A first search for sterile neutrinos in IceCube
IceCube studies of neutrino physics usually happen in the low-energy regime, where the inner and more dense DeepCore is the most relevant subdetector of the Antarctic observatory. However, searches for new physics beyond the Standard Model can also use the main IceCube array when the signature signal is expected at energies above 100 GeV.
This is the case for searches for light sterile neutrinos with IceCube. Sterile neutrinos could be a fourth type of neutrino that only interacts gravitationally and is able to answer questions such as why neutrinos have mass or if neutrinos are important contributors to the dark matter pool in the universe. A typical signature of light sterile neutrinos, those with mass around 1 eV, is expected to produce a strong disappearance of atmospheric muon neutrinos crossing the Earth. This effect results in a depletion at energies of a few TeV due to matter effects in neutrino oscillations.
The IceCube Collaboration has performed two independent searches, both with one year of data, searching for sterile neutrinos in the energy range between approximately 320 GeV and 20 TeV. IceCube has not found any anomalous disappearance of muon neutrinos and has placed new exclusion limits on the parameter space of the 3+1 model, a scenario with only one sterile neutrino. These results have been submitted today to Physical Review Letters.
IceCube has proven to be a great tool for the study of neutrino oscillations, i.e., their change from one flavor to another, in the energy regime from a few GeV up to 50 GeV. But the detailed properties of neutrinos have not yet been fully revealed, and the international scientific community is working on several experiments that will shed new light on what neutrinos can tell us about the universe.
We know that neutrinos traveling through matter will change their oscillation pattern because of interactions with atomic electrons and nucleons. These interactions, which depend on the energy of the neutrinos and on the density of the medium, create matter enhanced oscillations that can result in a strong disappearance of antineutrinos at specific energies.
Following the first observations of neutrino oscillations back in 1998, several experiments have measured oscillations patterns at different energies and for different neutrino types. And a few of them, including LSND and MiniBooNE, have found anomalies that cannot be explained with the current model of three neutrinos. However, theories that postulate the existence of sterile neutrinos could accommodate these results. Sterile neutrinos only interact gravitationally, and they would be harder to detect than the currently known neutrinos.
During the last few years, several experiments have been searching for sterile neutrinos. These searches have not been successful so far and have every time further constrained the sterile neutrino parameter space, namely, the relative mass and mixing angle of sterile neutrinos with the other three active neutrinos.
The sterile neutrino search announced by IceCube today used throughgoing neutrino-induced muon events, which are neutrinos that reach the detector after crossing the Earth, from the first year of data with the full detector, i.e., with 86 strings of sensors, and its results are confirmed by an independent study using the so-called IC59 data, or data taken when the detector was running with 59 strings. IceCube researchers selected atmospheric neutrinos with energies between 320 GeV and 20 TeV, which includes the energies where the existence of sterile neutrinos would introduce a new resonance effect in matter neutrino oscillations.
Sterile neutrino models predict a strong disappearance of muon antineutrinos for energies around a few TeV. And, although IceCube cannot differentiate neutrino and antineutrino interactions, if sterile neutrinos exist, it should be able to measure a significant disappearance in the total number of atmospheric muon neutrinos and antineutrinos reaching the South Pole after sailing through the Earth.
“IceCube’s search for sterile neutrinos is an example of something that experimental physicists strive for: taking a phenomenon that is weak and difficult to study and examining it using a method where its effects would be amplified,” says Ben Jones, who co-led this study while a graduate student at MIT. “By not seeing sterile neutrinos in this way, we have excluded much of the parameter space that has been inaccessible to previous sterile neutrino experiments,” adds Jones.
In fact, IceCube’s null result also excludes the allowed parameter space region for several experiments that had observed anomalies in the oscillation patterns, which were interpreted as hints of sterile neutrinos, at approximately the 99% confidence level.
“Not finding the characteristic depletion signal has allowed us to place constraints in some parameter regions that are an order of magnitude stronger than past experiments. This greatly increases the tension between experiments that claim observation and those that do not.“ explains Carlos Argüelles, an IceCube researcher who received his PhD at UW–Madison working on this study. “IceCube results have changed the parameter space where sterile neutrinos may exist, calling their existence into question and impacting future search strategies,” adds Argüelles.
To sum up, sterile neutrinos are not yet ruled out, but their existence is now more remote than ever.