University of Wisconsin-Madison

First observation of high-energy neutrino oscillations by DeepCore and IceCube

Even though neutrino oscillations are now an established fact within the scientific community, many questions remain to be answered. Among them, a precise measurement of the three neutrino squared-mass splittings and of the mixing angles, which describe the relationship between neutrino flavor and mass states. This set of observables, together with the energy and the distance traveled by neutrinos, determines the oscillation probability, i.e., the probability of the survival or disappearance of a given neutrino.

The IceCube Neutrino Observatory, which comprises the IceCube and DeepCore detectors, has been designed to contribute heavily to our understanding of these ghostly particles. In a paper submitted today to Physical Review Letters, the IceCube Collaboration has announced the first statistically significant detection of neutrino oscillations in the high-energy region (20–100 GeV), verifying the first but low-significance indication reported a few months ago by the ANTARES collaboration. Dr. Andreas Gross from Technische Universität München, the corresponding author of this publication, says, "The measurement of high-energy neutrino oscillations with more than a 5-sigma significance confirms that IceCube and DeepCore are ready to provide new insights into neutrino physics. The goal is now to collect more statistics and become more competitive with other experiments."

Signifi`cance contours for the IceCube+DeepCore atmospheric neutrino oscillation analysis, compared with the results of ANTARES, MINOS and SuperKamiokande. Image: IceCube Collaboration.
Signifi`cance contours for the IceCube+DeepCore atmospheric neutrino oscillation analysis, compared with the results of ANTARES, MINOS and SuperKamiokande. Image: IceCube Collaboration.

The analysis performed, using a two-neutrino flavor formalism, allows for a new estimation of the oscillation parameters and with maximum mixing favored. These results are in good agreement with previous measurements by the MINOS and SuperKamiokande detectors.

Data used for this analysis was collected from May 2010 to May 2011 by IceCube and DeepCore. The IceCube detector is an array with 86 strings of digital sensors deployed in Antarctica’s ice sheet at depths from 1,450 to 2,450 meters. The main array defines the high-energy detector, designed to detect neutrinos from hundreds of GeV to PeV energies. The DeepCore subdetector adds 8 additional strings near the center of this array, but only 6 were deployed during the data-taking period covered by this analysis. Having a denser core detector allows lowering the energy threshold to about 20 GeV.

More efficient event reconstruction methods are being tested, which together with new data sets will increase the sensitivity of the IceCube and DeepCore detectors to atmospheric neutrino oscillations. As a result of these improvements, the IceCube Collaboration is expecting to set further constraints on the oscillation parameters in the coming months.

Neutrinos as described by our theoretical models

Neutrinos are massless particles in the Standard Model, one of the most successful theoretical models of all times, but this characterization fails to describe real neutrinos. The observation of anomalies in atmospheric and solar neutrino fluxes, and later of neutrino oscillations, are good examples of how new physics may transpire when looking at our universe. In a simple extension of the Standard Model, neutrino oscillations can be explained thanks to a quantum phenomenon: that neutrino mass states are a mixing of the flavor states and thus have some probability of being detected as a given flavor. For atmospheric neutrinos reaching IceCube after passing through the entire Earth, the probability of muon neutrinos oscillating into tau neutrinos peaks around 25 GeV. DeepCore was designed to capture those high-energy incoming neutrinos, and data taken after full deployment should allow for setting more restrictive limits on the parameters that define neutrino oscillations.

+ info "Measurement of Atmospheric Neutrino Oscillations with IceCube," IceCube Collaboration: M.G. Aartsen et al. Submitted to Physical Review Letters, arXiv:1305.3909