The IceCube experiment has taken hits from three neutrinos carrying energies above the outlandishly high peta–electron volt range that suggest they may radiate from titanic explosions in the depths of space.
A belowground experiment at the South Pole has now discovered three of the highest-energy neutrinos ever found, particles that may be created in the most violent explosions of the universe. These neutrinos all have energies at the absurdly high scale of peta–electron volts—roughly the energy equivalent of one million times a proton’s mass. (As Albert Einstein showed in his famous E = mc2equation, energy and mass are equivalent, and such a large amount of mass converts to an extreme level of energy.) The experiment, called IceCube, reported the discovery of the first two—nicknamed Ernie and Bert—last year, and announced the third Monday here at the American Physical Society meeting. “Internally, it’s known as Big Bird,” said IceCube physicist Chris Weaver of the University of Wisconsin–Madison.
These neutrinos are valuable because they are extremely standoffish, rarely ever interacting with other particles, and are uncharged, so their direction is never swayed by magnetic fields in the universe. Thus, their trajectories should point straight back to their source, which astronomers think could be a variety of intense events such as humongous black holes accreting matter, explosions called gamma-ray bursts or galaxies forming stars at furious rates.
This penchant for noninteraction also makes neutrinos extremely difficult to detect. The IceCube experiment looks for the very rare occasions when neutrinos collide with atoms in a cubic kilometer of ice buried underneath the South Pole. Such shielding is necessary to filter out collisions from other particles, but does not inhibit neutrinos. The experiment capitalizes on the naturally pure ice there, using a region that extends twice the depth of the Grand Canyon underground. Thousands of light detectors are imbedded in the ice to catch the little blips of light created when neutrinos are caught. Such interactions are so infrequent that IceCube researchers had to search for two years to find these three high-energy neutrinos. During that time span the instrument also detected 34 neutrinos of somewhat lower energies. Some of these neutrinos are thought to be contamination created when charged particles called cosmic rays hit Earth’s atmosphere, but some portion of IceCube’s haul likely came directly from violent processes in the cosmos. Those particles are called astrophysical neutrinos. “It looks like we have reached compelling evidence for astrophysical neutrinos,” said U.W.–Madison physicist Albrecht Karle, a member of the IceCube team.
Written By: Clara Moskowitz
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