Phys. Rev. Lett. 123, 241803 (2019) - Search for Light Dark Matter Interactions Enhanced by the Migdal Effect or Bremsstrahlung in XENON1T

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Search for Light Dark Matter Interactions Enhanced by the Migdal Effect or Bremsstrahlung in XENON1T

E. Aprile et al. (XENON Collaboration)

Phys. Rev. Lett. 123, 241803 – Published 13 December 2019

Abstract

Direct dark matter detection experiments based on a liquid xenon target are leading the search for dark matter particles with masses above $\sim 5 GeV / c^{2}$ , but have limited sensitivity to lighter masses because of the small momentum transfer in dark matter-nucleus elastic scattering. However, there is an irreducible contribution from inelastic processes accompanying the elastic scattering, which leads to the excitation and ionization of the recoiling atom (the Migdal effect) or the emission of a bremsstrahlung photon. In this Letter, we report on a probe of low-mass dark matter with masses down to about $85 MeV / c^{2}$ by looking for electronic recoils induced by the Migdal effect and bremsstrahlung using data from the XENON1T experiment. Besides the approach of detecting both scintillation and ionization signals, we exploit an approach that uses ionization signals only, which allows for a lower detection threshold. This analysis significantly enhances the sensitivity of XENON1T to light dark matter previously beyond its reach.

Received 31 July 2019
Revised 15 October 2019
Corrected 16 October 2020

DOI:https://doi.org/10.1103/PhysRevLett.123.241803

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)

Dark matter Particle dark matter

Weakly interacting massive particles

Dark matter detectors Time-projection chambers

Particles & FieldsGravitation, Cosmology & Astrophysics

Corrections

16 October 2020

Correction: The second sentence of the caption to Figure 4 contained an error and has been fixed.

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Vol. 123, Iss. 24 — 13 December 2019

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Figure 1
Illustration of the ER signal production from BREM (green) and MIGD processes (pink) after elastic scattering between DM ( $χ$ ) and a xenon nucleus. The electrons illustrated in pink represent those involved in ionization, deexcitation, and Auger electron emission during a MIGD process.
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Figure 2
Median effective exposures of ER signals after event selections as a function of recoil energy for the $S 1 − S 2$ data (black line) and $S 2$ -only data (red line). The 68% credible regions of the effective exposures are also shown as the shaded regions. The expected event rate of DM-nucleus scattering from MIGD (BREM) for DM masses of 0.1 and $1.0 GeV / c^{2}$ are overlaid as well, in magenta (green) dashed and solid lines, respectively, assuming a spin-independent DM-nucleon interaction cross section of $10^{- 35} {cm}^{2}$ .
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Figure 3
Comparison of $c S 1$ and $c S 2_{b}$ spectra between the $S 1 − S 2$ data and the signal response model [17]. In upper panel (I), the distribution of the $S 1 − S 2$ data in ( $c S 2_{b}$ , $c S 1$ ) space is shown as light blue dots, along with the best-fit ER background model (black shaded region). The contours containing 90% of the expected signals from MIGD for 0.3, 0.5, and $1 GeV / c^{2}$ DM are shown in red dotted, dashed, and solid lines, respectively. Gray lines show isoenergy contours in ER energy. The events having lower $c S 2_{b}$ than what we expect for ER are mostly surface backgrounds [8], which have minimal impact to the results of this study. The lower panel (II) shows the projected $c S 1$ distribution of the $S 1 − S 2$ data, where $c S 2_{b}$ is within the $2 σ$ contour of ER model shown in (I). For comparison, the 68% credible region of $c S 1$ distribution from ER background model (blue shadow) is shown, which is mainly attributed to the systematic uncertainties of the model. The $c S 1$ distributions of the expected signals from MIGD for 0.3, 0.5, and $1 GeV / c^{2}$ DM with assumed spin-independent DM-nucleon cross sections of $2 \times 10^{- 28}$ , $10^{- 36}$ , and $10^{- 38} {cm}^{2}$ , respectively, are shown as well. The vertical dashed lines indicate the region of interest (3–70 PE). The inset, panel (III), shows the $c S 2_{b}$ distribution, with $c S 1$ in (3,10) PE, compared with the 68% credible region of the $c S 2_{b}$ spectra from the ER background model (blue shadow).
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Figure 4
Observed $S 2$ spectra for the $S 2$ -only data after the optimized selection described in Ref. [21]. The expected spectra of ER signals induced by MIGD for DM with mass of 0.1, 0.5, and $1.0 GeV / c^{2}$ are shown in green, blue, and red solid lines, respectively, assuming the spin-independent DM-nucleon interaction cross section of $1.2 \times 10^{- 37}$ , $1.5 \times 10^{- 39}$ , and $2.0 \times 10^{- 39} {c m}^{2}$ for 0.1, 0.5, and $1.0 GeV / c^{2}$ DM, respectively. The gray shaded region shows the conservative background model used in analysis of $S 2$ -only data. The arrows indicate the $S 2$ ROIs that are later used in inference for the three DM signals mentioned above. The $S 2$ threshold used for the $S 2$ -only data is denoted in the dashed black line.
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Figure 5
Limits on the SI (upper panel), SD proton-only (middle panel), and SD neutron-only (lower panel) DM-nucleon interaction cross sections at 90% C.L. using signal models from MIGD and BREM in the XENON1T experiment with the $S 1 − S 2$ data (blue contours and lines) and $S 2$ -only data (black contours and lines). The solid and dashed (dotted) lines represent the lower boundaries (also referred to as upper limits) and MIGD (BREM) upper boundaries of the excluded parameter regions. Green and yellow shaded regions give the $1 σ$ and $2 σ$ sensitivity contours for upper limits derived using the $S 1 − S 2$ data, respectively. The upper limits on the SI DM-nucleon interaction cross sections from LUX [35], EDELWEISS [30], CDEX [36], CRESST-III [37], NEWS-G [38], CDMSLite-II [39], and DarkSide-50 [40], and upper limits on the SD DM-nucleon interaction cross sections from CRESST [37, 41] and CDMSLite [42] are also shown. Note that the limits derived using the $S 1 − S 2$ and $S 2$ -only data are inferred using unbinned profile likelihood method [16] and simple Poisson statistics with the optimized event selection [21], respectively. The sensitivity contours for the $S 2$ -only data are not given since the background models used in the $S 2$ -only data are conservative [21].
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Figure 6
Limits on the SI-LM DM-nucleon interaction cross sections at 90% C.L. using signal models from MIGD and BREM in the XENON1T experiment with the $S 1 − S 2$ data (blue contours and lines) and $S 2$ -only data (black contours and lines). The description is the same as in Fig. 5. The upper limits on the SI DM-nucleon interaction cross sections from LUX [35] and XENON1T S2-only (elastic NR results) [21] are also shown.
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Physical Review Letters

Search for Light Dark Matter Interactions Enhanced by the Migdal Effect or Bremsstrahlung in XENON1T

E. Aprile et al. (XENON Collaboration)

Phys. Rev. Lett. 123, 241803 – Published 13 December 2019

Abstract

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