IceCube and the discovery of high-energy cosmic neutrinos

By transforming a cubic kilometer of natural Antarctic ice into a neutrino detector, the IceCube project created the opportunity to observe cosmic neutrinos. We describe the experiment and the complementary methods presently used to study the flux of the recently discovered cosmic neutrinos. In one method, events are selected in which neutrinos interacted inside the instrumented volume of the detector, yielding a sample of events dominated by neutrinos of electron and tau flavor. Alternatively, another method detects secondary muons produced by neutrinos selected for having traveled through the Earth to reach the detector, providing a pure sample of muon neutrinos. We will summarize the results obtained with the enlarged data set collected since the initial discovery and appraise the current status of high-energy neutrino astronomy. The large extragalactic neutrino flux observed points to a nonthermal universe with comparable energy in neutrinos, gamma rays and cosmic rays. Continued observations may be closing in on the source candidates. In this context, we highlight the potential of multimessenger analyses as well as the compelling case for constructing a next-generation detector larger in volume by one order of magnitude.