The IceCube Neutrino Observatory, embedded in a cubic kilometer of Antarctic ice, searches for weakly interacting particles called neutrinos that are able to travel undisturbed through the cosmos. Of interest are high-energy astrophysical neutrinos that can arise from cosmic ray interactions with matter or photons in astrophysical sources. Thus far, the dominant sources of the astrophysical neutrino flux still elude researchers.
The total energy spectrum of astrophysical neutrinos reflects the summed contribution from all sources in the universe and is therefore expected to constitute a rather smooth distribution. Any feature—like a break or curvature—in this smooth distribution is highly interesting because it may signal a significant change of the type of sources or collective changes of the dynamics within the contributing sources. Because the diffuse neutrino background encompasses all neutrino emission in the universe, it might also harbor evidence of neutrinos produced by new and unknown physics, such as the decay or annihilation of dark matter particles.
The IceCube Collaboration presented two complementary analyses of more than 10 years of data, sensitive to neutrino events from 1 TeV to 10 PeV, in a joint study published today in Physical Review Letters and Physical Review D. Both analyses found consistent results about the shape of the extragalactic diffuse neutrino spectrum. For the first time, a significant deviation of the spectrum from a simple smooth power law is identified. The studies were selected as an Editor’s Suggestion in Physical Review D and highlighted in Physics Magazine.
The researchers found a feature in the spectrum at around 30 TeV. This observed feature indicates a smaller neutrino flux towards lower energies than a simple extrapolation of previous measurements and thus potentially resolves a tension that has been pointed out in recent years about the intensities of the extragalactic neutrino and gamma-ray backgrounds.
Aswathi Balagopal V., a postdoctoral researcher at the University of Delaware and Vedant Basu, a postdoctoral researcher at the University of Utah, led one of the analyses while at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at the University of Wisconsin–Madison.
For this analysis, the researchers utilized a new event sample that consisted of medium-energy starting events (MESE), which allowed them to study the astrophysical neutrino flux with energies all the way down to 1 TeV. The other analysis combined existing data samples of tracks and cascades (a “combined fit”) for a more precise measurement. The MESE sample builds upon a previous IceCube analysis using a sample of high-energy starting events (HESE), where “starting events” are neutrinos interactions that occur within the instrumented volume of IceCube, which allowed measuring the cosmic neutrino spectrum above 60 TeV.
“We extended the concept of HESE to lower energies in order to understand the shape of the spectrum across several energy scales,” says Balagopal V.
“We found that at 33 TeV, the spectrum of cosmic neutrinos deviates from a simple power law, following a steeply falling ‘soft’ spectrum above this energy and a less steep, or ‘harder,’ spectrum below this energy for both analyses,” says Basu. “This result has great implications for the mechanisms of neutrino production at their sources, theorized to be sites where cosmic rays are accelerated to very high energies.”
Although the majority of astrophysical sources still remain to be resolved, this result will undoubtedly improve efforts to model neutrino production in order to obtain a consistent picture of the high-energy universe.
“At lower energies, it is trickier to separate the neutrinos of cosmic origin from those generated in the Earth’s atmosphere. That was the hard part,” says Albrecht Karle, a professor of physics at UW–Madison. “The observed feature in the spectrum will allow theorists to relate this finding to data from lower-energy photons and improve our understanding of the cosmic high-energy particle production sites.”
+ info “Evidence for a Spectral Break or Curvature in the Spectrum of Astrophysical Neutrinos from 5 TeV–10 PeV,” IceCube Collaboration: R. Abbasi et al., Physical Review Letters 136 (2026) 12, 121002, journals.aps.org, arXiv and “Improved measurements of the TeV- PeV extragalactic neutrino spectrum from joint analyses of IceCube tracks and cascades,” IceCube Collaboration: R. Abbasi et al., Physical Review D 113 (2026) 062002, journals.aps.org, arXiv