Limiting the Parameter Space for Unstable eV-scale Neutrinos Using IceCube Data

Abstract

This Letter extends a recent IceCube sterile neutrino search to include unstable sterile neutrinos within the context of a model termed 3+1+Decay, which expands upon the 3+1 model by introducing sterile neutrino decay to invisible particles with coupling constant $g^2$. The model is attractive since it reduces tension between oscillation experiments within the global fits and with constraints that come from cosmological observables. The analysis uses 10.7 years of up-going muon neutrino data with energy 500 GeV to 100 TeV and with improved reconstruction and modeling of systematics. The best-fit point is found to be $g^2 = 0$, $\sin^2(2θ_{24}) = 0.16$, and $Δm^{2}_{41} = 3.5$ eV$^2$, in agreement with the recent 3+1 sterile neutrino search. Values of $g^2 \geq π$ are excluded at 95\% confidence level. This result substantially limits decay parameter space indicated by recent global fits, disfavoring the decay scenario.

Figures (4)

• Figure 1: Left: Feynman diagram corresponding to the invisible decay mechanism of the sterile neutrino mass state $\nu_4$ into invisible particles $\psi$ and $\phi$, where $\psi$ is a right-handed light neutrino and $\phi$ is a light scalar. Right: diagram of the relative flavor components of each neutrino mass state in a generic 3+1 sterile neutrino model assuming the normal mass ordering. The mass squared splitting between the light states $\nu_{1,2,3}$ and $\nu_{4}$ is $\Delta m_{41}^2 \approx m_{4}^2 \gg \Delta m_{32}^2$.
• Figure 2: Left: pull terms of systematic parameters from the fits for the best-fit point (blue bars) and null (black bars) expressed in units of uncertainty ($\sigma$). The bounds for each systematic parameter are shown as gray-shaded regions. Right: corresponding pull differences between the best fit and null. The parameters can be grouped into categories: atmospheric, hadronic, cosmic ray, detector, high-energy flux, and cross-section model parameters. The atmospheric (Atm.), hadronic, and cosmic ray parameters describe the conventional atmospheric flux. The detector parameters describe uncertainties in the ice model parameterization and DOM response. The high-energy (HE) flux parameters describe the non-conventional flux that is described by a broken power law, and the cross-section (XS) terms describe the attenuation of high-energy neutrinos through the Earth. See Ref. IceCubeCollaboration:2024dxk for a complete description of the individual parameters.
• Figure 3: The result of the 3+1+Decay analysis (black) presented in nine bins of the coupling constant $g^2$. Each subplot shows the 90%, 95%, and 99% C.L. contours as dotted, dashed, and solid lines computed with Wilks' Theorem and three degrees of freedom. The best-fit point, shown as the black star, lies at $\Delta m^2 = 3.5~\textrm{eV}^2$, $\sin^2(2\theta_{24})=0.16$, and $g^2 = 0$. The $99\%$ C.L. Asimov sensitivities are shown as the solid fuchsia lines. Overlaid are the $95\%$ (green) and $99\%$ (blue) C.L. preferred regions of the short-baseline global fits (SBL) Hardin:2022muu.
• Figure 4: The result of this analysis (black) compared to the previous IceCube analysis presented in Ref. IceCubeCollaboration:2022tso (magenta) for $g^2 = 0$ (top) and $g^2 = 2.5\pi$ (bottom). The 90% (dotted), 95% (dashed), and 99% (solid) C.L. contours are shown. The best-fit points from this work and the previous IceCube analysis are shown as stars in their respective colors.