Flavoring Astrophysical Neutrinos: Flavor Ratios Depend on Energy

Abstract

Electromagnetic (and adiabatic) energy losses of pions and muons modify the flavor ratio (measured at Earth) of neutrinos produced by pion decay in astrophysical sources, $Φ_{ν_e}:Φ_{ν_μ}:Φ_{ν_τ}$, from 1:1:1 at low energy to 1:1.8:1.8 at high energy. The transition occurs over 1-2 decades of nuetrino energy, and is correlated with a modification of the neutrino spectrum. For gamma-ray bursts, e.g., the transition is expected at \~100 TeV, and may be detected by km-scale neutrino telescopes. Measurements of the transition energy and energy-width will provide unique probes of the physics of the sources. pion and muon energy losses also affect the ratio of $\barν_e$ flux to total neutrino flux, which may be measured at the W-resonance (6.3 PeV): It is modified from 1/6 (1/15) at low energy to 1/9 (practically 0) at high energy for neutrinos produced in pp ($pγ$) interactions.

v1: 4 pages, 1 figure; v2: added reference; v3: improved introduction, accepted to PRL; v4: added note about matter oscillations