UC Berkeley

At the IceCube Neutrino Observatory, neutrino interactions are observed across a broad energy range extending from ~ 5 GeV to ~ 1 PeV. Above ~ 100 TeV, neutrinos primarily originate from distant astrophysical objects whose identity is yet unknown. At lower energies, most neutrinos result from cosmic-ray interactions in Earth's atmosphere. In this work, two studies of contained neutrino interactions in IceCube are presented.

In the first study, three years of IceCube data recorded from May 2010 to May 2013 are analyzed to measure the flavor composition of astrophysical neutrinos in the energy range from 35 TeV to 1.9 PeV. The flavor composition is found to be consistent with the ratio of ~ (1/3 : 1/3 : 1/3) expected from the oscillation of neutrinos produced by complete pion decay in distant astrophysical sources. Limits are placed on non-standard flavor compositions that could arise in exotic physics scenarios. The energy spectrum of astrophysical neutrinos is also measured and is well-described by a falling power law with index 2.6+-0.15.

In the second study, five years of IceCube data from May 2011 to May 2016 are analyzed with improved background rejection and event classification techniques, and the inelasticity in charged-current muon neutrino interactions is reconstructed. Improved limits on the flavor composition of astrophysical neutrinos are obtained, and there is consistency with the expected flavor ratio of ~ (1/3 : 1/3 : 1/3). Additionally, the energy spectrum of astrophysical neutrinos is found to be consistent with a power-law with index 2.62+-0.07 in the energy range from 6.6 TeV to 2.2 PeV. Limits are placed on a power-law flux from a second population of astrophysical sources that may explain the harder power-law index of 2.13+-0.13 found in an IceCube analysis of up-going muons at higher energies.

The inelasticity distribution of charged-current muon neutrino interactions is also obtained and is found to be consistent with the calculation of Cooper-Sarkar et al. across an energy range from 1 TeV to 100 TeV. The inelasticity distribution can also be used measure the neutrino to antineutrino ratio of the atmospheric neutrino flux. A scaling factor on the muon neutrino to antineutrino flux ratio calculated by Honda et al. is found to be 0.77+0.44-0.25 in the energy range from 770 GeV to 21 TeV. Lastly, the inelasticity distribution can also be used to perform an indirect search for charged-current charm production by neutrinos. Assuming a leading-order calculation of the inelasticity distribution for charm production events, zero charm production can be excluded at 91% confidence level in the energy range from 1.5 TeV to 340 TeV. In the future, it is expected that the techniques developed for this analysis can be applied to search for neutrino interactions beyond the Standard Model in IceCube and next-generation neutrino telescopes.