This award provides continued funding for the UW-Madison group's scientific analysis of data collected with the South Pole IceCube neutrino observatory. IceCube will reach a total exposure of a km-squared-year of data within one year, a target neutrino astronomy has aspired to since the nineteen seventies. If typical estimates of their flux are valid, they will reach the sensitivity to observe the flux of cosmogenic neutrinos produced by the interactions of cosmic rays with microwave photons. If gamma ray bursts accelerate the observed extragalactic cosmic rays, they will detect the neutrinos produced when the protons and photons coexist in the fireball.
The group participates in all major scientific analyses; in this project they emphasize that their goal is to not only increase statistics and better understand the systematics of the detector, but to focus on improvements of the analysis methods made possible by a larger and better instrument. UW-Madison scientists contribute on average half of their time to service tasks required to maintain and operate the detector. IceCube will undoubtedly enter a phase of modifications and enhancements of capabilities resulting from experience operating the detector and from new science opportunities. The group actively participates in the R&D program that capitalizes on the current IceCube scientific program. The radio detection of ultra high-energy neutrinos is the main focus at this time.
Broader Impacts: The mystique of the South Pole environment and the compelling science are an alluring mix. The results of IceCube science will enhance scientific and technological understanding on many levels through broad dissemination of the results and experiences. IceCube is in a unique class of projects that inspire the innovative capacity of a new generation of American scientists and engineers.
The award covered the 2010–2013 scientific analysis at the University of Wisconsin–Madison of data taken with the IceCube neutrino detector. The IceCube detector, completed in 2010, transformed a cubic kilometer of mile-deep natural Antarctic ice into a neutrino detector. Neutrinos are tiny, nearly massless particles that travel at near light speed and are remarkably abundant in the Universe. IceCube looks for neutrinos created in or around powerful cosmic objects such as black holes or gamma-ray bursts with the aim of yielding important insights into the origins and nature of cosmic rays. Our group has produced the first evidence for a flux of very high-energy neutrinos from outside our solar system, some with energies more than a thousand times those produced by earthbound particle accelerators. This outstanding achievement has made IceCube a leading detector among an extensive group of diverse instruments attempting to pinpoint the still enigmatic sources of comic rays. Nature accelerates elementary particles, or cosmic rays, to energies in excess of 1020 electron volts, a macroscopic energy of 50 joules carried by a single particle. Because they are electrically charged, their paths are scrambled by magnetic fields during their journey through the Universe, and therefore their arrival directions on Earth do not reveal their origin. Neutrinos, with essentially no mass and no electric charge, are predicted to be produced alongside high-energy cosmic rays and thus represent the ideal particle for "extreme" astronomy. They can travel unscathed from the edge of the Universe and, hopefully, from the nuclear furnaces where cosmic rays are born. Unfortunately, their weak interaction with matter makes neutrinos very difficult to detect. Immense particle detectors, such as IceCube, are required in order to collect them in sufficient numbers for scientific analyses. The astrophysical neutrino flux determined by IceCube was from 28 detected events, two of them with energies above 1 PeV, or petaelectronvolts (1015 electron volts). We have shown, with a probability greater than 99.9999%, that this signal is not produced by atmospheric neutrinos, and that the properties of the observed flux follow our expectations for neutrinos coming from sources outside our solar system. This in itself was an exciting discovery. So, where do these neutrinos come from? We do not have a conclusive answer yet, but more data is already available and improvements to the analysis continue. These results are undoubtedly our most outstanding scientific achievement, but our group participates in almost all major analyses performed with IceCube. For example, in the last three years, we have mapped Galactic cosmic rays in the Southern Hemisphere confirming observations in the Northern Hemisphere of enigmatic structures in the sky map. We showed that these structures persist at high energy and are not associated with the sun. Further observations and theoretical modeling may allow us to finally identify the sources of the Galactic cosmic rays. Also, our failure to observe neutrinos in association with gamma-ray bursts (GRBs) has challenged the theory that GRBs are the sources of extragalactic cosmic rays. At UW–Madison, we share space with other scientists, engineers and technicians that operate the IceCube detector and thus are heavily involved in improving the performance of the detector. Our work has resulted in an increased sensitivity of the instrument, an improved high-energy angular resolution of up to 0.2° for muon tracks, and an energy resolution of about 15% for energies above 1 TeV. The success of IceCube followed on the heels of research and development that had started in the early 1990s, with the Antarctic Muon and Neutrino Detector Array (AMANDA). The UW–Madison group has established a solid and lengthy record for analyzing data from both IceCube and AMANDA, underscoring our capability to attract top students and postdoctoral researchers. The mystique of the South Pole environment and the compelling science of IceCube are an alluring mix, which give us a unique opportunity to enhance scientific and technological understanding on many levels. Highlights of these efforts, some of them partially supported by other funding awards, include a two-year outreach program that will bring IceCube research and researchers to every UW campus, webcasts from the South Pole, a high school research internship to explore IceCube data, and a full-scale planetarium show produced in collaboration with the Milwaukee Public Museum. We maintain ongoing schedules of talks and hands-on activities for broad audiences, often join in cooperation with public science programs at other institutions. IceCube is in a unique class of projects, like the space program and the Large Hadron Collider. It inspires an upcoming generation of American scientists and engineers to solve our most vexing problems by applying new knowledge of the Universe and creating innovative technology to improve lives and expand options for future explorations.
