The diffuse astrophysical neutrino flux — the background of high-energy neutrinos from unresolved sources across the universe — has been measured by IceCube over nearly two decades. The spectrum shows a break near 30 TeV: the flux above this energy follows a different power law than below it. The conventional interpretation requires multiple source populations, each dominating a different energy range.
Winter, Fiorillo, and Buson (arXiv:2603.15754) show that a single source class suffices. Multi-pion production in magnetic fields of tens of kilogauss — conditions characteristic of the cores of active galactic nuclei — generates a neutrino spectrum that naturally produces the observed spectral break without invoking multiple populations.
The mechanism is photomeson production. Protons accelerated in the AGN core interact with the intense photon field, producing pions that decay into neutrinos. At low proton energies, single-pion production dominates, and the neutrino spectrum follows the proton spectrum with a fixed energy ratio. At higher energies, multi-pion production becomes efficient — each proton-photon collision produces multiple pions — and the energy per neutrino decreases relative to the proton energy. This transition creates the spectral break.
The magnetic field strength matters because it determines the proton energy at which synchrotron cooling begins to compete with the interaction timescale. Tens of kilogauss places the cooling break in the right energy range to match the observed 30 TeV feature.
One source class. One mechanism. One spectral break explained by the natural transition from single-pion to multi-pion production in a magnetized environment. The simplest explanation consistent with the data is not a superposition of populations but a single population viewed through the energy-dependent lens of photomeson physics.