Constraining GRB as Source for UHE Cosmic Rays through Neutrino Observations

TL;DR

This paper examines how neutrino oscillation and decay reduce the detectability of neutrinos from GRBs, challenging their role as sources of UHE cosmic rays and suggesting alternative detection methods.

Contribution

It highlights the impact of neutrino oscillation and decay on neutrino flux from GRBs, refining constraints on GRBs as UHECR sources and proposing complementary detection approaches.

Findings

Neutrino oscillation reduces muon-neutrino flux from 2/3 to 1/3 at Earth.

Neutrino decay further decreases muon-neutrino ratio to 1/8 under normal hierarchy.

Alternative neutrino detection methods can better distinguish neutrino flavors.

Abstract

The origin of ultra-high energy cosmic rays (UHECR) has been widely regarded as one of the major questions in the frontiers of particle astrophysics. Gamma ray bursts (GRB), the most violent explosions in the universe second only to the Big Bang, have been a popular candidate site for UHECR productions. The recent IceCube report on the non-observation of GRB induced neutrinos therefore attracts wide attention. This dilemma requires a resolution: either the assumption of GRB as UHECR accelerator is to be abandoned or the expected GRB induced neutrino yield was wrong. It has been pointed out that IceCube has overestimated the neutrino flux at GRB site by a factor of $\sim 5$. In this paper we point out that, in addition to the issue of neutrino production at source, the neutrino oscillation and the possible neutrino decay during their flight from GRB to Earth should further reduce the detectability of IceCube, which is most sensitive to the muon-neutrino flavor as far as point-source identification is concerned. Specifically, neutrino oscillation will reduce the muon-neutrino flavor ratio from 2/3 per neutrino at GRB source to 1/3 on Earth, while neutrino decay, if exists and under the assumption of normal hierarchy of mass eigenstates, would result in a further reduction of muon-neutrino ratio to 1/8. With these in mind, we note that there have been efforts in recent years in pursuing other type of neutrino telescopes based on Askaryan effect, which can in principle observe and distinguish all three flavors with comparable sensitivities. Such new approach may therefore be complementary to IceCube in shedding more lights on this cosmic accelerator question.