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.
Edwin Cartlidge is a science writer based in Rome
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