High-energy neutrinos detected by the IceCube experiment in Antarctica are equally distributed among the three possible neutrino flavours, according to two independent teams of physicists. Their analyses overturn a preliminary study of data, which suggested that the majority of the particles detected were electron neutrinos. The latest result is in line with our current understanding of neutrinos, and appears to dash hopes that early IceCube data point to “exotic physics” beyond the Standard Model.

Located at the Amundsen–Scott South Pole Station, the IceCube Neutrino Observatory is a large array of photodetectors buried in ice. In late 2013 IceCube revealed that it had captured the first signals from neutrinos with extremely high energies, which suggests that the particles came from outside of our galaxy. While neutrinos generated inside the Sun and by cosmic rays colliding with the Earth’s atmosphere have been detected for many years, neutrinos from much farther away had remained elusive. As a result, the discovery was named the Physics World Breakthrough of the Year in 2013.

Neutrinos come in three different types or “flavours” – electron, muon and tau – and change or “oscillate” from one type to another as they travel across long distances. For neutrinos that have travelled arbitrarily large distances, we expect to see nearly equal numbers of each flavour when they reach Earth – that is, an electron:muon:tau ratio of about 1:1:1. Depending how the neutrinos were produced, there will be small deviations from this equal-flavour distribution. This deviation should give us information on how and where the neutrinos were produced.

Abundance of electron neutrinos

In 2014 Olga Mena, Sergio Palomares-Ruiz and Aaron Vincent of the University of Valencia in Spain did an independent analysis of IceCube data from 2010 to 2012, and concluded that the best-fit flavour ratio was 1:0:0 – an abundance of electron neutrinos with no muon or tau neutrinos present. If true, this unexpected result would mean that rare neutrino decays were taking place or that the detected particles were mixing with a fourth and very hypothetical “sterile” neutrino. In both cases, the discovery could have pointed to exotic physics beyond our current understanding.

IceCube detects neutrinos of all three flavours when they produce small showers of particles as they interact within the detector. However, muon neutrinos and a small fraction of tau neutrinos also produce a highly energetic muon that is visible as a track as it travels across the entire detector. “So if we observe such a track, we can tell an event was either a muon or tau neutrino, but not which,” says Gary Binder, who is a physicist at the University of California, Berkeley, and part of the IceCube collaboration. He explains, “We can’t tell for certain what flavour produced a given event, but we can do a statistical analysis on the distribution of showers and tracks to estimate the abundance of each flavour.”

Now, however, Binder and IceCube colleagues have analysed a much larger set of IceCube’s data, collected across 974 days from May 2010 to May 2013. They have identified 137 neutrinos with energies above 35 TeV and found that the neutrinos are equally distributed among the three flavours. “No matter what parameters we adjust, there’s just no way to get all electron neutrinos, so if we did observe that, it would surely indicate new physics affecting cosmic neutrinos as they travel over long distances,” says Binder.

A similar analysis was performed by an independent group in Italy, led by Francesco Vissani and Andrea Palladino at Gran Sasso Science Institute in L’Aquila and the Gran Sasso Laboratories in Assergi. This group focused on neutrinos with energies above 60 TeV, and it also found the data to be consistent with conventional astrophysical models.

Unravelling mysteries

Binder told physicsworld.com, “So far, the flavour ratio is consistent with the equal-flavour assumption, and we don’t have enough precision to measure the small deviations that could tell us about the astronomical objects producing cosmic neutrinos, but we expect that to change very soon.” He adds, that they “anticipate the information we gather from flavour studies will help us to resolve the mystery of how these neutrinos are produced and where they come from”.

Binder also points out that there are a number of exotic ideas that predict much larger deviations from the equal-flavour expectation. “Measuring the flavour ratio could give us clues to physics beyond the Standard Model, but so far we haven’t seen evidence for anything exotic yet,” he says.

Vissani adds that IceCube also measures the energy distribution of the neutrinos and this provides yet another important clue about their sources. He points out that “future IceCube analyses can show beyond any doubt that these neutrinos come from cosmic sources, by observing for the first time a tau neutrino”.

The research is published in two papers in Physical Review Letters – one from Palladino et al. and one from IceCube.