Impact of reactor antineutrinos on the neutrino floor in low-mass WIMP-like dark matter searches

TL;DR

This work addresses the irreducible neutrino background in direct dark matter searches by quantifying how reactor antineutrinos modify the neutrino floor for sub-10 GeV/$c^2$ WIMPs using germanium detectors. It develops a detailed WIMP signal model with SI germanium scattering, incorporating Lindhard quenching and realistic halo and form-factor treatments, and couples it to a comprehensive CE$\nu$NS background model that includes solar, geoneutrino, DSNB, atmospheric, and reactor sources. The authors compare discovery-limit and opacity-based neutrino-floor definitions under distance-dependent reactor fluxes, finding that proximity to a gigawatt-scale reactor within ~10 km can elevate the floor by up to several orders of magnitude, while baselines $\gtrsim 100$ km render the reactor effect negligible. The results have practical implications for site selection and experimental design of next-generation low-threshold dark matter experiments, highlighting the necessity of accurate, location-specific reactor modeling and potential mitigation strategies such as directional detection. Overall, reactor proximity emerges as a critical factor shaping the accessible WIMP parameter space in the low-mass regime and guiding future sensitivity projections.

Abstract

The sensitivity of conventional direct dark matter searches for weakly interacting massive particles (WIMPs) is ultimately limited by coherent elastic neutrino-nucleus scattering (CEvNS), which produces nuclear recoils indistinguishable from WIMP signals and defines the so-called neutrino floor. While the effects of solar neutrinos, geoneutrinos, diffuse supernova neutrinos, and atmospheric neutrinos have been extensively studied in this context, the contribution from reactor antineutrinos has received comparatively little attention. We present the first systematic evaluation of how reactor antineutrino fluxes, modeled as a function of reactor-detector distance, modify the neutrino floor for low-mass WIMP searches using SuperCDMS-like high-voltage germanium detectors. Both discovery-limit and opacity-based formulations of the neutrino floor are examined under consistent assumptions. We find that proximity to gigawatt-scale reactors within 10 km can raise the neutrino floor by up to a few orders of magnitude, significantly reducing the sensitivity to sub-10 GeV/c^2 dark matter. Beyond 100 km, the reactor contribution becomes negligible. These conclusions hold for both definitions of the neutrino floor and remain stable under reasonable variations in detector quenching, site-dependent geoneutrino flux, and reactor antineutrino flux uncertainties, emphasizing reactor proximity as a critical factor in site selection for future low-threshold dark matter experiments.

Figures (15)

• Figure 1: Comparison of nuclear-recoil (top) and electron-equivalent (bottom) spectra obtained using the Lindhard quenching model for various CE$\nu$NS components, shown alongside a representative WIMP signal (6 GeV/$c^2$, $\sigma_{\chi-n} = 4.4\times10^{-45}$ cm$^2$). The $^8$B solar neutrino spectrum closely resembles the WIMP-induced recoil spectrum over the entire energy range of interest, highlighting the challenge of distinguishing low-mass WIMP signals from solar neutrino backgrounds.
• Figure 2: Reactor antineutrino flux at 1, 10, and 50 km from a 3.6 GW$_\mathrm{th}$ reactor core. The total spectrum combines a parameterized model above 2 MeV with tabulated data below, as described in Sec. \ref{['subsec:reactor_antineutrino']}. The overall flux decreases with distance following the expected $1/L^2$ dependence.
• Figure 3: Comparison of differential CE$\nu$NS event rates from reactor antineutrinos and non-reactor (solar, atmospheric, DSNB, and geoneutrino) sources. Results are shown for detector locations at 1 km (upper left), 10 km (upper right), and 50 km (bottom) from a 3.6 GW$_\mathrm{th}$ reactor core. The reactor contribution begins to dominate over non-reactor backgrounds at reactor–detector separations below approximately 10 km.
• Figure 4: Impact of reactor antineutrinos on the discovery-limit neutrino floor for low-mass WIMPs. Results are shown for detector locations at 1, 5, 10, 50, and 100 km from a 3.6 GW$_\mathrm{th}$ reactor core, along with the no-reactor baseline.
• Figure 5: Effect of the discovery-limit–based neutrino floor on reactor antineutrino flux uncertainties. Results are shown for reactor–detector distances of 1 km and 10 km, assuming a nominal 8% systematic uncertainty in the reactor antineutrino flux, and compared with a scenario where this uncertainty is reduced by a factor of ten.