Search for inelastic WIMP-iodine scattering with COSINE-100

We report the results of a search for inelastic scattering of weakly interacting massive particles (WIMPs) off ${}^{127}\mathrm{I}$ nuclei using NaI(Tl) crystals with a data exposure of $97.7\mathrm{kg}·\mathrm{years}$ from the COSINE-100 experiment. The signature of inelastic $\mathrm{WIMP}\text{−}{}^{127}\mathrm{I}$ scattering is a nuclear recoil accompanied by a 57.6 keV $\gamma$-ray from the prompt deexcitation, producing a more energetic signal compared to the typical WIMP nuclear recoil signal. We found no evidence for this inelastic scattering signature and set a 90% confidence level upper limit on the WIMP-proton spin-dependent, inelastic scattering cross section of $1.2\times {10}^{-37}{\mathrm{cm}}^2$ at the WIMP mass $500\mathrm{GeV}/{\mathrm{c}}^2$.

Figure 1

Schematic of the COSINE-100 detector. The NaI(Tl) detectors are immersed in the 2200 l LAB-LS that is surrounded by layers of copper and lead shielding.

Figure 2

The expected energy spectra for $\mathrm{WIMP}\text{−}{}^{127}\mathrm{I}$ inelastic scattering in the COSINE-100 detector are shown for WIMP masses of 50, 500, and $5000\mathrm{GeV}/{\mathrm{c}}^2$. The spectra include the nuclear recoil energy and the deexcitation energy.

Figure 3

Measured energy spectrum in the ROI of crystal 4 (black points) and its background model (blue solid line) with the 68% (yellow band) and 95% (green band) confidence intervals are presented. The expected contributions to the background from ${}^{210}\mathrm{Pb}$ (red dashed line), ${}^{129}\mathrm{I}$ (green dotted-dashed line), and other components (black dotted line) are indicated. In the 50–70 keVee energy region, ${}^{210}\mathrm{Pb}$ and ${}^{129}\mathrm{I}$ are the dominant background components.

Figure 4

(a) The black-filled circles represent the data points showing the summed energy spectra from the five crystals. The solid blue line indicates the result of the fit with a $500\mathrm{GeV}/{\mathrm{c}}^2$ WIMP mass signal. The expected signal shape of the $500\mathrm{GeV}/{\mathrm{c}}^2$ WIMP mass is a red solid line, assuming WIMP-proton spin-dependent cross sections of $3.6\times {10}^{-36}{\mathrm{cm}}^2$, which is 30 times higher than the 90% confidence level upper limit. (b) It is an example of the posterior probability density function (PDF) and cumulated density function (CDF) for the 1.7 year COSINE-100 data and a WIMP mass of $500\mathrm{GeV}/{\mathrm{c}}^2$. The posterior PDF is scaled so that the maximum value is one. In this PDF, the best fit (i.e., the most probable cross section) points to a null signal. Therefore, we set the 90% confidence level upper limit. The exclusion limit at a 90% confidence level is obtained from the CDF matched with 0.9. The yellow and green areas represent the $1\sigma$ and $2\sigma$ confidence intervals, respectively.

Figure 5

The observed 90% confidence level exclusion limits on the WIMP-proton spin-dependent cross section of $\mathrm{WIMP}\text{−}{}^{127}\mathrm{I}$ inelastic interactions from the 1.7 year COSINE-100 data are shown, along with the $\pm 1\sigma$ and $\pm 2\sigma$ bands for the expected sensitivity under the background-only hypothesis. The limits are compared with a WIMP interpretation of the ELEGANTS-V upper limit of 57.6 keVee event rate for the $\mathrm{WIMP}\text{−}{}^{127}\mathrm{I}$ inelastic scattering hypothesis.