1 Introduction

The Deep Underground Neutrino Experiment (DUNE) will be made up of four 10-kton liquid argon time projection chambers underground in South Dakota as part of the DUNE/Long-Baseline Neutrino Facility (LNBF) program. DUNE will record and reconstruct neutrino interactions in the \(\sim \)GeV and higher range for studies of neutrino oscillation parameters and searches for new physics using neutrinos from a beam sent from Fermilab and using neutrinos from the atmosphere. DUNE’s dynamic range is such that it is also sensitive to neutrinos with energies down to about 5 MeV. Charged-current (CC) interactions of neutrinos from around 5 MeV to several tens of MeV create short electron tracks in liquid argon, potentially accompanied by gamma-ray and other secondary particle signatures. This regime is of particular interest for detection of the burst of neutrinos from a galactic core-collapse supernova. Such a detection would be of great interest in the context of multi-messenger astronomy. The sensitivity of DUNE is primarily to electron-flavor neutrinos from supernovae, and this capability is unique among existing and proposed supernova neutrino detectors for the next decades. Neutrinos and antineutrinos from other astrophysical sources, such as solar and diffuse supernova background neutrinos, are also potentially detectable. This low-energy (few to few tens of MeV) event regime has particular reconstruction, background and triggering challenges.

One of the primary physics goals of DUNE as stated in the technical design report (TDR) [1,2,3] is to “Detect and measure the \(\nu _{e}\) flux from a core-collapse supernova within our galaxy, should one occur during the lifetime of the DUNE experiment. Such a measurement would provide a wealth of unique information about the early stages of core collapse, and could even signal the birth of a black hole.” [4].

This paper will document selected studies from the DUNE TDR aimed at understanding DUNE’s sensitivity to low-energy neutrino physics, with an emphasis on supernova burst signals. Section 2 describes basic supernova neutrino physics, as well as prospects for astrophysics and particle physics from observation of a burst. Section 3 gives an overview of the landscape of supernova neutrino burst detection. Section 4 gives a brief description of the DUNE far detector. Section 5 describes the general properties of low-energy events in DUNE, including interaction channels, simulation and reconstruction tools, and backgrounds. The tools include MARLEY, a neutrino event generator specifically developed for this energy regime [5], and the SNOwGLoBES  fast event-rate calculation tool [6]. These are both open-source community tools, rather than DUNE-specific software. The studies described here make use of MARLEY and SNOwGLoBES with input from the full DUNE simulation-reconstruction chain. Section 5.3 describes the expected supernova signal in DUNE, and Sect. 5.4 describes burst triggering studies (as distinct from offline reconstruction studies.) Section 6 describes an example of a study of supernova flux parameter sensitivity in DUNE. Details on supernova pointing capabilities and solar neutrino capabilities will be described in separate publications.

2 Supernova neutrino bursts

The burst of neutrinos from the celebrated core-collapse supernova 1987A in the Large Magellanic Cloud, about 50 kpc from Earth, heralded the era of extragalactic neutrino astronomy. This single neutrino-based observation of a core collapse confirmed our basic understanding of its physical mechanism. Theoretical understanding of the process and of the potential to gain far deeper knowledge from a future observation has advanced considerably in the past decades.

2.1 Neutrinos from collapsed stellar cores: basics

A core-collapse supernovaFootnote 1 occurs when a massive star reaches the end of its life. As a result of nuclear burning throughout the star’s life, the central region of such a star gains an “onion” structure, with an iron core at the center surrounded by concentric shells of lighter elements (silicon, oxygen, neon, magnesium, carbon, etc). At temperatures of \(T\sim 10^{10}\) K and densities of \(\rho \sim 10^{10}\) g/cm\(^{3}\), the Fe core continuously loses energy by neutrino emission (through pair annihilation and plasmon decay [7]). Since iron cannot be further burned, the lost energy cannot be replenished throughout the volume and the core continues to contract and heat up, while also growing in mass thanks to the shell burning. Eventually, the critical mass of about \(1.4 M_{\odot }\) of Fe is reached, at which point a stable configuration is no longer possible. As electrons are absorbed by the protons in nuclei and some iron is disintegrated by thermal photons, the degeneracy pressure support is suddenly removed and the core collapses essentially in free fall, reaching speeds of about a quarter of the speed of light.Footnote 2

The collapse of the central region is suddenly halted after \(\sim 10^{-2}\) s, as the density reaches nuclear (or super-nuclear) values. The central core rebounds and an outward-moving shock wave is formed. The extreme physical conditions of this core, in particular the densities of order \(10^{12}{-}10^{14}\) g/cm\(^{3}\), create a medium that is opaque even for neutrinos. As a consequence, the core initially has a trapped lepton number. The gravitational energy of the collapse at this stage is stored mostly in the degenerate Fermi sea of electrons (\(E_{F}\sim 200\) MeV) and electron neutrinos, which are in equilibrium with the former. The temperature of this core is not more than 30 MeV, which means the core is relatively cold.

At the next stage, the trapped energy and lepton number both escape from the core, carried by the least interacting particles, which in the standard model are neutrinos. Neutrinos and antineutrinos of all flavors are emitted in a time span of a few seconds (their diffusion time). The resulting central object then settles to a neutron star, or a black hole. A tremendous amount of energy, some \(10^{53}\) ergs, is released in \(10^{58}\) neutrinos with energies \(\sim 10\) MeV. A fraction of this energy is absorbed by beta reactions into the material behind the shock wave that then blasts away the rest of the star, creating, in many cases, a spectacular explosion. Yet, from the energetics point of view, this visible explosion is but a tiny perturbation on the total event. Over 99% of all gravitational binding energy of the \(1.4 M_{\odot }\) collapsed core – some 10% of its rest mass – is emitted in neutrinos.

2.2 Stages of the explosion

The core-collapse neutrino signal starts with a short, sharp “neutronization” (or “break-out”) burst primarily composed of \(\nu _e\) from \(e^- + p \rightarrow \nu _e + n\). These neutrinos are messengers of the shock front breaking through the neutrinosphere (the surface of neutrino trapping): when this happens, iron is disintegrated, the neutrino scattering rate drops and the lepton number trapped just below the original neutrinosphere is suddenly released. This quick and intense burst is followed by an “accretion” phase lasting some hundreds of milliseconds, depending on the progenitor star mass, as matter falls onto the collapsed core and the shock is stalled at the distance of \(\sim 200\) km. The gravitational binding energy of the accreting material is powering the neutrino luminosity during this stage. The later “cooling” phase over \(\sim \)10 s represents the main part of the signal, over which the proto-neutron star sheds its trapped energy.

The flavor content and spectra of the neutrinos emitted from the neutrinosphere change throughout these phases, and the supernova’s evolution can be followed with the neutrino signal.

The physics of neutrino decoupling and spectra formation is far from trivial, owing to the energy dependence of the cross sections and the roles played by both CC and neutral-current (NC) reactions. Detailed transport calculations using methods such as MC or Boltzmann solvers have been employed. It has been observed that flux spectra coming out of such simulations can typically be parameterized at a given moment in time by the following ansatz (e.g., [10, 11]):

$$\begin{aligned} \phi (E_{\nu }) = \mathcal {N} \left( \frac{E_{\nu }}{\langle E_{\nu } \rangle }\right) ^{\alpha } \exp \left[ -\left( \alpha + 1\right) \frac{E_{\nu }}{\langle E_{\nu } \rangle }\right] \ , \end{aligned}$$

where \(E_{\nu }\) is the neutrino energy, \(\langle E_\nu \rangle \) is the mean neutrino energy, \(\alpha \) is a “pinching parameter”, and \(\mathcal {N}\) is a normalization constant related to the total luminosity. Large \(\alpha \) corresponds to a more “pinched” spectrum (suppressed tails at high and low energy). This parameterization is referred to as a “pinched-thermal” form. The different \(\nu _e\), \(\overline{\nu }_e\) and \(\nu _x~(x = \mu , \tau , \bar{\mu },\bar{\tau }\)) flavors are expected to have different average energy and \(\alpha \) parameters and to evolve differently in time.

The initial spectra get further processed by flavor transitions, and understanding these oscillations is very important for extracting physics from the detected signal (see Sect. 2.4.1).

In general, one can describe the neutrino flux as a function of time by specifying the three pinching parameters in successive time slices. Figure 1 gives an example of pinching parameters as a function of time for a specific model, and Fig. 2 shows the spectra for the three flavors as a function of time corresponding to this parameterized description. We have verified that the time-integrated spectrum for each flavor is expected to be reasonably well approximated by the pinched-thermal form as well.

Fig. 1
figure 1

Expected time-dependent flux parameters for a specific model for an electron-capture supernova [8]. No flavor transitions are assumed. The top plot shows the luminosity as a function of time, the second plot shows average neutrino energy, and the third plot shows the \(\alpha \) (pinching) parameter. The vertical dashed line at 0.02 s indicates the time of core bounce, and the vertical lines indicate different eras in the supernova evolution. The leftmost time interval indicates the infall period. The next interval, from core bounce to 50 ms, is the neutronization burst era, in which the flux is composed primarily of \(\nu _e\). The next period, from 50 to 200 ms, is the accretion period. The final era, from 0.2 to 9 s, is the proto-neutron-star cooling period. The general features are qualitatively similar for most core-collapse supernova models

Fig. 2
figure 2

Examples of time-dependent neutrino spectra for the electron-capture supernova model [8] parameterized in Fig. 1, on three different timescales. The x-axis for all plots indicates time in seconds and the y-axis indicates neutrino energy in MeV. The z-axis color-shading units are neutrinos per cm\(^2\) per millisecond per 0.2 MeV. Note the different z scales in the panels. Core bounce is at \(t=0\). Top: \(\nu _e\). Center: \(\bar{\nu }_e\). Bottom: \(\nu _x\). Flavor transition effects are not included here; note they can have dramatic effects on the spectra. Figure modified from Ref. [9]

2.3 Astrophysical observables

A number of astrophysical phenomena associated with supernovae are expected to be observable in the supernova neutrino signal, providing a remarkable window into the event. In particular, the supernova explosion mechanism, which in the current paradigm involves energy deposition into the stellar envelope via neutrino interactions, is still not well understood, and the neutrinos themselves will bring the insight needed to confirm or refute the paradigm.

There are many other examples of astrophysical observables:

  • The initial burst, primarily composed of \(\nu _e\) and called the “neutronization” or “breakout” burst, represents only a small component of the total signal. However, flavor transition effects can manifest themselves in an observable manner in this burst, and flavor transformations can be modified by the “halo” of neutrinos generated in the supernova envelope by scattering [12].

  • The formation of a black hole would cause a sharp signal cutoff (e.g., [13,14,15]).

  • Shock wave effects (e.g., [16]) would cause a time-dependent change in flavor and spectral composition as the shock wave propagates.

  • The standing accretion shock instability (SASI) [17, 18], a “sloshing” mode predicted by three-dimensional neutrino-hydrodynamics simulations of supernova cores, would give an oscillatory flavor-dependent modulation of the flux.

  • Turbulence effects [19, 20] would also cause flavor-dependent spectral modification as a function of time.

The supernova neutrino burst is prompt with respect to the electromagnetic supernova signal and therefore can be exploited to provide an early warning to astronomers [