Primordial black hole probes of heavy neutral leptons

TL;DR

This paper investigates how primordial black hole evaporation could emit heavy neutral leptons, and how multimessenger signals like neutrinos and gamma rays could improve constraints on these particles' properties.

Contribution

It introduces a multimessenger analysis combining neutrino and gamma-ray observations to probe heavy neutral leptons emitted from evaporating primordial black holes.

Findings

IceCube and HAWC could significantly improve constraints on HNL mixing and mass.

The analysis is sensitive to PBH explosions at about 10^{-4} parsecs from Earth.

Constraints are particularly strong for HNL masses between 0.1 and 1 GeV.

Abstract

Primordial black holes (PBH), while still constituting a viable dark matter component, are expected to evaporate through Hawking radiation. Assuming the semi-classical approximation holds up to near the Planck scale, PBHs are expected to evaporate by the present time, emitting a significant flux of particles in their final moments, if produced in the early Universe with an initial mass of $\sim 10^{15}$ g. These ``exploding'' black holes will release a burst of Standard Model particles alongside any additional degrees of freedom, should they exist. We explore the possibility that heavy neutral leptons (HNL), mixing with active neutrinos, are emitted in the final evaporation stages. We perform a multimessenger analysis. We calculate the expected number of active neutrinos from such an event, including contributions due to the HNL decay for different assumptions on the mixings, that could be visible in IceCube. We also estimate the number of gamma-ray events expected at HAWC. By combining the two signals, we infer sensitivities on the active-sterile neutrino mixing and on the sterile neutrino mass. We find that, for instance, for the scenario where $U_{\tau 4}\neq 0$, IceCube and HAWC could improve current constraints by a few orders of magnitude, for HNLs masses between 0.1 - 1 GeV, and a PBH explosion occurring at a distance of $\sim 10^{-4}$ pc from Earth.