Low-Mass Dark Matter Search with the DarkSide-50 Experiment

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 $1.8–6\mathrm{GeV}/c^2$.

Figure 1

Acceptance of the basic cuts described in the text as a function of the number of PE in the pulses.

Figure 2

(Filled symbols) DarkSide-50 (DS-50) experimental $N_{e^{-}}$ spectra obtained during the last 100 days of data taking and (open symbols) during the short period where the getter was off for maintenance. Both the single- and double-electron peaks are seen to be strongly enhanced in the absence of argon purification. The smooth black curve shows a weighted sum of the G4DS one-, two-, and three-electron responses.

Figure 3

Spectrum showing cosmogenic ${}^{37}\mathrm{Ar}$ contributions and their decay as discussed in the text. (Black) First 100 days of present exposure. (Dark blue) Last 500 days. (Red and cyan, respectively) The contributions to the dark blue spectrum from events with only an $S2$ pulse and from events with a single $S1$ and a single $S2$ pulse. (Inset) Normalized difference of black minus dark blue, showing the two peaks from ${}^{37}\mathrm{Ar}$ decay.

Figure 4

Data and MC fit of the $N_{e^{-}}$ spectrum for the ${}^{241}\mathrm{AmBe}$ run in DarkSide-50. The dashed line shows the lower edge of fit range.

Figure 5

Data and MC fit of the $N_{e^{-}}$ spectrum from the ${}^{241}\mathrm{Am}$ ${}^{13}\mathrm{C}$ run in DarkSide-50. The dashed line shows the lower edge of fit range.

Figure 6

The measured ionization yield $Q_y$ for nuclear recoils in LAr as a function of the reduced energy parameter $\epsilon$. Also shown is the Bezrukov model fit to the ${}^{241}\mathrm{AmBe}$ and ${}^{241}\mathrm{Am}$ ${}^{13}\mathrm{C}$ data (see text).

Figure 7

The DarkSide-50 $N_{e^{-}}$ spectra at low recoil energy from the analysis of the last 500 days of exposure compared with a G4DS simulation of the background components from known radioactive contaminants. Also shown are the spectra expected for recoils induced by dark matter particles of masses 2.5, 5, and $10\mathrm{GeV}/c^2$ with a cross section per nucleon of ${10}^{-40}{\mathrm{cm}}^2$ convolved with the no energy quenching fluctuation model and detector resolution. The $y$-axis scales on the right-hand side are approximate event rates normalized at $N_{e^{-}}=10e^{-}$.

Figure 8

Ninety percent upper limits on spin-independent DM-nucleon cross sections from DarkSide-50 in the range above $1.8\mathrm{GeV}/c^2$. See the text for additional details.