A new study published in Physical Review Letters by the IceCube Collaboration reports evidence that the energy spectrum of astrophysical neutrinos is not a simple straight line.
Astrophysical neutrinos are tiny, nearly massless particles produced when high-energy cosmic rays interact with matter or radiation near sources such as active galactic nuclei, gamma-ray bursts, and supernova remnants. Because they barely interact with anything, they travel from the edges of the observable universe in straight lines, carrying information about the environments that produced them.
Analyzing more than a decade of data, the study found a break in the spectrum near 30 TeV, comparable to the energies seen at the Large Hadron Collider. This rules out the single power law with a statistical significance greater than 4σ, meaning the chance of the result being a fluke is less than about 1 in 16,000.
Phys.org spoke to co-authors Aswathi Balagopal V. from the University of Delaware, Vedant Basu from the University of Utah, and Albrecht Karle from the University of Wisconsin–Madison to learn more about the IceCube collaboration’s work.
“What I find personally most interesting is that neutrinos act as cosmic messengers from the furthest edges of space,” said Basu. “They allow us to probe the dynamics of extreme environments at energies we simply cannot replicate on Earth.”
The total flux of these neutrinos at Earth, known as the diffuse astrophysical neutrino flux, is the combined emission from every neutrino source in the observable universe. Mapping how this flux changes with energy helps researchers understand what kinds of sources dominate and how cosmic rays are accelerated to such high energies.
Revisiting the spectrum
Since IceCube first detected high-energy astrophysical neutrinos in 2013, the collaboration has been working to characterize how their flux behaves across different energies. For years, the data have been well described by a single power law, a simple model where the number of neutrinos falls off smoothly as energy increases. But there have been hints of something more complicated.
Earlier IceCube analyses pointed to a possible excess or break in the spectrum near 30 TeV, where the neutrino flux seemed to behave differently than the high-energy tail would predict. None of those hints, however, were statistically strong enough to confirm a real feature.
The current work revisits the question with more data, refined event selection, and improved treatment of systematic uncertainties. The goal is to test whether the spectrum follows a single power law or shows additional structure.
Two independent analyses, one detector
The IceCube Neutrino Observatory uses 5,160 optical sensors buried in a cubic kilometer of Antarctic glacial ice at the South Pole. When a neutrino occasionally interacts with a nucleus in the ice, it produces a shower of charged particles that travel faster than light moves through ice, emitting a faint blue glow called Cherenkov light that the sensors pick up.
“As neutrinos interact very rarely, a large detector volume is required, with a transparent medium to transmit the Cherenkov light signals,” explained Balagopal V. “This is why IceCube uses 1 cubic kilometer of very clear ice, readily available in Antarctica. The detector is also buried 1.5 km under the surface, which reduces the backgrounds from cosmic ray air showers.”
To test the shape of the neutrino spectrum, the team ran two independent analyses on overlapping but distinct datasets. The first, called the Combined Fit, merged two existing datasets. One was a large sample of track-like events, produced when muon neutrinos travel through the ice and leave a long line of light. The other was a sample of cascade events, the more compact showers produced when other neutrino types interact.
The second analysis, called Medium Energy Starting Events (MESE), focused on neutrinos that interact inside the detector itself, providing a cleaner sample that naturally captures all three flavors of neutrinos: electron, muon, and tau. Each analysis fit four possible spectral models to the data: a single power law, a power law with an exponential cutoff, a log parabola, and a broken power law.
“Each analysis independently measured the spectrum. Both analyses approached the measurement in two ways and obtained very similar results,” said Karle.
A break near 30 TeV
Both analyses landed on the same answer. The broken power law was the preferred model, with the single power law rejected at greater than 4σ significance in each case. The data favors a spectrum that is harder at low energies than at high energies, with the transition occurring near 30 TeV.
“A ‘harder’ spectrum is one where the flux decreases less with increasing energy, or in other words, where the slope of the spectrum is less steeply falling,” explained Basu. “What this means for our results is that we see a lower flux of neutrinos at lower energies than would be predicted by simply extrapolating the steep spectrum at high energies,” added Balagopal V.
The spectral index is the number that describes how steeply the neutrino flux falls with energy. A larger index means a steeper drop, with proportionally fewer high-energy neutrinos.
In the MESE analysis, the low-energy index was 1.72 and the high-energy index was 2.84, meaning the spectrum becomes noticeably steeper above the break. The Combined Fit returned 1.31 and 2.74 for the same parameters, telling the same story. These two analyses work together: MESE provides tighter constraints on the low-energy slope, while the Combined Fit better constrains the high-energy slope.
The team also tested a log parabola model, which captures spectral curvature rather than a sharp break. While it also outperformed the single power law, the broken power law remained the preferred fit in both analyses.
What comes next
A break in the diffuse neutrino spectrum carries implications for the sources producing these particles. The authors note that it may signal a change in the populations or dynamics of the contributing sources, and that the diffuse neutrino background could even reveal evidence of new physics, such as neutrinos produced by the decay or annihilation of dark matter.
The result also eases a long-standing tension between IceCube measurements and the diffuse extragalactic gamma-ray background. Extrapolating the older single power law down to the 1–10 TeV range implied more neutrinos than were consistent with the observed gamma-ray flux. The new measurement narrows that mismatch by predicting fewer neutrinos at low energies than the old model did.
“These results are an important first step in better resolving and understanding the neutrino spectrum at TeV-PeV scales and tying into the broader multi-messenger picture with complementary measurements of the MeV-GeV gamma ray spectrum,” said Karle. “More refined neutrino spectral analyses, with improved modeling, are already underway and will go a long way to illuminating the dynamics of neutrino sources in the high-energy universe.”
Written for you by our author Tejasri Gururaj, edited by Lisa Lock, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
If this reporting matters to you, please consider a donation (especially monthly). You’ll get an ad-free account as a thank-you.
Publication details
R. Abbasi et al, Evidence for a Spectral Break or Curvature in the Spectrum of Astrophysical Neutrinos from 5 TeV to 10 PeV, Physical Review Letters (2026). DOI: 10.1103/2gh9-d4q7. On arXiv: DOI: 10.48550/arxiv.2507.22233
© 2026 Science X Network
Citation:
IceCube detects break in cosmic neutrino spectrum, ruling out simple power-law model (2026, May 29)
retrieved 29 May 2026
from https://phys.org/news/2026-05-icecube-cosmic-neutrino-spectrum-simple.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.