Neutrino nonradiative decay and the diffuse supernova neutrino background

We revisit the possibility that neutrinos undergo nonradiative decay. We investigate the potential to extract information on the neutrino lifetime-to-mass ratio from the diffuse supernova neutrino background. To this aim, we explicitly consider the current uncertainties on the core-collapse supernova rate and the fraction of failed supernovae. We present predictions in a full $3\nu$ framework in the absence and presence of neutrino nonradiative decay, for the Super-Kamiokande+Gd, the JUNO, the Hyper-Kamiokande, and the DUNE experiments, that should observe the diffuse supernova neutrino background in the near future. Our results show the importance of a $3\nu$ treatment of neutrino decay and of identifying the neutrino mass ordering to break possible degeneracies between diffuse supernova neutrino background predictions in the presence of decay and standard physics.

Figure 1

Core-collapse supernova rate as a function of redshift. The figure shows the piecewise parametrization by [64, 68], used in [21, 22] (blue), and the one from [67] (pink) with the band showing the core-collapse supernova rate uncertainty. The dashed line shows the older evolving core-collapse supernova rate, employed in the DSNB study of [55], including neutrino nonradiative decay.

Figure 2

Scenario I to III (top to bottom) for the BH fraction as well as the progenitor dependence of a supernova that left either a neutron star or a black hole. The parameters (neutrino luminosity, average energies, and pinching) of the corresponding fluences are given in Table 6 (Appendix pp1).

Figure 3

Decay patterns for $3\nu$ flavors. Upper left: normal mass ordering in the strongly-hierarchical (SH) case. The branching ratios are equal to $1/4$ for ${\nu }_3$ (${\overline{\nu }}_3$) and $1/2$ for ${\nu }_2$ (${\overline{\nu }}_2$). Upper right: normal mass ordering in the quasi-degenerate case (QD). The branching ratios are equal to $1/2$ for ${\nu }_3$ or ${\overline{\nu }}_3$. Lower: inverted mass ordering case. The branching ratios are equal to $1/3$ for ${\nu }_2$ (${\overline{\nu }}_2$) and $1/2$ for ${\nu }_1$ (${\overline{\nu }}_1$). In all cases the lifetime-to-mass ratio of the decaying eigenstates is taken equal, i.e., ${\tau }_2/m_2={\tau }_3/m_3$ (NO) or ${\tau }_2/m_2={\tau }_1/m_1$ (IO).

Figure 4

No $\nu$ decay: DSNB fluxes for electron neutrinos (top), electron anti-neutrinos (middle) and all neutrino flavors added (bottom) as a function of neutrino energy, with the $\mathrm{\Lambda }\mathrm{CDM}$ model and the core-collapse supernova rate Eq. (8). Three different scenarios are taken for the BH fraction. The NO results correspond to the fiducial model ($f_{\mathrm{BH}}=0.21$), the band showing the uncertainty from $R_{SN}$ (see Table 1). The IO results are for the fiducial model without the $R_{SN}$ uncertainty.

Figure 5

Comparison of the DSNB ${\overline{\nu }}_e$ fluxes in presence of $\nu$ decay, with $2\nu$ (dotted) or $3\nu$ (full lines), for NO, SH (top), and IO (bottom). Only $(\tau /m{)}_{\mathrm{short}}$ and $(\tau /m{)}_{\text{medium}}$ are presented for clarity. The DSNB fluxes in absence of decay are also shown. The results correspond to the fiducial model.

Figure 6

Neutrino decay in $3\nu$ framework: DSNB ${\overline{\nu }}_e$ fluxes for NO with the SH decay pattern. Three values of the lifetime-over-mass ratio are considered. The lines show the results for the fiducial model, whereas the bands come from the uncertainty on core-collapse supernova rate. The DSNB fluxes in absence of neutrino decay are given for comparison.

Figure 7

Neutrino decay in $3\nu$ framework: DSNB ${\overline{\nu }}_e$ and ${\nu }_e$ fluxes in NO, with the QD decay pattern. Three values of the lifetime-over-mass ratio are considered. The lines show the results for the fiducial model, whereas the bands come from the uncertainty on core-collapse supernova rate. The DSNB fluxes in absence of neutrino decay are given for comparison.

Figure 8

Neutrino decay in $3\nu$ framework: DSNB ${\overline{\nu }}_e$ fluxes for IO. Three values of the lifetime-over-mass ratio are considered. The lines show the results for the Fiducial model, whereas the bands come from the uncertainty on core-collapse supernova rate. The DSNB fluxes in absence of neutrino decay are given for comparison.

Figure 9

Comparison of the expected DSNB events, as a function of positron energy, in the effective $2\nu$ (dotted) and the $3\nu$ frameworks (full lines). The results are for the HK detector and a running time of 20 years for NO (upper) and IO (lower figure). The events for no decay are also shown for comparison.

Figure 10

No decay case: Expected DSNB events, as a function of positron energy, for the fiducial model in the SK-Gd experiment and a running time of 10 years. The band corresponds to the current uncertainty on $R_{SN}$. Backgrounds from invisible muons, NC and CC atmospheric neutrinos are shown (from [80]). Spallation due to cosmogenic backgrounds (producing for example ${}^9\mathrm{Li}$) and accidentals [12] are not shown.

Figure 11

Expected DSNB ${\overline{\nu }}_e$ events associated with inverse beta-decay, as a function of positron energy in SK-Gd for a running time of 10 years. The cases are NO (upper figure) and IO (lower figure). The results correspond to the Fiducial model with the shortest $\tau /m$ (dot-dashed line), the intermediate $\tau /m$ (dotted) and the long $\tau /m$ (dashed) (decay patterns in Fig. 3). For no decay and for the case of decay with $\tau /m={10}^9\mathrm{s}/\mathrm{eV}$, the bands come from the uncertainty in the core-collapse supernova rate. The black line corresponds to the summed backgrounds shown in Fig. 10.

Figure 12

Expected DSNB ${\overline{\nu }}_e$ events associated with inverse beta-decay, as a function of positron energy in HK-Gd for a running time of 20 years (see Table 4). The cases are NO and SH (upper figure) an IO (lower figure). The results correspond to the Fiducial model with the shortest $\tau /m$ (dot-dashed line), the intermediate $\tau /m$ (dotted) and the long $\tau /m$ (dashed). For the cases of no decay case and of $\tau /m={10}^9\mathrm{s}/\mathrm{eV}$, the bands come from the uncertainty in the core-collapse supernova rate (see text). Backgrounds are not shown here.

Figure 13

Expected DSNB ${\overline{\nu }}_e$ events associated with inverse beta-decay, as a function of positron energy in HK-Gd for a running time of 20 years (see Table 4). The case is NO and QD decay pattern. The results correspond to the fiducial model with the shortest $\tau /m$ (dot-dashed line), the intermediate $\tau /m$ (dotted) and the long $\tau /m$ (dashed). For the cases of no decay case and of $\tau /m={10}^9\mathrm{s}/\mathrm{eV}$, the bands come from the uncertainty in the core-collapse supernova rate (see text). Backgrounds are not shown here.

Figure 14

Expected DSNB ${\overline{\nu }}_e$ events associated with inverse beta-decay, as a function of positron energy in JUNO for a running time of 20 years. The cases are NO (upper figure) and IO (lower figure). The results correspond to the fiducial model with the shortest $\tau /m$ (dot-dashed line), the intermediate $\tau /m$ (dotted) and the long $\tau /m$ (dashed) (decay patterns in Figure 3). For no decay and for the case of decay with $\tau /m={10}^9\mathrm{s}/\mathrm{eV}$, the bands come from the uncertainty in the core-collapse supernova rate.

Figure 15

Number of events associated with ${\nu }_e$ scattering on ${}^{40}\mathrm{Ar}$, as a function of neutrino energy, for the DUNE detector. The running time is 20 years. The cases are NO (upper figure) and IO (lower figure).