Impact of neutrino decays on the supernova neutronization-burst flux

The discovery of nonzero neutrino masses invites one to consider decays of heavier neutrinos into lighter ones. We investigate the impact of two-body decays of neutrinos on the neutronization burst of a core-collapse supernova—the large burst of ${\nu }_e$ during the first 25 ms post-core-bounce. In the models we consider, the ${\nu }_e$, produced mainly as a ${\nu }_3({\nu }_2)$ in the normal (inverted) mass ordering, are allowed to decay to ${\nu }_1({\nu }_3)$ or ${\overline{\nu }}_1({\overline{\nu }}_3)$ and an almost massless scalar. These decays can lead to the appearance of a neutronization peak for a normal mass ordering or the disappearance of the same peak for the inverted one, thereby allowing one mass ordering to mimic the other. Simulating supernova-neutrino data at the Deep Underground Neutrino Experiment (DUNE) and the Hyper-Kamiokande (HK) experiment, we compute their sensitivity to the neutrino lifetime. We find that, if the mass ordering is known and depending on the nature of the physics responsible for the neutrino decay, DUNE is sensitive to lifetimes $\tau /m\lesssim {10}^6\mathrm{s}/\mathrm{eV}$ for a Galactic supernova sufficiently close by (around 10 kpc), while HK is sensitive to lifetimes $\tau /m\lesssim {10}^7\mathrm{s}/\mathrm{eV}$. These sensitivities are far superior to existing limits from solar-system-bound oscillation experiments. Finally, we demonstrate that using a combination of data from DUNE and HK, one can, in general, distinguish between decaying Dirac neutrinos and decaying Majorana neutrinos.

Figure 1

Luminosities, the average neutrino energies, and pinching parameter $\alpha$—see Eq. (3.2)—of the neutrinos emitted during the neutronization-burst phase—first 25 ms or so—and slightly beyond. These results come from hydrodynamical simulations of the Garching group for a $25M_{\odot }$ progenitor model [71].

Figure 2

Expected differential flux of electron neutrinos ${\nu }_e$ (left) and antineutrinos ${\overline{\nu }}_e$ (right) at the surface of the Earth as a function of time after bounce, for a supernova explosion 10 kpc away, for the NMO (blue) and IMO (red) and $E_{\nu }=12\mathrm{MeV}$. The values of the neutrino oscillation parameters are the best-fit ones tabulated in Ref. [80]. The neutronization burst exists roughly for the first 25 ms post-core-bounce.

Figure 3

Expected event rates as a function of time in DUNE (40 ktons of liquid argon), for different values of the lifetime of the heaviest neutrino mass eigenstate, for the NMO (left) and the IMO (right), for a supernova explosion 10 kpc away. We include all events that deposit at least 4 MeV of reconstructed energy in the detector. The values of the neutrino oscillation parameters are the best-fit ones tabulated in Ref. [80]. The neutrinos are assumed to be Dirac fermions, and the neutrino decay is described by Eq. (2.1).

Figure 4

Expected event rates as a function of energy in DUNE (40 ktons of liquid argon), for different values of the lifetime of the heaviest neutrino mass eigenstate, for the NMO (left) and the IMO (right), for a supernova explosion 10 kpc away. We include all events that arrive during the neutronization burst (first 25 ms). The values of the neutrino oscillation parameters are the best-fit ones tabulated in Ref. [80]. The neutrinos are assumed to be Dirac fermions, and the neutrino decay is described by Eq. (2.1).

Figure 5

$\sqrt{\mathrm{\Delta }{\chi }^2}$ as a function of the hypothetical ${\nu }_3$ lifetime over mass, obtained when comparing simulated DUNE data consistent with the IMO and effectively stable neutrinos with the hypothesis that the neutrino mass ordering is normal and the ${\nu }_3$ lifetime is finite, for different distances between the Earth and the supernova explosion. Data are binned in both time and energy, as described in the text. We assume a 40% uncertainty in the overall neutrino flux, and marginalize over the neutrino oscillation parameters (except for the mass ordering). The values of the neutrino oscillation parameters—best-fit values and uncertainties—are tabulated in Ref. [80]. The neutrinos are assumed to be Dirac fermions, and the neutrino decay is described by Eq. (2.1).

Figure 6

$\sqrt{\mathrm{\Delta }{\chi }^2}$ as a function of the hypothetical ${\nu }_3$ lifetime over mass, obtained when comparing simulated DUNE data consistent with effectively stable neutrinos with the hypothesis that the ${\nu }_3$ lifetime is finite, for different distances between the Earth and the supernova explosion. The neutrino mass ordering is assumed to be normal and known. Data are binned in both time and energy, as described in the text. We assume a 40% uncertainty in the overall neutrino flux, and marginalize over the neutrino oscillation parameters (except for the mass ordering). The values of the neutrino oscillation parameters—best-fit values and uncertainties—are tabulated in Ref. [80]. The neutrinos are assumed to be Dirac fermions, and the neutrino decay is described by Eq. (2.1). The colored shaded region is excluded by supernova cooling bounds using data from SN1987A observations [48, 49].

Figure 7

Expected event rates from a supernova explosion 10 kpc away as a function of time in DUNE (left, 40 ktons of liquid argon) and HK (right, 374 ktons of water), for decaying Dirac (blue) and Majorana ${\nu }_3$ (red), for ${\tau }_3/m_3={10}^5\mathrm{s}/\mathrm{eV}$. We include all events that deposit at least 4 MeV (3 MeV) of reconstructed energy in DUNE (HK). The values of the neutrino oscillation parameters are the best-fit ones tabulated in Ref. [80], and the mass ordering is normal. Dirac neutrino decay is described by Eq. (2.1), while Majorana neutrino decay is described by Eq. (2.5).

Figure 8

Expected event rates from a supernova explosion 10 kpc away as a function of energy in DUNE (left, 40 ktons of liquid argon) and HK (right, 374 ktons of water), for decaying Dirac (blue) and Majorana ${\nu }_3$ (red), for ${\tau }_3/m_3={10}^5\mathrm{s}/\mathrm{eV}$. We include all events that arrive during the neutronization burst (first 25 ms). The horizontal scales do not start at $E=0$, reflecting the energy thresholds for the experiments. The values of the neutrino oscillation parameters are the best-fit ones tabulated in Ref. [80], and the mass ordering is normal. Dirac neutrino decay is described by Eq. (2.1), while Majorana neutrino decay is described by Eq. (2.5).

Figure 9

$\sqrt{\mathrm{\Delta }{\chi }^2}$ as a function of the hypothetical ${\nu }_3$ lifetime over mass, obtained when comparing simulated DUNE or HK data consistent with a decaying Dirac ${\nu }_3$ from a supernova explosion 10 kpc away, for ${\tau }_3/m_3={10}^5\mathrm{s}/\mathrm{eV}$, with different hypotheses regarding the ${\nu }_3$ lifetime and the nature of the neutrinos. The neutrino mass ordering is assumed to be normal and known. Data are binned in both time and energy, as described in the text. We assume a 40% uncertainty in the overall neutrino flux—independent at DUNE and HK—and marginalize over the neutrino oscillation parameters (except for the mass ordering). The values of the neutrino oscillation parameters—best-fit values and uncertainties—are tabulated in Ref. [80]. Dirac neutrino decay is described by Eq. (2.1), while Majorana neutrino decay is described by Eq. (2.5).