Physicists using the IceCube Neutrino Observatory – a cubic-kilometer-sized detector sunk into the ice sheet at the South Pole – have announced a new observation of high-energy neutrinos that originated beyond the Solar System, and beyond our Milky Way Galaxy. The first evidence for astrophysical neutrinos was announced by the team in November 2013. The results published this week in the journal Physical Review Letters are the first independent confirmation of this discovery.
A graphic of one of the highest-energy neutrino events is superimposed on a view of the IceCube Neutrino Observatiory at NSF’s Amundsen-Scott South Pole Station. Image credit: IceCube Collaboration.
Because neutrinos have almost no mass and no electric charge, they can be very hard to detect and are only observed indirectly when they collide with other particles to create muons, telltale secondary particles. What’s more, there are different kinds of neutrinos produced in different astrophysical processes.
The IceCube Neutrino Observatory records a hundred thousand neutrinos every year, most of them produced by the interaction of cosmic rays with the Earth’s atmosphere. Billions of atmospheric muons created in the same interactions also leave traces in the detector.
And from all of these, physicists are looking for only a few dozen astrophysical neutrinos, which will expand our current understanding of the Universe.
More than 35,000 neutrinos were found in data recorded by the IceCube between May 2010 and May 2012.
However, only 21 of those neutrino events were clocked at energy levels indicative of astrophysical sources.
“This is an excellent confirmation of IceCube’s recent discoveries, opening the doors to a new era in particle physics,” said Dr Vladimir Papitashvili of the NSF’s Division of Polar Programs.
The latest observations were made by pointing the IceCube detector through the Earth to observe the Northern Hemisphere sky.
The observed high-energy neutrinos are a brand-new neutrino sample, with only one event in common with the first results announced in 2013, which searched for high-energy neutrinos that had interacted with the ice inside IceCube during the same data-taking period.
The current search looked for muon neutrinos only. These neutrinos produce a muon when they interact with the ice and have a characteristic signature in IceCube, called a track, that makes them easy to identify.
The same shape is expected for an atmospheric muon, but by looking only at the Northern Hemisphere, physicists know that a detected muon could have only been produced by a neutrino interaction.
“Looking for muon neutrinos reaching the detector through the Earth is the way IceCube was supposed to do neutrino astronomy and it has delivered. This is as close to independent confirmation as one can get with a unique instrument,” said Prof Francis Halzen of the University of Wisconsin-Madison, principal investigator of IceCube.
But while the new observations confirm the existence of astrophysical neutrinos, actual point sources of high-energy neutrinos remain to be identified.
“While the neutrino-induced tracks recorded by the IceCube detector have a good pointing resolution, within less than a degree, the IceCube team has not observed a significant number of neutrinos emanating from any single source,” said Prof Albrecht Karle, also from the University of Wisconsin-Madison.
The neutrinos observed in the latest search, however, have energy levels identical to those seen when the observatory sampled the sky of the Southern Hemisphere.
“That suggests that many of the potential sources of the highest-energy neutrinos are generated beyond the Milky Way,” Prof Karle said.
“If there were a significant number of sources in our own Galaxy, the IceCube detector would light up when observing the plane of our Galaxy – the region where most neutrino-generating sources would likely be found.”
“The plane of the Galaxy is where the stars are. It is where cosmic rays are accelerated, so you would expect to see more sources there. But the highest-energy neutrinos we’ve observed come from random directions,” Prof Karle said. “It is sound confirmation that the discovery of cosmic neutrinos from beyond our Galaxy is real.”
In addition, the new high-energy neutrino sample, when combined with previous IceCube measurements, allows the most accurate measurements to date of the energy spectrum and neutrino-type composition of the extraterrestrial neutrino flux. Those results are published in an accompanying paper in the Astrophysical Journal.
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M. G. Aartsen et al. 2015. Evidence for Astrophysical Muon Neutrinos from the Northern Sky with IceCube. Phys. Rev. Lett. 115, 081102; doi: 10.1103/PhysRevLett.115.081102
M. G. Aartsen et al. 2015. A Combined Maximum-likelihood Analysis of the High-energy Astrophysical Neutrino Flux Measured with IceCube. ApJ 809, 98; doi: 10.1088/0004-637X/809/1/98
