Neutrinos are weakly interacting particles that are able to travel unhindered through the cosmos. The IceCube Neutrino Observatory, embedded in a cubic kilometer of Antarctic ice, searches for neutrinos and captures them at different energies. By measuring the entire spectrum of astrophysical neutrinos, scientists can gain insights into the origin and acceleration mechanisms of highly energetic particles called cosmic rays.
But of particular interest to IceCube are the most energetic neutrinos that come from extremely powerful sources. At these energies, the Earth absorbs most neutrinos, so IceCube can only see neutrinos coming from the Southern Hemisphere. Previously, IceCube has searched for extremely high-energy (EHE) neutrinos, but has yet to see neutrino “events” above around 10 PeV.
In a new study submitted to Physical Review D, the IceCube Collaboration presents an analysis designed to extend IceCube’s sensitivity at very high energies, in the energy range between other IceCube analyses and the EHE search. Using nine years of IceCube data, two complementary techniques were combined to reject the large atmospheric muon background in the search. After applying both techniques, two neutrino events were identified, but they were not coincident with any astrophysical sources.
The analysis fills the gap between 1 PeV and 10 PeV and uses a new event selection that targets high-energy, downgoing events—from the southern sky above IceCube—that are not affected by the Earth’s absorption. However, the southern sky presents its own challenges, including a large background of atmospheric muons that are created when cosmic rays hit the upper atmosphere and produce secondary particle showers that can reach the surface of the ice. To reduce the overwhelming background, the researchers combined two largely independent techniques.
The first method took advantage of the surface array of IceCube, called IceTop, to veto neutrino events that left traces on the surface of the ice. They selected the region where the effectiveness was very high and required the events to have fewer than two associated IceTop hits. For the second method, an algorithm was used to measure the stochasticity, or randomness, of muon energy losses to suppress backgrounds.
“At high energies, single muons created by astrophysical neutrinos lose energy in ice stochastically,” explains Yang Lyu, colead on the study and a recent PhD graduate who attended the University of California, Berkeley (UC Berkeley). “On the other hand, atmospheric muon bundles have many muons along one track, and therefore, the energy loss patterns along the track are ‘smoothed out.’ Eventually, we select only events that have high stochasticity values.”
The researchers found two neutrino candidate events in the signal region from which they determined whether the data better fit a single power law (SPL) or an SPL with an exponential cutoff scenario. The cutoff represents a transition in the shape of the neutrino energy spectrum, presumably in the PeV region. Although there was not enough evidence to reject the hypothesis, the measured SPL and SPL with cutoff are consistent with results from previous analyses.
“This analysis was the first dedicated search for downgoing muons from neutrinos, and the first to combine the stochasticity variable and the IceTop surface veto to reject the large number of atmospheric muon background events,” says Spencer Klein, senior scientist at Lawrence Berkeley National Laboratory and research physicist at UC Berkeley, colead on the study and Lyu’s PhD advisor. “The results from this search, with more data and a few improvements, will be integrated into a global analysis of the neutrino spectrum, which makes use of data from many analyses. This global fit should give us a better idea of the maximum energy of astrophysical neutrinos.”
+ info “Probing the PeV Region in the Astrophysical Neutrino Spectrum using νµ from the Southern Sky,” IceCube Collaboration: R. Abbasi et al. Submitted to Physical Review D. arxiv.org/abs/2502.19776