Light Dark Matter Search with Ionization Signals in XENON1T

Abstract: We report constraints on light dark matter (DM) models using ionization signals in the XENON1T experiment. We mitigate backgrounds with strong event selections, rather than requiring a scintillation signal, leaving an effective exposure of $(22 \pm 3)$ tonne-days. Above $\sim!0.4\,\mathrm{keV}\mathrm{ee}$, we observe $<1 \, \text{event}/(\text{tonne} \times \text{day} \times \text{keV}\text{ee})$, which is more than one thousand times lower than in similar searches with other detectors. Despite observing a higher rate at lower energies, no DM or CEvNS detection may be claimed because we cannot model all of our backgrounds. We thus exclude new regions in the parameter spaces for DM-nucleus scattering for DM masses $m_\chi$ within $3-6\,\mathrm{GeV}/\mathrm{c}^2$, DM-electron scattering for $m_\chi > 30\,\mathrm{MeV}/\mathrm{c}^2$, and absorption of dark photons and axion-like particles for $m_\chi$ within $0.186 - 1 \, \mathrm{keV}/\mathrm{c}^2$.

• The paper demonstrates an S2-only approach that relaxes scintillation (S1) requirements, extending sensitivity to interactions as low as 0.186 keV.
• The analysis sets new exclusion limits on dark matter interactions, constraining DM-nucleus scatterings between 3-6 GeV/c² and DM-electron scatterings above 30 MeV/c².
• The study’s findings narrow the parameter space for dark photons and axion-like particles, guiding future strategies in low-mass dark matter searches.

Light Dark Matter Search with Ionization Signals in XENON1T

The paper under review presents a comprehensive analysis of light dark matter (DM) detection utilizing ionization signals within the XENON1T experiment. This document represents a significant contribution to the ongoing efforts to constrain the parameter space of lighter DM candidates, expanding the understanding of potential DM interactions with matter. The XENON1T experiment, a key player in DM detection, is housed in a dual-phase xenon time projection chamber (TPC) and is recognized for its ability to achieve the most stringent detection limits on DM-nucleus interactions for masses above 6 GeV/c$^2$.

Methodology Overview

The XENON1T experiment employs a sophisticated detection mechanism involving the use of ionization (S2) signals. By relaxing traditional requirements for scintillation signals (S1), this analysis is able to extend sensitivity to much lower energy interactions, down to 0.186 keV for electronic recoils. This S2-only approach is crucial in probing DM candidates with masses below 6 GeV/c$^2$ and allows for the exploration of interaction models involving dark photons and axion-like particles.

Results and Claims

The study reports significant results:

• For DM-nucleus scatterings, the analysis places new limits on DM masses between 3-6 GeV/c$^2$.
• For DM-electron scatterings, constraints are provided for masses above 30 MeV/c$^2$.
• In addition, for dark photons and axion-like particles, the study restricts the parameter space for masses between 0.186 and 1 keV/c$^2$.

A robust event selection process mitigates background noise, achieving a background event rate of fewer than 1 event per tonne-day-keV$_{ee}$, significantly lower than that reported in similar experimental setups. Despite observing some elevated event rates at the very low energy end, no conclusive DM or coherent elastic neutrino-nucleus scattering (CEvNS) detections are claimed, primarily due to the inability to fully model the observed backgrounds.

Theoretical and Practical Implications

The constraints established in this research carry profound implications for particle physics. The reduction in viable parameter space challenges several theoretical models of DM, particularly those predicting interactions at the energy scales assessed in this study. Practically, the techniques developed may inform future experiment designs, emphasizing the need for precision in background modeling and novel signal discrimination methods.

Speculation on Future Developments

Future exploration of the light DM domain will likely benefit from increased detector sensitivity and larger exposure times, as seen in prospective upgrades such as XENONnT and LZ projects. These advancements are expected to push the boundaries of current detection capabilities further into unexplored low-mass regimes. Furthermore, integration of independent calibration techniques would improve background understanding and signal modeling, potentially leading to the first direct detections of lighter DM candidates.

The XENON1T findings underscore the necessity of adapting detection strategies for low-mass DM searches, an increasingly pertinent area as null results continue to refine the scope of viable DM parameter spaces. The broadened reach of this study exemplifies the evolving landscape of DM research, where subtle shifts in methodological approaches can yield new insights into the dark sector of particle physics.