University of Wisconsin-Madison

Research Highlights

Neutrino Astronomy

IceCube was built to search for very high energy neutrinos created in the most extreme cosmic environments. The improved performance at EeV energies has opened a window to search for cosmogenic neutrino interactions, produced by the interaction of extragalactic cosmic rays with cosmic microwave background photons.

We have observed the first astrophysical high-energy neutrino flux ever, with significance at the 5.7 sigma level after analyzing three years of data (2010-2013). The importance of the result, first published in Science, has been acknowledged by the community, e.g., Physics World, Scientific American, Nature, and the editors of the Physical Review journals from the American Physical Society, but also by mainstream media such as The New York Times, BBC News, or Smithsonian magazine.

We have established the world’s best limit on a flux of cosmogenic neutrinos. In this search, we discovered three neutrino events with energies at the PeV level. These are the highest energy neutrinos ever detected.


IceCube has detected the highest energy neutrinos ever recorded, with energies reaching above 2 PeV. From left to right, Bert, Ernie and Big Bird, with energies of 1.0, 1.1 and 2.2 PeV.
IceCube has detected the highest energy neutrinos ever recorded, with energies reaching above 2 PeV. From left to right, Bert, Ernie and Big Bird, with energies of 1.0, 1.1 and 2.2 PeV.

The search for very high energy neutrinos is primarily driven by the quest for discovering the origin and nature of cosmic rays. The dozens of astrophysical neutrinos observed so far do not allow us to identify any individual source.

The surprisingly high level of the neutrino flux observed implies that a significant fraction, possibly all, of the energy in the non-thermal universe is generated in hadronic accelerators. The non-thermal universe contains collapsed objects such as black holes or neutron stars. High-energy neutrinos, which are unique fingerprints of hadron acceleration, therefore represent a discovery potential for either revealing new sources or unveiling new insight on the energy generation in known sources.

We have also performed all-sky surveys looking for extended regions of neutrino emission, searches using point-source as well as stacked-source catalogs, and searches for faint point sources. No evidence of neutrino emission has been found yet, but some ideas on the origin of cosmic rays have been challenged.

Arrival directions of the 37 very high energy events found in IceCube after analyzing three years of data (2010–2013). The grey line denotes the equatorial plane. The color map shows the test statistic (TS) for the point-source clustering test at each location. No significant clustering was observed.
Arrival directions of the 37 very high energy events found in IceCube after analyzing three years of data (2010–2013). The grey line denotes the equatorial plane. The color map shows the test statistic (TS) for the point-source clustering test at each location. No significant clustering was observed.

As GRBs were the most promising candidate sources of ultra-high-energy cosmic rays (UHECRs), we have also undertaken a multiwavelength search for neutrino emission in coincidence with GRBs. We conclude that the GRB contribution to the observed astrophysical neutrino flux cannot be larger than about 1%.

Cosmic Ray Physics

IceCube is a powerful neutrino telescope but also a huge muon detector that registers more than 100 billion muons per year, produced by the interaction of cosmic rays in the Earth’s atmosphere.

We measured the cosmic-ray anisotropy for the first time in the Southern Hemisphere. The arrival direction distribution is not isotropic but shows significant structure on several angular scales, at the level of 10-3 in relative amplitude, similar to previous observations in the Northern Hemisphere. We have also observed structure on scales between 15 degrees and 30 degrees with lower amplitudes. The origin of these anisotropies is still unknown.

Using IceTop, the IceCube air shower array sensitive to cosmic rays between 100 TeV and 1 EeV, we have extended searches for cosmic-ray anisotropy at the 10-3 level to PeV energies. We have shown that this anisotropy persists to PeV energies and even increases in amplitude.

Cosmic-ray anisotropy sky maps from IceCube. The two maps at 20 TeV median energy, on the left, show the anisotropy structures at larger and smaller angular scales, the latter derived from the first by filtering out features at scales larger than about 60 degrees. Note that the amplitude of small-scale anisotropy structures is smaller than that at large scale. Looking at the anisotropy maps at the different energies, the large-scale anisotropy topology changes substantially between 20 TeV and 400 TeV, but then mainly increases in amplitude when reaching the PeV scale.
Cosmic-ray anisotropy sky maps from IceCube. The two maps at 20 TeV median energy, on the left, show the anisotropy structures at larger and smaller angular scales, the latter derived from the first by filtering out features at scales larger than about 60 degrees. Note that the amplitude of small-scale anisotropy structures is smaller than that at large scale. Looking at the anisotropy maps at the different energies, the large-scale anisotropy topology changes substantially between 20 TeV and 400 TeV, but then mainly increases in amplitude when reaching the PeV scale.

We have performed studies of the chemical composition of cosmic rays using the combined IceTop shower and IceCube muon data. We found a transition from light to heavier nuclei as energy increases, which could be attributed to the end of the galactic cosmic-ray spectrum.

We have also reported a measurement of the all-particle cosmic-ray energy spectrum in the energy range from 1.6 PeV to 1.3 EeV using data from IceTop, which exhibits clear deviations from power law behavior.

Neutrino Physics

The DeepCore subdetector allows IceCube to extend the measurement of the neutrino flux from 100 TeV down to 10 GeV. At these energies, we can observe atmospheric neutrino oscillations and perform searches for sterile neutrinos.

We have measured the atmospheric oscillation parameters with a precision compatible with and comparable to those of the dedicated oscillation experiments, such as MINOS, T2K, or Super-Kamiokande.

Oscillation parameters measured with three years of IceCube data (2011–2014).  The 90% confidence contours are shown in comparison with the ones of the most sensitive experiments. At the top and on the right side of the figure, the log-likelihood profiles for individual oscillation parameters are given. Normal mass hierarchy is assumed.
Oscillation parameters measured with three years of IceCube data (2011–2014). The 90% confidence contours are shown in comparison with the ones of the most sensitive experiments. At the top and on the right side of the figure, the log-likelihood profiles for individual oscillation parameters are given. Normal mass hierarchy is assumed.

Dark Matter

We have produced the world’s best limits on the spin-dependent cross section for weakly interacting dark matter particles. They are derived from the failure to observe the annihilation into neutrinos of dark matter particles gravitationally trapped by the Sun. IceCube can only perform indirect searches for dark matter, i.e., looking for the neutrino signature of dark matter annihilation. Other searches have addressed annihilations in the galactic halo and galaxy clusters.

Shows 90% confidence level upper limits on the spin-dependent cross section for hard and soft annihilation channels over a range of WIMP masses using IceCube data from June 2010 to May 2011. Results are shown in comparison to recent results by other accelerator, cosmological, and direct dark matter search constraints.
Shows 90% confidence level upper limits on the spin-dependent cross section for hard and soft annihilation channels over a range of WIMP masses using IceCube data from June 2010 to May 2011. Results are shown in comparison to recent results by other accelerator, cosmological, and direct dark matter search constraints.

The neutralino, the lightest weakly interacting massive particle (WIMP) in many supersymmetric models, is the usual test candidate in dark matter searches. The Sun is the golden channel for searches of the annihilation of WIMPS since the expected signature is not subject to any astrophysical ambiguities. As seen in the graphic, IceCube has set limits on a combination of mass and cross-section, i.e., for a given mass we exclude a cross-section higher than a certain value. These are the most stringent upper limits yet for spin-dependent interactions of dark matter particles with ordinary matter.

Glaciology

To realize the full potential of IceCube, the properties of light propagation in the Antarctic ice must be well understood. We have made the most detailed measurements ever of these properties using both light sources deployed on board the IceCube sensors and a dedicated borehole laser probe called the “dust logger.” We also observed that light propagates preferentially in the direction of the movement of the South Pole glacier.

We have shown that ice layers tilt by as much as 10% across IceCube, likely following the topography of the underlying bedrock almost two miles down.


Movie of dust isochrons changing with depth in IceCube as measured with the dust logger. The Z-axis is shown in meters.
Movie of dust isochrons changing with depth in IceCube as measured with the dust logger. The Z-axis is shown in meters.

By comparing the laser data to ice core measurements, we were able to reconstruct a detailed climate record of the last glacial period. We found evidence for the Tova volcano eruption 74,000 years ago, which had never been observed in ice-core studies. The results have played a role in the designation of the South Pole as the site of the next major American ice coring mission.