Background decomposition of the CONUS+ run 1 data

Abstract

The CONUS+ experiment is measuring the coherent elastic neutrino nucleus scattering (CE$\nu$NS) process using reactor anti-neutrinos as a source and four low energy threshold point-contact high-purity germanium spectrometers for their detection. It achieved the first measurement of coherent neutrino scattering at a nuclear reactor in run 1 of the experiment with a detection energy threshold of 160 eV${}_{ee}$. This work presents the decomposition of the background spectra of the three detectors used in the run 1 analysis and the development of the corresponding background model with Geant4-based Monte Carlo simulations. The background model is used as the underlying input for the likelihood fit of the analysis. It is shown that reactor-correlated backgrounds are subdominant in all energy regions, specifically in the region of interest for CE$\nu$NS searches below 350 eV${}_{ee}$ where their contribution is one order of magnitude below the expected CE$\nu$NS signal. Furthermore, cosmic ray muons and neutrons are identified as the dominant background source below 1 keV${}_{ee}$ contributing approximately 75 - 90 \% of the recorded background rate. The final background model predicts an average rate of (47.5 $\pm$ 3.2) d${}^{-1}$ kg${}^{-1}$ in reactor on measurement in the energy region between [0.4, 1.0] keV${}_{ee}$, which is in excellent agreement with the average measured value of (48.0 $\pm$ 0.6) d${}^{-1}$ kg${}^{-1}$. Similar agreement is found in all energy regions of both reactor on and off measurements.

25 pages, 24 figures