Billions of years ago, an ultra-high-energy cosmic ray––maybe a proton––escaped the environment of a powerful, extragalactic source. It did not travel very far and soon interacted with relic, low-energy photons that permeate every corner of the universe. A superenergetic neutrino was then created, which traveled through the universe to interact in the Antarctic ice and produce a cone of blue light. The sensors of the IceCube Neutrino Observatory recorded this event. It was the highest energy neutrino—the first cosmogenic neutrino––ever detected!
Wouldn’t that be a nice story? The reality is that even though IceCube has detected dozens of astrophysical neutrinos, we are still waiting for a neutrino of this kind.
The IceCube Collaboration has, once more, looked for these extremely high-energy neutrinos. And now, after analyzing nine years of IceCube data, it’s clearer than ever that the discovery of cosmogenic neutrinos is waiting for the next generation of neutrino detectors. Still, and with an improved technique developed by IceCube, scientists set the most stringent limits on the existence of cosmogenic neutrinos to date. As a result, the idea that ultra-high-energy cosmic rays are mostly protons is vanishing. These results were published in the journal Physical Review D last week.
The IceCube search for the highest energy neutrinos found two cosmogenic neutrino candidates. The first one had already been identified in a previous search and had a deposited energy of 2.6 PeV––scientists estimated that the actual energy was around 9 PeV. A second PeV neutrino was recorded in December 2016 with an estimated energy of around 6 PeV. Both of them were found to be compatible with the measured astrophysical neutrino flux.
When selecting neutrino candidates, scientists used a lower energy cut for track signatures––mostly produced by muon neutrinos and at extreme energies also tau neutrinos––than for cascade signatures––mostly produced by electron neutrinos and hadronic showers. The reason is that in track-like events, the secondary muon or tau carries most of the neutrino energy when leaving the detector. For cascade-like events, however, most of the energy is rapidly released.
Using typical cosmogenic models and assuming that most ultra-high-energy cosmic rays are protons, this selection should have found between four and seven high-energy neutrinos. IceCube researchers introduced a parameter to account for the astrophysical neutrino background, which reported improved limits on the cosmogenic neutrino flux by almost a 50%.
“We have developed a new model-independent analysis, what scientists call a differential limit, that for the first time takes into account that we can distinguish cosmogenic neutrino candidates from the astrophysical neutrinos IceCube keeps detecting,” explains Shigeru Yoshida, who leads the IceCube team at Chiba University, in Japan, and was a main analyzer of this work. By using what we have learned about astrophysical neutrinos, IceCube researchers have set the tightest bound on the flux of extremely high-energy neutrinos. “And, as had happened before, the search came with a bonus: the highest energy cascade we have detected to date.”
This nonobservation of cosmogenic neutrinos points once more to ultra-high-energy cosmic rays with a high rate of heavy nuclei, which also implies that the flux is at least one order of magnitude smaller. And, if that’s correct, only the next-generation detectors, such as the ten times larger IceCube-Gen2, would be sensitive to cosmogenic neutrinos.
+ info “Differential limit on the extremely-high-energy cosmic neutrino flux in the presence of astrophysical background from nine years of IceCube data,” IceCube Collaboration: M.G. Aartsen et al. Physical Review D 98, 062003, and arxiv.org/1807.01820.