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Abstract

The CONUS experiment is looking for coherent elastic neutrino nucleus scattering
(CEvNS) at the 3.9GW_th nuclear power plant in Brokdorf, Germany, at a distance
of 17m to the reactor core. Four high-purity low threshold germanium spectrometers
(in total 4 kg) are deployed within a massive shield. Detector properties and
the electronics noise are characterized in detail. This includes the active volume, the
stability of the energy scale, the time difference distribution of the events and a complete
study of correlations to environmental parameters. Potential reactor-correlated
backgrounds were examined by dedicated neutron and ray measurements outside
of the shield. With the help of Monte Carlo (MC) simulations it was shown that the
reactor neutron-induced background is neglible within the shield. For the CEvNS
analysis, the MC was further expanded by including all non reactor-correlated background
components, especially the muon-induced one, which is dominant at the
shallow depth of the experiment. From an exposure of 248 kg.d reactor ON and
59 kg.d reactor OFF data the first upper limit for CEvNS with reactor antineutrinos
is derived in a likelihood analysis. Quenching, the incomplete conversion of recoil
energy to ionization energy registered by the detectors, is the dominant systematic
uncertainty. From the evaluated upper limit and the Standard Model prediction of
the expected number of counts, a wide range of the quenching parameters found in
literature can be excluded. For the central value in literature of 0.16, the upper limit
with 90% C.L. is only a factor of two away from the Standard Model expectation.