In what is called the dawn of the era of neutrino astronomy, researchers of the IceCube collaboration have detected very high-energy neutrinos from extraterrestrial sources, possibly from outside our solar system. In a paper published in Science on November 22, they share the results of the experimental observations from 2010 to 2012.
Apart from several low-energy neutrinos, they observed 28 events of high-energy neutrinos interacting with the detectors, and of these, two have energies in the range of petaelectronvolts (PeV). A petaelectron volt is equal to 1,000,000 gigaelectronvolt and is among the higher ranges of energy of the cosmic rays. The studies show a statistical measure of four sigma, which means the data is not merely a coincidence but has statistical significance.
Talking about the significance of the results in an email to The Hindu, Tyce De Young, Associate Professor, Pennsylvania State University and a member of the IceCube collaboration says: “These are the highest energy neutrinos ever observed, and except for a few, which were detected from a nearby supernova in 1987, the only ones that have ever been seen coming from outside of our solar system.”
Unlike earlier detectors which were designed to detect neutrinos indirectly, the IceCube detectors are designed to detect neutrinos of very high energy — up to thousands of PeVs from higher energy cosmic rays. IceCube consists of over 5,160 detectors buried deep into the ice in the South Pole in Antarctica, and the size of the entire ensemble is about one cubic kilometre.
This extraordinary size is what makes it sensitive to the elusive neutrino, which interacts extremely weakly with matter. When asked why they believe these neutrinos are from outside the solar system, Nathan Whitehorn, of the IceCube collaboration from the University of Madison, notes in an email to The Hindu: “Our evidence comes principally from having relatively few muon-type neutrinos (atmospheric neutrinos are exclusively of this kind), having them reach much higher energies than should be produced locally, and arriving mostly from the southern hemisphere.”
The last point is interesting, because neutrinos produced in the atmosphere are made in the interactions of cosmic rays with air molecules and are produced as part of the particle showers created in these collisions. “For neutrinos coming from the southern sky (directly above IceCube), the detector should also see the rest of the particles in the shower along with atmospheric neutrinos. But it does not, which implies there was no associated shower and thus that the events are extremely likely — though not certainly — to be coming from beyond the atmosphere,” says Dr Whitehorn.
When asked to comment on the results, Shrihari Gopalakrishnan, particle physicist from Institute of Mathematical Sciences, Chennai, says that such high-energy neutrinos have never been measured before and they carry information about things we may not be capable of knowing about otherwise.
The neutrinos form a unique window. “We do not know the mechanism which has accelerated these particles to this energy, and that is important. There are various hypotheses — it could be active galactic nuclei, cores of quasars or massive black holes in the cores of galaxies.”