Analysis of CONUS+ run 2: Detector performance, background model and CE$ν$NS measurement

Abstract

Coherent elastic neutrino-nucleus scattering (CE$ν$NS) provides a uniquely sensitive probe of neutrino properties, but its detection requires pushing energy thresholds to the sub-keV scale while controlling backgrounds. The CONUS+ experiment, located at the Leibstadt nuclear power plant, reported the first observation of CE$ν$NS using reactor antineutrinos in 2025. Following a detector upgrade that replaced three of the original 1 kg high-purity germanium detectors with larger 2.42 kg diodes, this paper presents the analysis of the CONUS+ run 2 dataset, including the detector characterization, the background model, the signal prediction and expected sensitivity, and the extraction of the CE$ν$NS signal. The new detectors reach a background level per unit mass reduced by approximately a factor of two relative to the original detectors, while reaching energy thresholds between 140 and 170 eV$_{ee}$. With an exposure of 1296 kg$\cdot$days reactor-on and 86 kg$\cdot$days reactor-off, a blind likelihood analysis fitting directly the number of signal counts yields $N_s = 1555 \pm 152$ CE$ν$NS events, corresponding to a significance of 10.2$\,σ$. The result is in agreement with the Standard Model expectation of $1701 \pm 198$ events, with $N_s/N_{\mathrm{SM}} = 0.91 \pm 0.14$.