New Evidence for DM-like Anomalies in Neutron Multiplicity Spectra

Abstract: Subterrestrial neutron spectra show weak but consistent anomalies at multiplicities ~100 and above. The origin of the excess events remains ambiguous, but, in principle, it could be a signature of Dark Matter WIMP annihilation-like interaction with a massive Pb target. However, since the results of the available measurements are below the 5-sigma discovery level, and the observed anomalous structures are on a significant muon-induced background, an independent verification at even greater depth is needed. For that purpose, we have launched NEMESIS 1.4 - a new dedicated experiment consisting of a 1134 kg Pb target and 14 He-3 detectors with PE moderators and a fully digital readout. NEMESIS 1.4 has been taking data at the deepest level (1.4 km, 4000 m.w.e.) of the Pyhasalmi mine, Finland, since November 2022. We describe the idea behind the new setup, compare the first results with the previous data and Monte Carlo simulations, and give the outlook for further research. If the existence of the anomalies is unambiguously confirmed and the model interpretation positively verified, this will be the first Indirect Detection of Dark Matter in the laboratory.