Determining the Neutrino Mass Hierarchy from the Next Galactic Supernova ✦
The next galactic core-collapse supernova will provide a unique opportunity to determine the neutrino mass ordering. Two observable signals are analyzed: the neutronization burst of electron neutrinos (a sharp peak over ~20–30 ms) and the rise time of the electron antineutrino flux during the accretion phase. The neutronization burst is nearly model-independent and gives a clear distinction via its presence or absence. Realistic simulations for several progenitor masses with event calculations for the DUNE, Hyper-Kamiokande, and JUNO detectors at a distance of 10 kpc yield sensitivities of >6σ (DUNE) and >4σ (Hyper-K) for discriminating normal and inverted hierarchy. For the rise time, to circumvent progenitor-dependent degeneracies, cumulative and ratio-based observables at characteristic times of 20 ms and 100 ms are used, giving confidence levels of ~5σ (Hyper-K) and ~3σ (JUNO). The combined use of the neutronization burst and accretion-phase data from different detectors will be crucial for an unambiguous determination of the neutrino mass ordering from the next supernova.
Context
Supernova explosions are grand cosmic events in which a massive star, having exhausted its nuclear fuel, collapses, giving birth to a neutron star or a black hole. In a matter of seconds, energy comparable to the total radiation of an entire galaxy is released, and 99% of it is carried away by neutrinos—particles predicted by Wolfgang Pauli back in 1930. Supernova explosions also serve as the main factories of heavy elements and cosmic dust, spreading them throughout the Galaxy. Studying the neutrino signal from the next supernova, which is expected in the Milky Way at any moment, will not only unravel the explosion mechanism but also answer a fundamental question: how are the masses of the three types of neutrinos ordered—normally or inversely? This knowledge is critically important for understanding dark matter and the expansion of the Universe, since neutrinos, though nearly elusive, contribute to the cosmological balance.
Methods
The analysis used results from hydrodynamic simulations of supernova core collapse from the Garching Group with progenitor masses from 12 to 27 solar masses. It is important to note that the fate of the collapsing core depends on the mass limit established by Subrahmanyan Chandrasekhar. The initial neutrino fluxes underwent flavor conversion according to three scenarios: standard MSW oscillations for normal (NO) and inverted (IO) hierarchy, as well as a full exchange (FE) scenario, which could arise from fast collective effects in the dense medium where entropy and density reach extreme values. Using the SNOwGLoBES package, expected events were calculated in detectors: DUNE (liquid argon, 40 kt), Hyper-Kamiokande (water, 187 kt), and JUNO (liquid scintillator, 20 kt), located at a distance of 10 kpc from the supernova. The statistical analysis included the chi-squared method for both time-dependent spectra and integral characteristics—cumulative events and their ratio at 20 and 100 ms—thus circumventing model uncertainties.
Results
The neutronization burst, a sharp peak of electron neutrinos lasting about 20–30 milliseconds, turned out to be an almost model-independent indicator: for the inverted hierarchy it is clearly visible, while for the normal hierarchy it is practically absent. In the DUNE detector, the sensitivity to hierarchy discrimination exceeds 6σ for all considered models, and in Hyper-Kamiokande—at least 4σ. This means that even a single event will give a reliable answer. For the accretion phase, the analysis of the rise in the electron antineutrino flux, caused by the faster increase in the luminosity of heavy-lepton neutrinos, showed that using the ratio of cumulative events at 20 and 100 ms, the significance in HK reaches ∼5σ, and in JUNO—∼3σ. Thus, combining data from several detectors and different explosion phases will provide multiple cross-checks of the result. Interestingly, the FE scenario, predicted in some models of collective oscillations, somewhat reduces the contrast but does not destroy it completely, leaving an opportunity to test the theory of collective effects itself.
Implications
The results confirm that the next galactic supernova will most likely finally clarify the neutrino mass hierarchy—a task that accelerator and reactor experiments have not yet been able to solve. Back in the mid-20th century, the pioneer of neutrino astrophysics John Wheeler discussed the possibility of obtaining information about processes in stellar interiors precisely through neutrinos. This measurement has far-reaching consequences: it will constrain the spectrum of allowed theoretical models in particle physics, influence our understanding of the Big Bang and the subsequent evolution of the Universe. Just as the cosmic microwave background serves as an echo of the hot Universe, the neutrino background carries information about the first moments of creation. Success in determining the hierarchy will also shed light on the nature of collective neutrino oscillations, which represent new, not yet fully understood physics.
Future development
Further improvement of three-dimensional hydrodynamic simulations including magnetic fields and rotation will allow refinement of neutrino luminosities and reduction of systematic errors. Planned upgrades of detectors, including Hyper-Kamiokande and DUNE, as well as the commissioning of new neutrino telescopes such as IceCube-Gen2, will increase statistical significance. Of particular interest is the theoretical investigation of fast collective oscillations—a phenomenon that could radically change the picture of neutrino flavors in the supernova interior. Joint analysis of neutrino data with gravitational-wave and electromagnetic observations within the framework of multimessenger astronomy will become standard upon detection of the next galactic supernova.
Impact
Determining the neutrino mass hierarchy will impact elementary particle physics, nuclear physics, astrophysics, and cosmology. In astrophysics, it will improve understanding of supernova explosion mechanisms and the formation of compact objects—neutron stars and black holes. In cosmology, it will refine models of large-scale structure formation and the contribution of neutrinos to dark matter.
Next steps
A key step will be the development and testing of rapid alert protocols for all operating neutrino observatories, so that upon detecting a burst, immediate pointing of optical, X-ray, and gravitational-wave instruments is ensured. It is also necessary to continue theoretical studies of collective neutrino oscillations under realistic conditions of a collapsing core.
Key open problems
The neutrino mass hierarchy problem is closely linked to other fundamental questions: why fermion masses differ so greatly, whether a sterile neutrino exists, and what the nature of dark matter is. The neutrino signal from a supernova can also provide unique information about the equation of state of nuclear matter at supranuclear densities, which is important for understanding the structure of neutron stars and gravitational-wave signals from their mergers.
🎯 In the first 20 milliseconds of the neutronization burst, so many neutrinos are emitted that on Earth, despite tens of thousands of light-years distance, the DUNE detector will register about 100 events. For comparison, a year of observing solar neutrinos in a similar detector yields roughly the same number.
Key numbers
- total number of emitted neutrinos: ~10^58
- duration of the neutronization burst: 20–30 ms
- significance level of hierarchy separation in DUNE: ≳6σ
- distance to the galactic supernova: 10 kpc
- mass of the Hyper-Kamiokande detector: 187 kt