Scientists in Antartica detect cosmic neutrinos from outside our galaxy

By Ryan Whitwam

Few particles in the universe are as strange and interesting as the neutrino. These elementary particles have no charge and so little mass that they can usually slip through matter without leaving a trace. While you read this sentence, several thousand neutrinos have shot right through your body unimpeded by all the squishy stuff inside. Naturally, detecting these particles is not easy, but a team of researchers working in Antarctica has managed to detect the rarest neutrinos of all — cosmic neutrinos from beyond the Milky Way.

Neutrinos can be created in a number of ways. For example, as a product of nuclear decay or in a nuclear reaction. The sun pumps out a steady supply of neutrinos that (mostly) shoot straight through Earth. The highest energy neutrinos are created when cosmic rays strike the Earth’s atmosphere and rain neutrinos down on us, as well as in supernovae and around black holes. That means neutrino sources can point astronomers to some very interesting stellar phenomena. First we have to spot the particles, though.

The IceCube Neutrino Observatory near the south pole is perfectly positioned to filter out the noise as it searches for neutrinos. IceCube consists of 86 shafts dug 8,000 feet down into the ice. Optical detectors in these shafts look for the minuscule blink of light resulting from a neutron striking one of the nuclei in the ice, but only one out of every few trillion neutrinos will collide with anything in these shafts. The light is actually produced by particles called muons released when a neutrino impacts the nucleus.

In the past, IceCube has shown that it can detect neutrinos originating within our galaxy, but what scientists really wanted to find were neutrinos coming from sources outside the Milky Way. These so-called “cosmic neutrinos” would be very high energy and indicative of energetic cosmic events far away in space. The facility’s location in Antarctica allowed scientists to point the instrument downward, so only neutrinos passing through the Earth first would be detected. That filters out all the high-energy neutrinos produced by cosmic rays.

Neutrinos of the proper energy were found several years ago, but now the team has spent two years combing through data to prove that IceCube can tell the difference between neutrinos originating inside our galaxy and those coming from outside. To do that, the team looked for neutrinos of the same energy that seem to come from all directions at the same rate. That can only happen if the source is outside our galaxy.

The team detected more than 35,000 neutrinos between May 2010 and May 2012. Of those, 20 had enough energy to suggest they came from another galaxy. Those 20 nuetrinos came from various directions, but at the same rate observed in previous runs. The authors say that confirms it. Improved detection of these rare neutrinos could help astronomers find new objects to study — objects that are much easier to observe than the neutrinos they create.