this, and indeed the IceCube collaboration makes no such claim in its paper. Migneco, who is former co-ordinator of the KM3NeT neutrino telescope that is currently under construction in the waters off Sicily, says that a facility in the northern hemisphere, such as KM3NeT, “may help in solving the puzzle since it will have a better view of this region of the sky”.
IceCube itself will also concentrate on trying to resolve this issue. “Now that we know what we are looking for, we will probably find lots of other events fairly easily,” says Halzen. “Statistics is the key. Especially those from muon neutrinos, the tracks of which help determine where they come from.”
What might the data tell us? “It would be disappointing if we didn’t manage to pinpoint the sources of cosmic rays this way,” says Halzen. “But there are likely to be surprises as well. One surprise would be identifying the source of cosmic rays and finding out it is nothing that theorists have thought of in the last 100 years.”
The findings are described in Science.