SENSEI: First Direct-Detection Constraints on Sub-GeV Dark Matter from a Surface Run

The Sub-Electron-Noise Skipper CCD Experimental Instrument (SENSEI) uses the recently developed Skipper-CCD technology to search for electron recoils from the interaction of sub-GeV dark matter particles with electrons in silicon. We report first results from a prototype SENSEI detector, which collected 0.019 g day of commissioning data above ground at Fermi National Accelerator Laboratory. These commissioning data are sufficient to set new direct-detection constraints for dark matter particles with masses between $\sim 500\mathrm{keV}$ and 4 MeV. Moreover, since these data were taken on the surface, they disfavor previously allowed strongly interacting dark matter particles with masses between $\sim 500\mathrm{keV}$ and a few hundred MeV. We discuss the implications of these data for several dark matter candidates, including one model proposed to explain the anomalously large 21-cm signal observed by the EDGES Collaboration. SENSEI is the first experiment dedicated to the search for electron recoils from dark matter, and these results demonstrate the power of the Skipper-CCD technology for dark matter searches.

Figure 1

Recorded spectrum after selection cuts for the 0.019 g day of commissioning data. Gaussian fits to the peaks show there are 140,302, 4676, 131, and 1 event(s) with 1, 2, 3, and 4 electrons, respectively. No events are seen for 5–100 electrons. The Gaussian width of the peaks are $\sim 0.14e^{-}$.

Figure 2

The 90% C.L. constraints on the DM-electron scattering cross sections ${\overline{\sigma }}_e$ as a function of DM mass $m_{\chi }$ from a commissioning run above ground at Fermilab using the SENSEI prototype detector. We show different DM form factors, $F_{\mathrm{DM}}(q)=1$, $\alpha m_e/q$, and $(\alpha m_e/q{)}^2$. The purple, blue, green, and red lines correspond to the constraints from the 1-, 2-, 3-, or 4-electron bin, respectively. The black line is the minimum of these. The blue shaded regions are the current constraints from XENON10, XENON100, and DarkSide-50. For large cross sections, the DM is stopped in Earth’s crust (atmosphere) and does not reach the noble-liquid (SENSEI prototype) detectors: the dark-shaded regions (labeled $|g_p|=|g_e|$) show order-of-magnitude estimates of the excluded parameter regions assuming the interaction between DM and ordinary matter is mediated by a heavy dark photon (left), an electric dipole moment (middle), or an ultralight dark photon (right). The light-shaded regions (labeled $g_p=0$) are order-of-magnitude estimates of the 90% C.L. excluded parameter regions assuming a mediator that couples only to electrons.

Figure 3

The 90% C.L. constraint on ${\overline{\sigma }}_e$ versus $m_{\chi }$ for $F_{\mathrm{DM}}(q^2)=(\alpha m_e/q{)}^2$ from a surface run at Fermilab using the SENSEI prototype detector (orange region, bounded below by a solid line and above by a dashed line that is the same as the $|g_p|=|g_e|$ line in the right plot of Fig. 2). We assume that $\chi$ couples to an ultralight dark-photon mediator, and ${\mathrm{\Omega }}_{\chi }=0.01{\mathrm{\Omega }}_{\mathrm{DM}}$, which may explain the 21-cm signal observed by EDGES. Other 90% C.L. constraints are described in the text. The SENSEI surface run disfavors a small region of previously open parameter space for ${\overline{\sigma }}_e\gtrsim {10}^{-25}{\mathrm{cm}}^2$ and $m_{\chi }$ greater than a few hundred MeV.

Figure 4

90% C.L. constraints on dark-photon DM absorption by electrons, in the kinetic mixing ($\epsilon$) versus dark-photon mass ($m_{A^{\prime }}$) parameter space from a surface run at Fermilab using the SENSEI prototype detector (orange). 90% C.L onstraints from XENON10, XENON100, CDMSlite, DAMIC, and a measurement of the Rydberg constant are also shown. The direct-detection constraints are absent at large $\epsilon$, where the $A^{\prime }$ is absorbed in Earth’s crust or atmosphere before reaching the detector. The SENSEI constraint closes a gap in laboratory probes of the high-$\epsilon$ region. We assume the $A^{\prime }$ to be DM, irrespective of any (model-dependent) production process.