SENSEI: Direct-Detection Results on sub-GeV Dark Matter from a New Skipper CCD

We present the first direct-detection search for sub-GeV dark matter using a new $\sim 2$-gram high-resistivity Skipper CCD from a dedicated fabrication batch that was optimized for dark matter searches. Using 24 days of data acquired in the MINOS cavern at the Fermi National Accelerator Laboratory, we measure the lowest rates in silicon detectors of events containing one, two, three, or four electrons, and achieve world-leading sensitivity for a large range of sub-GeV dark matter masses. Data taken with different thicknesses of the detector shield suggest a correlation between the rate of high-energy tracks and the rate of single-electron events previously classified as “dark current.” We detail key characteristics of the new Skipper CCDs, which augur well for the planned construction of the $\sim 100$-gram SENSEI experiment at SNOLAB.

Figure 1

A copper-Kapton flex circuit is laminated to a silicon-aluminum pitch adapter that is glued and wirebonded to the Skipper CCD (top right); this is placed in a copper tray (top left), where a copper leaf-spring (bottom left) maintains constant pressure for consistent thermal contact between the CCD and the tray when closed inside the module (shown transparent, bottom right).

Figure 2

Single-electron event rate, $R_{1e^{-}}$ (after subtracting the spurious charge) versus the rate of events with energies 500 eV to 10 keV with extra shielding (red) and without extra shielding for one image with the amplifier off during exposure (green) and when combined with three images with the amplifier on during exposure (black).

Figure 3

The pixel charge spectra (after selection cuts) used for the $1e^{-}$ and $2e^{-}$ analyses. A double-Gaussian fit is shown for the spectrum with $1e^{-}$ cuts. There are no $3e^{-}$ or $4e^{-}$ events.

Figure 4

90% C.L. constraints (cyan solid line) on: DM-$e^{-}$ cross section, ${\overline{\sigma }}_e$, versus DM mass, $m_{\chi }$, for two DM form factors, $F_{\mathrm{DM}}(q)=1$ (top left) and $F_{\mathrm{DM}}(q)=(\alpha m_e/q{)}^2$ (top right); DM-nucleus cross section, ${\overline{\sigma }}_n$, for a light mediator (bottom left); and the kinetic-mixing parameter, $\epsilon$, versus the dark-photon mass, $m_{A^{\prime }}$, for dark-photon-DM absorption (bottom right). Constraints are shown on DM-$e^{-}$ scattering also from the SENSEI prototype [14, 15], $\mathrm{XENON}10/100$ [39], DarkSide-50 [40], EDELWEISS [41], CDMS-HVeV [42], XENON1T [43], DAMIC [44], solar reflection (assuming DM couples only to $e^{-}$) [45]; constraints on DM-nucleus scattering from SENSEI, $\mathrm{XENON}10/100$/1T [12] and LUX [46]; and constraints on absorption from SENSEI [14, 15], DAMIC [44, 47], EDELWEISS [41], $\mathrm{XENON}10/100$, CDMSlite [9], and the Sun [9, 48, 49]. Orange regions are combined benchmark model regions for heavy [2, 5, 50, 51, 52, 53, 54] and light [2, 5, 55, 56] mediators.