Nearly degenerate heavy sterile neutrinos in cascade decay: mixing and oscillations

TL;DR

This paper investigates how nearly degenerate heavy sterile neutrinos can exhibit mixing and oscillations detectable as quantum beats in decay products, with implications for particle physics and cosmology.

Contribution

It introduces non-perturbative methods to analyze cascade decay dynamics of sterile neutrinos, revealing observable oscillation effects similar to meson mixing.

Findings

Detection of quantum beats indicating neutrino mixing

Distinct decay signatures for nearly degenerate sterile neutrinos

Potential impact on cosmological neutrino number estimates

Abstract

Some extensions beyond the Standard Model propose the existence of nearly degenerate heavy sterile neutrinos. If kinematically allowed these can be resonantly produced and decay in a cascade to common final states. The common decay channels lead to mixing of the heavy sterile neutrino states and interference effects. We implement non-perturbative methods to study the dynamics of the cascade decay to common final states, which features similarities but also noteworthy differences with the case of neutral meson mixing. We show that mixing and oscillations among the nearly degenerate sterile neutrinos can be detected as \emph{quantum beats} in the distribution of final states produced from their decay. These oscillations would be a telltale signal of mixing between heavy sterile neutrinos. We study in detail the case of two nearly degenerate sterile neutrinos produced in the decay of pseudoscalar mesons and decaying into a purely leptonic "visible" channel: $\nu_h \rightarrow e^+ e^- \nu_a$. Possible cosmological implications for the effective number of neutrinos $N_{eff}$ are discussed.