Observing EeV neutrinos through Earth: GZK and the anomalous ANITA events

Abstract

Tau neutrinos are unique cosmic messengers, especially at extreme energies. When they undergo a charged-current interaction, the short lifetime of the produced tau gives rise to secondary tau neutrinos that carry a significant fraction of the primary neutrino energy. This effect, known as tau neutrino regeneration, has not been applied to its full potential in current generation neutrino experiments. In this work, we present an updated calculation of tau neutrino regeneration, and explore its implications for two scenarios: the recent anomalous ANITA events and the cosmogenic neutrino flux. For the former, we investigate the idea of localized emission and find that the maximum secondary neutrino flux allowed by IceCube measurements implies a primary flux that is incompatible with the ANITA observation, regardless of the assumed source energy spectrum. For the latter, we study the prospect of detecting the cosmogenic neutrino flux of regenerated PeV neutrinos with current and next generation neutrino detectors.

A Search for IceCube Events in the Direction of ANITA Neutrino Candidates

“…If the intrinsic spectrum were to contain contributions from energies below 1 EeV, such as the power-law spectra tested in the analyses presented in Section 3, this would introduce a component to which IceCube might be sensitive but that could not produce events at ANITA consistent with the AAE; thus, this additional component would strengthen the constraints displayed in Figure 5. Additionally, if the spectrum consisted of neutrinos of energy greater than 1 EeV, the secondary ν τ spectrum would have a similar shape to that shown in Figure 5, as discussed in Safa et al (2020); therefore, the limits on the flux normalization would be constant for fluxes of higher energy, while the energy required to produce such a flux would scale with the injected energy. For that reason, these limits are conservative and severely constrain any incident spectrum that could produce an observable event at ANITA consistent with an AAE.…”

“…However, even in the case of non-power-law neutrino emission, the limits we can set on muon neutrinos in the TeV-PeV energy range can constrain the generic fluxes of incident tau neutrinos with EeV energies. As shown in Safa et al (2020), any incident flux with an EeV ν τ component that traverses large Earth chord lengths will result in a secondary flux of lower-energy neutrinos, to which IceCube would be sensitive. We use the same prescription here to analyze how constraining our limits are on a generic point-source flux that includes EeV neutrinos.…”

Signatures of secondary leptons in radio-neutrino detectors in ice

“…If the tau decays into a hadronic channel, the outgoing hadrons will induce a hadronic cascade. We note that ignoring regeneration results is an underestimation of the neutrino flux at low energies, but that detailed calculation lies outside the scope of the present paper and has been done previously elsewhere [44,45]. Its study is, however, in principle possible using PROPOSAL and NuRadioMC combined.…”

Addressing RD(*) , R

“…This poses considerable difficulty for interpretation of such events within the SM framework due to the low survival rate (≲10 −6 ) of EeVenergy neutrinos over long chord lengths in Earth ∼5000 km, even after accounting for the probability increase due to ν τ regeneration [60,113,114]. Moreover, as pointed out in earlier studies [33,112,115,116], the strength of isotropic cosmogenic neutrino flux needed to account for the two events is in severe tension with the upper limits set by Pierre Auger [117,118] and IceCube [119,120]. Therefore, a BSM explanation with an anisotropic astrophysical source with some exotic generation and propagation mechanism of upgoing events is desirable to solve the ANITA anomaly, provided it stands further scrutiny after more data is released from future ANITA flights.…”