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Unknown origin of the low energy excess (LEE) background

Identify the physical origin of the low energy excess (LEE) background observed as a sharp rise of events below a few hundred electron-volts in low-threshold detectors, including the cryogenic CaWO4 and Al2O3 targets considered by NUCLEUS, in order to enable its mitigation and prevent sensitivity loss in coherent elastic neutrino–nucleus scattering measurements at reactors.

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Background

Multiple experiments operating at sub‑keV thresholds report a pronounced excess of events below a few hundred eV, commonly termed the low energy excess (LEE). The paper notes that this excess does not appear to be tied to particle‑induced backgrounds and may relate to fundamental aspects of detector design, directly limiting sensitivity to reactor CEνNS.

Because NUCLEUS targets thresholds in the 10–100 eV region, understanding and mitigating the LEE is crucial for achieving the projected signal‑to‑background ratios and for validating background models in the sub‑keV regime.

References

Reaching energy thresholds below the 100\,eV range however led to the identification of another source of background, which is yet of unknown origin and commonly called the low energy excess (LEE).

— Particle background characterization and prediction for the NUCLEUS reactor CE$ν$NS experiment (2509.03559 - Abele et al., 3 Sep 2025) in Section 1 Introduction