Calibration of the liquid argon ionization response to low energy electronic and nuclear recoils with DarkSide-50

Abstract

DarkSide-50 has demonstrated the high potential of dual-phase liquid argon time projection chambers in exploring interactions of WIMPs in the GeV=c 2 mass range. The technique, based on the detection of the ionization signal amplified via electroluminescence in the gas phase, allows us to explore recoil energies down to the sub-keV range. We report here on the DarkSide-50 measurement of the ionization yield of electronic recoils down to ∼180 eV er , exploiting 37 Ar and 39 Ar decays, and extrapolated to a few ionization electrons with the Thomas-Imel box model. Moreover, we present a model-dependent determination of the ionization response to nuclear recoils down to ∼500 eV nr , the lowest ever achieved in liquid argon, using in situ neutron calibration sources and external datasets from neutron beam experiments.

Search for dark matter-nucleon interactions via Migdal effect with DarkSide-50

“…We use the ionization signal S2 since it has significantly lower detection threshold than S1 thanks to the gain of the electroluminescence. The region of interest (ROI) is defined as where the ionization response is calibrated [39] and backgrounds are well-understood. We characterize the strength of the ionization signal by the number of electrons that are extracted into the gas-region at the top of the TPC.…”

“…The major sources of background events in the ROI considered in the background calculation are 39 Ar and 85 Kr decays occurring in the LAr bulk (internal background) and γ-rays and X-rays from radioactive contaminants in the PMTs and stainlesssteel cryostat (external background). Backgrounds originating from radiogenic and cosmogenic neutrons, as well as coherent elastic neutrino-nucleus scattering from solar and atmospheric neutrinos, are negligible in comparison.…”

“…The calibration of the detector and its response to ER and NR energy deposits has been performed in [39]. The ionization response to electronic recoils has been measured down to 180 eV er and a fit to the data with a function of the Thomas-Imel box model form allows an extrapolation down to O(100 eV).…”

“…This is the lowest threshold ever reached in liquid argon and corresponds to 3 ionization electrons. The ER and NR ionization models are constrained by fitting the 241 Am 9 Be and 241 Am 13 C neutron sources data, βdecay data of 39 Ar, and electron captures of 37 Ar obtained during the DS-50 calibration campaign, and by external datasets from the SCENE [44], ARIS [45] and Joshi et al [46] experiments. Details can be found in [39].…”

“…Exploiting data from the full exposure of the DS-50 experiment, we performed a search for LDM by analysing the ionization signals induced by DM particles scattering off nuclei, enhanced by the Migdal effect. The Migdal detection channel, together with the new calibration [39], data selection, and background model [40], improves significantly the sensitivity of DS-50 to spin-independent DM-nucleon interactions for sub-GeV masses. This analysis sets the world best limit on the spinindependent DM-nucleon cross section for masses below 3.6 GeV/c 2 and down to 40 MeV/c 2 .…”