We present the results of a search for dark matter weakly interacting massive particles (WIMPs) in the mass range below $20\mathrm{GeV}/c^2$ using a target of low-radioactivity argon with a 6786.0 kg d exposure. The data were obtained using the DarkSide-50 apparatus at Laboratori Nazionali del Gran Sasso. The analysis is based on the ionization signal, for which the DarkSide-50 time projection chamber is fully efficient at $0.1\mathrm{keV}\text{ee}$. The observed rate in the detector at $0.5\mathrm{keV}\text{ee}$ is about $1.5\mathrm{event}/\mathrm{keV}\text{ee}/\mathrm{kg}/\mathrm{d}$ and is almost entirely accounted for by known background sources. We obtain a 90% C.L. exclusion limit above $1.8\mathrm{GeV}/c^2$ for the spin-independent cross section of dark matter WIMPs on nucleons, extending the exclusion region for dark matter below previous limits in the range $1.8–6\mathrm{GeV}/c^2$.

This report provides an extensive review of the experimental programme of direct detection searches of particle dark matter. It focuses mostly on European efforts, both current and planned, but does it within a broader context of a worldwide activity in the field. It aims at identifying the virtues, opportunities and challenges associated with the different experimental approaches and search techniques. It presents scientific and technological synergies, both existing and emerging, with some other areas of particle physics, notably collider and neutrino programmes, and beyond. It addresses the issue of infrastructure in light of the growing needs and challenges of the different experimental searches. Finally, the report makes a number of recommendations from the perspective of a long-term future of the field. They are introduced, along with some justification, in the opening overview and recommendations section and are next summarised at the end of the report. Overall, we recommend that the direct search for dark matter particle interactions with a detector target should be given top priority in astroparticle physics, and in all particle physics, and beyond, as a positive measurement will provide the most unambiguous confirmation of the particle nature of dark matter in the Universe.

We present new constraints on sub-GeV dark-matter particles scattering off electrons based on 6780.0 kg d of data collected with the DarkSide-50 dual-phase argon time projection chamber. This analysis uses electroluminescence signals due to ionized electrons extracted from the liquid argon target. The detector has a very high trigger probability for these signals, allowing for an analysis threshold of three extracted electrons, or approximately 0.05 keVee. We calculate the expected recoil spectra for dark matter-electron scattering in argon and, under the assumption of momentum-independent scattering, improve upon existing limits from XENON10 for dark-matter particles with masses between 30 and $100\mathrm{MeV}/c^2$.

The DarkSide-50 direct-detection dark matter experiment is a dual-phase argon time projection chamber operating at Laboratori Nazionali del Gran Sasso. This paper reports on the blind analysis of a $(16660\pm 270)\mathrm{kg}\mathrm{d}$ exposure using a target of low-radioactivity argon extracted from underground sources. We find no events in the dark matter selection box and set a 90% C.L. upper limit on the dark matter–nucleon spin-independent cross section of $1.14\times {10}^{-44}{\mathrm{cm}}^2$ ($3.78\times {10}^{-44}{\mathrm{cm}}^2$, $3.43\times {10}^{-43}{\mathrm{cm}}^2$) for a WIMP mass of $100\mathrm{GeV}/c^2$ ($1\mathrm{TeV}/c^2$, $10\mathrm{TeV}/c^2$).