The intellectual merit of this work is that it measures high-energy neutrino production in the universe and tests the standard model of particle physics thus increasing our fundamental knowledge of the high-energy universe. The origin of the highest energy cosmic-rays is one of the most compelling mysteries in modern physics. Air shower detectors have observed events with energies up to 3x10^20 electron volt (eV). However, despite decades of work, their sources, nuclear composition and acceleration mechanisms remain unknown. Some possible sources include active galactic nuclei, gamma-ray bursters, and/or supernova remnants. Observation of astrophysical neutrinos will tell us that hadrons must be accelerated in these sources; the neutrino flux and spectrum will significantly constrain theoretical models.
This award will support a search for the diffuse flux of neutrinos produced in these cosmic accelerators, using data from the recently completed IceCube detector. The work will be in two areas. The first is to search for a diffuse flux of electron and tau neutrinos. The group has demonstrated the power of using the outer strings and the top and bottom of IceCube to veto entering muon tracks by using it to measure the atmospheric electron neutrino flux from 50 gigaelectronvolt (GeV) to 10 teraelectronvolt (TeV). They will extend this technique to neutrinos with energies from 10 TeV up to ~ 1 petaelectronvolt (PeV). They will also search for a diffuse flux of high-energy tau neutrinos, through their distinctive topology - two separated cascades. Second, they will use these neutrinos to make a measurement of the neutrino-nucleon cross-section at an energy around 50 TeV - by measuring neutrino absorption in the Earth.
Broader Impacts: The observation of extra-terrestrial neutrinos will have broad significance for astrophysics. The observation of hadronic accelerators in the universe would answer one of the 11 questions posed in the National Research Council study on the Physics of the Universe: "Where do ultra-high energy particles come from" The Education and Outreach component will communicate this forefront research to students and the general public, through presentations at elementary through high schools and public lectures.
