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Detecting the diffuse supernova neutrino background in the future water-based liquid scintillator detector Theia

Julia Sawatzki, Michael Wurm, and Daniel Kresse
Phys. Rev. D 103, 023021 – Published 21 January 2021
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  • INTRODUCTION
  • DETECTOR TECHNOLOGY
  • DSNB MODEL
  • BACKGROUND MODELING
  • BACKGROUND REDUCTION
  • DETECTION POTENTIAL
  • COMPARISON WITH OTHER TECHNIQUES
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
    • References

    Abstract

    A large-scale neutrino observatory based on water-based liquid scintillator (WbLS) will be excellently suited for a measurement of the diffuse supernova neutrino background (DSNB). The WbLS technique offers high signal efficiency and effective suppression of the otherwise overwhelming background from neutral-current interactions of atmospheric neutrinos. To illustrate this, we investigate the DSNB sensitivity for two configurations of the future Theia detector by developing the expected signal and background rejection efficiencies along a full analysis chain. Based on a statistical analysis of the remaining signal and background rates, we find that a rather moderate exposure of 190  kt·yrs will be sufficient to claim a (5σ) discovery of the faint DSNB signal for standard model assumptions. We conclude that, in comparison with other experimental techniques, WbLS offers the highest signal efficiency of more than 80% and best signal significance over background.

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    • Received 29 July 2020
    • Accepted 7 December 2020

    DOI:https://doi.org/10.1103/PhysRevD.103.023021

    © 2021 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Novae & supernovae
    1. Techniques
    Neutrino detection
    Gravitation, Cosmology & Astrophysics

    Authors & Affiliations

    Julia Sawatzki*

    • Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany

    Michael Wurm†

    • Institut für Physik, Johannes Gutenberg Universität Mainz, Staudinger Weg 7, 55128 Mainz, Germany

    Daniel Kresse‡

    • Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany and Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany

    • *julia.sawatzki@ph.tum.de
    • †michael.wurm@uni-mainz.de
    • ‡danielkr@mpa-garching.mpg.de

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    Issue

    Vol. 103, Iss. 2 — 15 January 2021

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    Images

    • Figure 1
      Figure 1

      Basic detector geometries of Theia100 (left) and Theia25 (right).

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    • Figure 2
      Figure 2

      Differential ν¯e DSNB flux arriving at Earth with neutrino energy Eν for the fiducial model (“W18−BH2.7−α2.0” with best-fit SFH from [43]; blue), lying between the low-flux model (“S19.8−BH2.3−α2.0” with −1σ SFH from [43]; light blue) and the high-flux model (“W20−BH3.5−α2.0” with best-fit SFH from [43]; dark blue). See Ref. [11] for a more detailed description of the flux models.

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    • Figure 3
      Figure 3

      The visible scintillation energy spectrum expected for the DSNB signal and its ample backgrounds. The presented spectra include reactor neutrinos, cosmogenic Li9, fast neutrons as well as atmospheric neutrino charged-current (CC) and neutral-current (NC) interaction rates. We assume a basic event selection of IBD-like coincidence signals (with only a single accompanying neutron capture). The energy scale is based on the number of scintillation photons detected. The upper axis lists the corresponding visible scintillation energy. Expected rates according to the three DSNB models are indicated in blue (cf. Fig. 2).

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    • Figure 4
      Figure 4

      The visible scintillation energy spectra expected for the DSNB signal and backgrounds after the application of basic discrimination techniques: We apply a 2.5 m fiducial volume cut to reduce the background by fast neutrons and a veto of Li9 based on the coincidence with the preceding parent muons. Expected rates according to the three DSNB models are indicated in blue (cf. Fig. 2).

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    • Figure 5
      Figure 5

      Number of Cherenkov rings reconstructed for atmospheric NC events as a function of the prompt visible scintillation energy. The frequent occurrence of two or more rings allows for an efficient discrimination against the single-ring DSNB positrons. The grey boxes indicate the limits of the observation window.

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    • Figure 6
      Figure 6

      The Cherenkov-to-scintillation (C/S) ratio offers a powerful tool to discriminate prompt positrons of DSNB events (blue) and hadronic prompt events of atmospheric NC reactions (black). Atmospheric NC events lead to a significantly reduced emission of Cherenkov photons. The lower plot presents a zoom-in for C/S values greater than 0.5. The gray shaded area indicates the limits of the observation window. The red line corresponds to the C/S cut threshold reaching 82% signal efficiency.

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    • Figure 7
      Figure 7

      The optimum choice of a cut on the C/S ratio depends on the optimization of the signal-to-background (S/B) ratio. The rate of surviving DSNB events as a function of the residual rate of atmospheric NC events is indicated by the solid line. While not shown, the corresponding C/S cut threshold is increasingly relaxed from left to right. The dashed line indicates the corresponding significance of the signal over background S/S+B (scale on the right y axis). The maximum of the curve (82% signal efficiency at 3.5% residual background, indicated by the grey line) is chosen for the further analysis.

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    • Figure 8
      Figure 8

      The visible scintillation energy spectrum expected for DSNB signal and backgrounds after all selection cuts. The background components include IBDs from reactor and atmospheric neutrinos as well as a residual of IBD-like NC interactions of atmospheric neutrinos. The signal dominates with respect to the backgrounds over the entire observation window (white region). Expected rates according to the three DSNB models are indicated in blue (cf. Fig. 2).

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    • Figure 9
      Figure 9

      Significance of DSNB detection as a function of exposure. The curves correspond to a variation of the relative uncertainty in the atmospheric NC background rates from 5% to 20%. The upper black (blue) horizontal axis scale indicates the operation time of Theia25 (Theia100).

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    • Figure 10
      Figure 10

      Significance of DSNB detection as a function of exposure for the different DSNB flux models considered in our work. The black line corresponds to the fiducial model, whereas the high-flux and low-flux models are given in gray. The maximum exposure required for a 3σ detection (in case of the low-flux model) is ∼280  kt·yrs. The upper black (blue) horizontal axis scale indicates the operation time of Theia25 (Theia100).

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    • Figure 11
      Figure 11

      Projections for the signal rates (left panel) and signal significance (right panel) of the relevant DSNB observatories over the next two decades. Optimistic scenarios correspond to dashed lines. The optimistic sum includes Theia100, and a second tank for Gd-loaded HyperK. DUNE is not added to the overall sum, due to different neutrino channel. Once initiated, Theia100 and HK-Gd soon dominate the scene regarding both collected signal statistics and significance of the detection. Theia25 makes a slower start but provides an increasingly relevant contribution over ten years of data taking. See the text for a more detailed discussion.

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