First Probe of Sub-GeV Dark Matter beyond the Cosmological Expectation with the COHERENT CsI Detector at the SNS

The COHERENT Collaboration searched for scalar dark matter particles produced at the Spallation Neutron Source with masses between 1 and $220\mathrm{MeV}/{\mathrm{c}}^2$ using a CsI[Na] scintillation detector sensitive to nuclear recoils above $9{\mathrm{keV}}_{\mathrm{nr}}$. No evidence for dark matter is found and we thus place limits on allowed parameter space. With this low-threshold detector, we are sensitive to coherent elastic scattering between dark matter and nuclei. The cross section for this process is orders of magnitude higher than for other processes historically used for accelerator-based direct-detection searches so that our small, 14.6 kg detector significantly improves on past constraints. At peak sensitivity, we reject the flux consistent with the cosmologically observed dark-matter concentration for all coupling constants ${\alpha }_D<0.64$, assuming a scalar dark-matter particle. We also calculate the sensitivity of future COHERENT detectors to dark-matter signals which will ambitiously test multiple dark-matter spin scenarios.

Figure 1

The expected spectrum of coherent DM scattering signal in CsI from both galactic and SNS-produced DM for a mass of $25\mathrm{MeV}/c^2$, near our optimal sensitivity. Though the rate for galactic DM is higher, the recoil energies are far below threshold while we select 26% of DM produced at the SNS.

Figure 2

The observed, SSBkg-subtracted recoil spectra in the DM timing ROI (top) and the background control sample (bottom) compared to the best-fit prediction with no DM. The expected DM distribution at the 90% limit is stacked on the SM prediction for $m_{\chi }=25\mathrm{MeV}/c^2$.

Figure 3

Constraint of DM parameter space for COHERENT CsI data compared to other experimental data, assuming ${\alpha }_D=0.5$ (top). The parameters that give the observed relic abundance for scalar DM are also shown. We also show constraints on the DM-nucleon scattering cross section, averaged over the DM halo velocity distribution compared to constraints of the DM-quark coupling of light DM (bottom).

Figure 4

The values of ${\alpha }_D$ for which we can reject the cosmologically observed DM concentration as a function of DM mass compared to constraints shown in Fig. 3. All parameter space below the contour is excluded. We include scenarios where DM is scalar, a Majorana fermion, or a pseudo-Dirac fermion. A dashed line at ${\alpha }_D=0.5$, the value assumed in Fig. 3, is also drawn.