Search for low-mass dark matter WIMPs with 12 ton-day exposure of DarkSide-50

We report on the search for dark matter weakly interacting massive particles (WIMPs) in the mass range below $10\text{ }\text{ }\mathrm{GeV}/{\mathrm{c}}^{2}$, from the analysis of the entire dataset acquired with a low-radioactivity argon target by the DarkSide-50 experiment at Laboratori Nazionali del Gran Sasso. The new analysis benefits from more accurate calibration of the detector response, improved background model, and better determination of systematic uncertainties, allowing us to accurately model the background rate and spectra down to $0.06\text{ }\text{ }{\mathrm{keV}}_{er}$. A 90% C.L. exclusion limit for the spin-independent cross section of $3\text{ }\text{ }\mathrm{GeV}/{\mathrm{c}}^{2}$ mass WIMP on nucleons is set at $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}43}\text{ }\text{ }{\mathrm{cm}}^{2}$, about a factor 10 better than the previous DarkSide-50 limit. This analysis extends the exclusion region for spin-independent dark matter interactions below the current experimental constraints in the [1.2, 3.6] $\mathrm{GeV}/{\mathrm{c}}^{2}$ WIMP mass range.