A measurement of the diffuse astrophysical muon neutrino flux using multiple years of IceCube data

The IceCube Collaboration has observed a high-energy astrophysical neutrino flux using neutrino candidates with interaction vertices contained within the instrumented volume. A complementary measurement can be done with charged current muon neutrinos where the interaction vertex can be outside the instrumented volume. Due to the large muon range the effective area is significantly larger but the field of view is limited to the Northern Hemisphere. IceCube data from 2009 through 2012 have been analyzed by a likelihood approach with reconstructed muon energy and zenith angle as observables. While the majority of these events are atmospheric neutrinos, the highest energy events are incompatible with that interpretation. Assuming the astrophysical muon neutrino flux to be isotropic the data is well described by an unbroken power law with a normalization of 0.66 +0.40 -0.30 10 -18 GeV -1 cm -2 s -1 sr -1 at 100 TeV neutrino energy and a spectral index of = 1.91 0.20. A purely atmospheric origin of the observed events can be rejected with a significance of 4.3 . The found spectrum is harder compared to other IceCube measurements of astrophysical neutrinos which are dominantly based on neutrino-induced cascades. These measurements are sensitive to lower neutrino energies. Investigations about the nature of the observed differences are ongoing.