CAPTURING COSMIC NEUTRINOS: WHAT THE GHOST PARTICLE

Jim Madsen, IceCube Associate Director for Education and Outreach Chair at U. W. River Falls Physics Department talked about how  Physics World’s 2013 Breakthrough of the Year award went to the IceCube Neutrino Observatory at the South Pole for making the first observations of high-energy cosmic neutrinos.  Neutrinos are produced in the most energetic and extreme phenomena in the universe.  They are neutral and point back to their source.  Neutrinos can turn into protons and vice versa by emitting an electron.  Neutrinos are called a “Ghost Particle” because they are an invisible particle that carries away energy and they are hard to capture.  When a neutrino interacts with something, the particle charges and emits light.  Then you can tell the direction of the particle, how much energy it has and the time it happens. The idea to explore the universe by detecting neutrinos goes back more than five decades. Reines and Cowan detected neutrinos in a nuclear reactor in 1956.  Even early promoters knew that mapping the cosmos with high-energy neutrinos would require a detector of unprecedented size—a cubic kilometer of pristine transparent material.  The initial idea was to deploy a grid of light sensors in water, and smaller neutrino telescopes are currently operating in the Mediterranean Sea, and in Lake Baikal in Russia.  But so far, only the aptly named international IceCube Collaboration has constructed a cubic-kilometer-scale detector, and in the last year they isolated convincing evidence for high-energy neutrinos produced in outer space.  The IceCube Neutrino Observatory, the result of decades of design and seven seasons of construction in the South Pole ice sheet, is an incredible example of creativity, perseverance, and a bit of luck.  The IceCube Project is conducted in the South Pole because they have the infrastructure and the ice is incredibly clear.  The IceCube Collaboration, which consists of 43 institutions around the world and over 300 scientists, delivered a detector on time, on budget, and exceeding design performance specifications.  Dedicated teams worked hard to deploy over 5,000 light sensors to depths between 1,450 and 2,450 meters below the surface.  With some equally amazing ingenuity, scientists were able to find about one dozen cosmic high-energy neutrinos per year out of the roughly one hundred billion events recorded annually.  A few things to remember is that the universe is immense and mostly unexplained and science and technology enable both to advance.  The next steps are to continue to optimize analysis and detector performance and high and low energy extensions are under consideration.