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Spectral Decline of PeV Neutrinos from Starburst Galaxies

Starburst galaxies represent one of the most plausible origins of the cosmic high-energy neutrino flux recently discovered by IceCube. At $\sim$ PeV energies, the neutrino flux from starburst galaxies is expected to exhibit a characteristic spectral break due to cosmic-rays escaping the galaxy. We examine the 'smearing' of this spectral break by a population of starburst galaxies with varying properties. We incorporate galaxy distribution w.r.t. star-formation rate and redshift. Our results (i) show characteristic spectral softening in IceCube's energy band; (ii) resolve the conflicting observations of soft neutrino spectrum and diffuse gamma-ray flux observed by Fermi-LAT; (iii) constrain the properties of the magnetic fields in starburst galaxies.