Astrophysical neutrino flavor ratios in the presence of sterile neutrinos

Abstract

Astrophysical objects such as active-galactic nuclei (AGN) and gamma-ray bursts (GRBs) can be sources of high energy, astrophysical neutrinos. The decay of charged pions produces electron and muon-flavor neutrinos from the primary decay of the pion, and from the secondary decay of the resulting charged lepton. At low energies we expect the flavor ratio $\Phi_{\nu_e}:\Phi_{\nu_\mu}:\Phi_{\nu_\tau}$ to be $1:2:0$ at the source. We are interested in the flavor ratios as measured on Earth after the neutrinos propagate over cosmic distance scales from the source. If we only consider vacuum flavor transition probabilities between the three active flavors then we expect a measured flavor ratio of $1:1:1$ up to small corrections from $\theta_{13}$ and non-maximal $\theta_{23}$. When we include mixing with two additional flavors of sterile neutrinos then we see corrections to this ratio up to $\sim 30%$. Furthermore, if we consider energy-loss of the charged leptons involved in the pion decay from cosmic sources then these flux ratios depend on the neutrino energy. We examine these energy-dependent flavor ratios using a specific model for sterile neutrino mixing, and compare to expected ratios when only three neutrino flavors are considered. At energies $E_\nu >$ 1 TeV the flavor ratios observed in experiments such as IceCube can probe the existence of sterile neutrinos.