Bounds on heavy sterile neutrinos revisited

TL;DR

Revisits bounds on a heavy sterile neutrino $\nu_h$ mixing with active flavors in the MeV–hundreds of MeV window, emphasizing $\nu_h$–$\nu_\mu$ mixing in charged-current processes. The study reanalyzes accelerator-neutrino data and Super-Kamiokande results to derive updated production- and decay-based bounds, and surveys cosmological bounds from Big Bang Nucleosynthesis. It delivers new limits on $|(VU)_{\mu h}U_{ah}|$ across $m_h$ from about 8 to 390 MeV, plus a separate bound from Super-Kamiokande for 8–105 MeV, and discusses how upcoming experiments (K2K, MiniBooNE, MINOS) could substantially tighten these limits. Collectively, the work narrows the viable parameter space for light-to-medium mass sterile neutrinos and outlines concrete experimental strategies to detect $\nu_h$ decays in visible channels and time-delayed signatures in neutrino facilities.

Abstract

We revise the bounds on heavy sterile neutrinos, especially in the case of their mixing with muon neutrinos in the charged current. We summarize the present experimental limits, and we reanalyze the existing data from the accelerator neutrino experiments and from Super-Kamiokande to set new bounds on a heavy sterile neutrino in the range of masses from 8 MeV to 390 MeV. We also discuss how the future accelerator neutrino experiments can improve the present limits.

Figures (6)

• Figure 1: The exclusion plot for $|(VU)_{\mu h}|^2$ based on the energy spectrum of muons in pion decays. The excluded region is indicated in gray color (magenta color online). The bounds are taken from the analysis reported i) in Ref.prodpion2 for the dash-dotted line indicated as "[1]"; ii) in Ref.prodpion1 for the dotted line "[2]"; iii) in Ref.prodpion4 for the solid line shown as "[3]"; iv) in Ref.prodpion5 for the dashed line "[4]"; v) in Ref.prodpion7 for the dashed-double dotted line labelled "[5]". The bounds are 90% C.L., except for the one marked "[5]", which is 95% C.L.
• Figure 2: The same as in Fig. \ref{['figure:accpion']} but for $\nu_h$ from kaon decays. The bounds are taken from Ref. Asano81 for the dash dotted line indicated as "[1]" at 2 $\sigma$ and from the data of the experiment by R. S. Hayano et al., Ref. Hayano82, for the solid line labelled "[2]" at 90% C.L.
• Figure 3: The bounds on $|(VU)_{\mu h} (VU)_{e h}|$ versus $m_h$ obtained from searches of $\nu_h$ decays. The excluded region is indicated in gray color (magenta color online). The limits are taken for the lines labelled as i) "[1]" (dashed-dotted line) from the data in Ref. Berg83 ; ii) "[2]" (dashed-double dotted line) from the experiment in Ref. Bar93; iii) "[3]" (solid line) from Refs. Bernardi86Bernardi88; iv) "[4]" and "[5]" (dashed lines) from the analysis in Ref. Bernardi88; v) "[6]" (dotted line) from the experiment reported in Ref. Bernardi86. The limits are at 90% C.L..
• Figure 4: The same as in Fig. \ref{['figure:decayCC']} but for the bounds on $|(VU)_{\mu h} U_{a h}|$. For the lines labelled as "[1]"-"[4]", the limits are obtained from a reanalysis of the data reported in the references as indicated for Fig. \ref{['figure:decayCC']}.
• Figure 5: The strongest bound (indicated as line "[3]" in Fig. 4 from the reanalysis of data from Ref. Bernardi86Bernardi88 (diagonally hatched region with dashed-dotted contours). The limits from big bang nucleosynthesis are indicated with the light blue-light gray region with continuous contours, and the previous bounds from experimental searches are shown as dark gray regions with dashed contours. For simplicity, here we take $VU = U$.