Comprehensive constraints on heavy sterile neutrinos from core-collapse supernovae

Sterile neutrinos with masses up to $\mathcal{O}(100)\mathrm{MeV}$ can be copiously produced in a supernova (SN) core through the mixing with active neutrinos. In this regard, the SN 1987A detection of neutrino events has been used to put constraints on active-sterile neutrino mixing, exploiting the well-known SN cooling argument. We refine the calculation of this limit including neutral current interactions with nucleons, which constitute the dominant channel for sterile neutrino production. We also include, for the first time, the charged current interactions between sterile neutrinos and muons, relevant for the production of sterile neutrinos mixed with muon neutrinos in the SN core. Using the recent modified luminosity criterion, we extend the bounds to the case where sterile states are trapped in the stellar core. Additionally, we study the decays of heavy sterile neutrinos, affecting the SN explosion energy and possibly producing a gamma-ray signal. We also illustrate the complementarity of our new bounds with cosmological bounds and laboratory searches.

Figure 1

Upper panel: Radial profiles of the temperature $T$ (solid black line) and density $\rho$ (dashed line) in the SN core. Middle panel: Radial profile for the degeneracy parameters of neutrons ${\eta }_n$ (solid black line), protons ${\eta }_p$ (dashed black), electrons ${\eta }_e$ (dotted black), and muons ${\eta }_{\mu }$ (dot-dashed red). Lower panel: Radial profiles of the electron fraction $Y_e$ (solid line) and muon fraction $Y_{\mu }$ (dashed line). All panels refer to the postbounce time $t_{\mathrm{pb}}=1\mathrm{s}$.

Figure 2

Sterile neutrino luminosity as a function of $|U_{\alpha 4}{|}^2$ for $\alpha =\mu$, $m_4=10\mathrm{MeV}$ and $m_4=120\mathrm{MeV}$ (black and red lines, respectively) and the same for $\alpha =\tau$ (orange and blue lines, respectively). The horizontal dotted line corresponds to the limit value of $L_{\nu }=3\times {10}^{52}\mathrm{erg}{\mathrm{s}}^{-1}$. Dashed lines are used for values of the mixing where the sterile neutrino luminosity exceeds the luminosity of the species ${\nu }_{\alpha }$ mixed with ${\nu }_4$.

Figure 3

Branching ratios for the relevant decay channels listed in Table 2 as a function of the sterile neutrino mass $m_4$ for a mixing with ${\nu }_{\mu }$ (upper panel) and with ${\nu }_{\tau }$ (lower panel).

Figure 4

Cooling bound on the sterile neutrino parameter space $(m_4,|U_{\alpha 4}{|}^2)$ for $\alpha =\mu$ (solid line) and $\alpha =\tau$ (dashed line). The dot-dashed black line is the bound for $|U_{\tau 4}{|}^2$ that would be obtained neglecting the $\nu N\to N{\nu }_4$ process as in Ref. [38].

Figure 5

Explosion energy bound on the sterile neutrino parameter space $(m_4,|U_{\alpha 4}{|}^2)$ for $\alpha =\mu$ (solid line) and $\alpha =\tau$ (dashed line).

Figure 6

Number of produced positrons per SN for $\alpha =\mu$, $\tau$ (black and red lines, respectively) and $m_4=20\mathrm{MeV}$ (solid lines) and $m_4=120\mathrm{MeV}$ (dashed lines). The dotted line corresponds to the limit value of $N_{\mathrm{pos}}=2.5\times {10}^{53}$.

Figure 7

511 keV line bound on the sterile neutrino parameter space $(m_4,|U_{\alpha 4}{|}^2)$ for $\alpha =\mu$ (solid line) and $\alpha =\tau$ (dashed line).

Figure 8

Bound on the sterile neutrino parameter space $(m_4,|U_{\alpha 4}{|}^2)$ from the nondetection of gamma-rays from SN 1987A for $\alpha =\mu$ (solid line) and $\alpha =\tau$ (dashed line).

Figure 9

Bound on the sterile neutrino parameter space $(m_4,|U_{\alpha 4}{|}^2)$ from diffuse gamma-ray emission for $\alpha =\mu$ (solid line) and $\alpha =\tau$ (dashed line).

Figure 10

Overview of the bounds from SNe (green region), cosmology [63, 90, 91] (blue region), and experiments [92] (gray region) for sterile neutrinos mixed with muon neutrinos (upper panel) and tau neutrinos (lower panel). The dashed lines represent the sensitivities of the future experiments DUNE [93, 94] (black), SHiP [95] (red), and MATHUSLA [96] (brown). The hatched area below the dotted blue line represents the region of the parameter space in which sterile neutrinos are produced in the early Universe out of equilibrium, whose viability is more model dependent.