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  • Open Access

Search for inelastic WIMP-iodine scattering with COSINE-100

G. Adhikari et al. (COSINE-100 Collaboration)
Phys. Rev. D 108, 092006 – Published 16 November 2023
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  • INTRODUCTION
  • EXPERIMENT
  • WIMP−I127 INELASTIC SCATTERING SIGNALS
  • DATA ANALYSIS
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    We report the results of a search for inelastic scattering of weakly interacting massive particles (WIMPs) off I127 nuclei using NaI(Tl) crystals with a data exposure of 97.7  kg·years from the COSINE-100 experiment. The signature of inelastic WIMP−I127 scattering is a nuclear recoil accompanied by a 57.6 keV γ-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 1.2×10−37  cm2 at the WIMP mass 500  GeV/c2.

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    • Received 19 July 2023
    • Accepted 27 October 2023

    DOI:https://doi.org/10.1103/PhysRevD.108.092006

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Particle dark matter
    Particles & Fields

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    Vol. 108, Iss. 9 — 1 November 2023

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    Images

    • Figure 1
      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.

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    • Figure 2
      Figure 2

      The expected energy spectra for WIMP−I127 inelastic scattering in the COSINE-100 detector are shown for WIMP masses of 50, 500, and 5000  GeV/c2. The spectra include the nuclear recoil energy and the deexcitation energy.

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    • Figure 3
      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 Pb210 (red dashed line), I129 (green dotted-dashed line), and other components (black dotted line) are indicated. In the 50–70 keVee energy region, Pb210 and I129 are the dominant background components.

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    • Figure 4
      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  GeV/c2 WIMP mass signal. The expected signal shape of the 500  GeV/c2 WIMP mass is a red solid line, assuming WIMP-proton spin-dependent cross sections of 3.6×10−36  cm2, 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  GeV/c2. 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 1σ and 2σ confidence intervals, respectively.

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    • Figure 5
      Figure 5

      The observed 90% confidence level exclusion limits on the WIMP-proton spin-dependent cross section of WIMP−I127 inelastic interactions from the 1.7 year COSINE-100 data are shown, along with the ±1σ and ±2σ 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 WIMP−I127 inelastic scattering hypothesis.

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