‘Tis the season for good tidings from the North Pole, but this week some particle physicists are giddy about a humongous gift at the South Pole. The world’s largest detector for high-energy neutrinos was completed December 18 when scientists lowered the last of 5,160 sensors more than a mile beneath the ice of the Antarctic plateau.
The IceCube Neutrino Observatory will hunt for tiny particles that are common in the universe, but rarely interact with other matter. In fact, trillions of neutrinos pass through a person’s body each second. They rain down onto Earth as cosmic rays strike the upper atmosphere. Neutrinos also shoot out of the violent insides of stellar explosions, churn regularly from the sun and may even arise from the ambient leftovers of the Big Bang.
IceCube is tuned to find high-energy neutrinos like the ones bursting from active galactic nuclei, which are bright sources that are likely the radiation from a black hole gobbling the mass around it, and gamma ray bursts, intense beams of light from a star collapsing into a black hole. The $279 million observatory is a full cubic kilometer in volume, or 1,000 times bigger than the Super-Kamiokande neutrino detector in Japan. While IceCube is less sensitive than the Super-K, scientists will need the huge volume to see long streaks of muons, exotic leftovers from collisions between neutrinos and water nuclei.
IceCube’s sensors are designed to detect a flash of blue light when neutrinos collide with a water molecule. Ice at the South Pole is remarkably pure, so impinging neutrinos will almost certainly interact with water, not a different molecule. And because each new snowfall adds weight, packing down the ice below, there are a lot of molecules for a neutrino to hit.
Unlike most physics experiments, IceCube began taking data while under construction. Since 2005, it has already seen neutrinos with energies as high as 100 trillion electron-volts, seven times the maximum power that will be produced by collisions between protons at the Large Hadron Collider near Geneva, Switzerland.
Astrophysicists have a long list of scientific questions for IceCube to investigate during its planned 15-year life. For example, physicists believe that supernovas accelerate protons, but evidence is circumstantial. Seeing high-energy neutrinos, which should spew out of the bursting stars along with protons, would confirm theories of how stars explode.
“People have known for a long time it must be there, but to see it, and measure the right number, is an important thing to do,” says project spokesperson Tom Gaisser, a physicist at the University of Delaware in Newark.
Also on the wish list for scientists: finding neutrinos produced when hypothesized dark matter particles annihilate in the sun. Dark matter is thought to be much more abundant than ordinary matter in the universe, but has not yet been detected.