The IceCube Neutrino Observatory is the first, and so far the only, cubic-kilometer neutrino telescope. IceCube is a flagship experiment in neutrino and multimessenger astronomy thanks to the discovery of very high energy cosmic neutrinos and the detection of the first likely source of high-energy neutrinos, a blazar that was also observed with gamma rays and lower energy photons. IceCube is also a multipurpose research facility with outstanding precision measurements in neutrino physics and exceptional contributions to cosmic ray physics, dark matter searches, and glaciology.
Neutrino Astronomy and Multimessenger Astrophysics
Neutrino astronomy has emerged as a means to reveal the sources of the highest energy particles in our universe, the so-called ultra-high-energy cosmic rays (UHECR). IceCube’s detection of the first high-energy astrophysical neutrino flux confirmed cosmic neutrinos as the key messengers to reveal an unobstructed view of the universe at wavelengths where it is opaque to light.
Energy and wavelength spectra vs distance of the visible universe. About a fifth of the universe cannot be explored using photon-based telescopes.
The large neutrino flux measured by IceCube also implies that a significant fraction, possibly all, of the energy in the nonthermal universe is generated in powerful hadronic accelerators powered by objects such as black holes or neutron stars.
The figure shows that the astrophysical neutrino flux (black line) observed by IceCube matches the corresponding gamma-ray flux (red line) observed by Fermi. The black data points are combined IceCube results showing the flux of cosmic neutrinos interacting inside the detector. Also shown, shaded in blue, is the best fit to the flux of cosmic muon neutrinos penetrating the Earth. (see paper)
In July 2018, IceCube, gamma-ray telescopes Fermi and MAGIC, and several other experiments announced the detection of neutrinos and photons from blazar TXS 0506+056. These results constitute the first-ever identification of a likely source of extragalactic neutrinos and of high-energy cosmic rays.
This breakthrough detection is the outcome of a multimessenger collaboration with detectors and scientists across the globe and in space. Follow-up observations by gamma-ray, X-ray and optical telescopes were triggered by a real-time neutrino alert from IceCube on September 22, 2017.
IceCube has developed a powerful real time follow-up program that targets the detection of transient sources. This multimessenger program sends alerts of single and clusters of high-energy neutrino events (multiplets), typically within one minute of the event detection. In collaboration with other observatories, we aim to identify the electromagnetic counterpart of a rapidly fading source or coincident gravitational waves. Single event alerts are distributed publicly as GCN alerts, while multiple alerts are distributed through individual agreements with optical, X-ray, and gamma-ray observatories. Searches for bursts of low-energy neutrinos from nearby supernovas are performed, and above threshold detection is announced rapidly within the SNEWS network.
Seven years after its completion, IceCube has isolated more than 100 high-energy cosmic neutrinos, with energies between 100 TeV and 10 PeV, from more than a million atmospheric neutrinos and hundreds of billions of cosmic-ray muons. In order to filter out this huge atmospheric background, our searches for astrophysical neutrinos focus on high-energy events that start in the detector or that originate in the Northern Hemisphere.
Distribution of the median expected neutrino energy assuming the best-fit spectral index of 2.16. The black crosses correspond to experimental data and blue/red to the conventional atmospheric / astrophysical expectation weighted to the best-fit spectrum.
We have also conducted searches for cosmogenic neutrinos produced in the interactions of cosmic rays with microwave photons. Their energies typically exceed 100 PeV, but so far we have not observed any neutrino above 10 PeV. IceCube currently has the world’s best limit on the flux of cosmogenic neutrinos, which places very strong constraints on the sources of ultra-high-energy cosmic rays (UHECR). Proton-dominated sources are already disfavored.
The PeV neutrinos observed in IceCube, the highest energy neutrinos to date, have a thousand times the energy of the highest energy neutrinos produced with earthbound accelerators and a billion times the energy of the neutrinos detected from supernova SN1987 in the Large Magellanic Cloud, the only neutrinos that had been detected on Earth from outside the solar system prior to IceCube’s breakthrough. However, the most surprising property of these cosmic neutrinos is their large flux rather than their high energy or their origination outside our galaxy.
The large neutrino flux observed implies that the total energy density of neutrinos in the high-energy universe is similar to that of gamma rays, but no gamma ray has ever been observed above 10 TeV. The explanation for this nonobservation is revealing. Since the universe is not transparent to the highest energy photons, primary PeV gamma rays are expected to produce lower energy photons after their interaction with the microwave background, resulting in a photon flux in the GeV-TeV energy range. Data from the Fermi satellite is consistent with this expectation, suggesting that neutrinos and gamma rays may originate in common sources. The observation of neutrino and gamma-ray emission from TXS0506+06 is the first evidence that blazars, and possibly other sources of gamma rays, are the sources of both gamma-ray and neutrino emission at the highest energies.
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.
Once we measured the arrival directions of the atmospheric muons, an anisotropy at the level of 10-3 was revealed that shows significant structure on multiple angular scales. We have also observed structure on scales between 15 degrees and 30 degrees with even 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 measurements of the cosmic-ray anisotropy at the 10-3 level to PeV energies. We have shown that this anisotropy persists to PeV energies, although with phase reversal. No time-dependent variations are observed, in agreement with current theoretical models.
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 may be associated with 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.
The DeepCore subdetector allows IceCube to extend the measurement of the neutrino flux from 100 TeV down to below 10 GeV. At these energies, we 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. (see paper here)
Oscillation parameters measured with three years of DeepCore data (2012-2014). The 90% confidence contours are shown in comparison with those 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.
IceCube has also conducted two searches for light sterile neutrinos, one using high-energy atmospheric neutrinos detected in IceCube and another using low-energy neutrinos detected in DeepCore. None of the searches found evidence for anomalous muon neutrino disappearance and the limits exclude at 99% confidence level the allowed region from appearance experiments such as LSND and MiniBooNE. The search with low-energy neutrinos provided exclusion limits for the sterile neutrino oscillation parameters, with a new world-best limit on the |Uτ4|2 mixing matrix element.
Results from the IceCube search for light sterile neutrinos using high-energy atmospheric neutrinos. The 99% (red solid line) CL contour is shown with bands containing 68% (green) and 95% (yellow) of the 99% contours in simulated pseudo-experiments, respectively. The contours and bands are overlaid on 90% CL exclusions from previous experiments and the MiniBooNE / LSND 99% CL allowed region.
We have produced the world-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 also perform indirect searches for dark matter by looking for the neutrino signature of dark matter annihilation in the galactic halo, galaxy clusters, and the center of the Earth.
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 May 2011 to May 2014. Results are shown in comparison to recent results by several other direct and indirect dark matter search experiments. Accelerator and cosmological search constraints have been applied to the allowed model parameter space.
The neutralino, the lightest weakly interacting massive particle (WIMP) in many supersymmetric models, is the usual test candidate in dark matter searches. The Sun provides 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.
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 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.
We have also studied the ice stability at the bottom of the glacier, which was an uncharted region of the Antarctic ice sheet when IceCube was built. Ice sheets can deform mechanically due to their own weight, and the lower layers may not move at the same speed as the top ones, thus potentially introducing strong strains on the cables that could reduce IceCube’s longevity. Several years of measurements with 50 inclinometers deployed in the deeper sections of the array have shown a very small shearing effect, with a tilt consistently below 0.01 degrees per year. However, we also deployed a sensor to an additional 100 meters in depth, which is indicating an increasing strain below the IceCube instrumented volume.
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 Toba 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.