Projected WIMP Sensitivity of the XENONnT Dark Matter Experiment (2007.08796v2)

Abstract: XENONnT is a dark matter direct detection experiment, utilizing 5.9 t of instrumented liquid xenon, located at the INFN Laboratori Nazionali del Gran Sasso. In this work, we predict the experimental background and project the sensitivity of XENONnT to the detection of weakly interacting massive particles (WIMPs). The expected average differential background rate in the energy region of interest, corresponding to (1, 13) keV and (4, 50) keV for electronic and nuclear recoils, amounts to $12.3 \pm 0.6$ (keV t y)$^{-1}$ and $(2.2\pm 0.5)\times 10^{-3}$ (keV t y)$^{-1}$, respectively, in a 4 t fiducial mass. We compute unified confidence intervals using the profile construction method, in order to ensure proper coverage. With the exposure goal of 20 t$\,$y, the expected sensitivity to spin-independent WIMP-nucleon interactions reaches a cross-section of $1.4\times10^{-48}$ cm$^2$ for a 50 GeV/c$^2$ mass WIMP at 90% confidence level, more than one order of magnitude beyond the current best limit, set by XENON1T. In addition, we show that for a 50 GeV/c$^2$ WIMP with cross-sections above $2.6\times10^{-48}$ cm$^2$ ($5.0\times10^{-48}$ cm$^2$) the median XENONnT discovery significance exceeds 3$\sigma$ (5$\sigma$). The expected sensitivity to the spin-dependent WIMP coupling to neutrons (protons) reaches $2.2\times10^{-43}$ cm$^2$ ($6.0\times10^{-42}$ cm$^2$).

• The paper presents a detailed sensitivity projection showing that XENONnT can achieve spin-independent cross-sections as low as 1.4×10⁻⁴⁸ cm² for 50 GeV/c² WIMPs with a 20 tonne-year exposure.
• The paper employs a sophisticated Geant4-based simulation framework to model detector responses and accurately estimate backgrounds from intrinsic radioactivity, radon, krypton, and neutrons.
• The paper demonstrates that the advanced dual-phase TPC and innovative neutron veto system substantially improve background rejection, marking a significant advancement in dark matter detection.

Projected WIMP Sensitivity of the XENONnT Dark Matter Experiment

The paper presents an in-depth paper on the projected sensitivity of the XENONnT experiment, a significant advancement in dark matter direct detection endeavors, particularly focusing on the search for weakly interacting massive particles (WIMPs). XENONnT is a follow-up to the XENON1T experiment and promises substantial improvements in detecting WIMPs due to its enhanced xenon mass and refined background reduction techniques.

Experiment and Detector Design

XENONnT is constructed to utilize 5.9 tonnes of instrumented liquid xenon, located at the INFN Laboratori Nazionali del Gran Sasso. The experimental setup includes a dual-phase liquid-gas xenon time projection chamber (TPC), similar to XENON1T but with several enhancements such as increased detector mass and advanced purification techniques. This design ensures high sensitivity to both electronic and nuclear recoils, which are critical for WIMP detection.

The experiment's primary innovation includes a dedicated neutron veto system with gadolinium-loaded water that substantially reduces neutron-related backgrounds by detecting coincident events within the xenon and surrounding water. With the targeted liquid xenon mass of 8.4 tonnes contained in a vacuum-insulated cryostat, XENONnT can achieve unprecedented levels of background suppression, allowing for deeper explorations into WIMP-nucleon interactions.

Simulation Framework and Background Estimation

The paper meticulously details the simulation framework used to estimate the experiment's sensitivity to WIMPs. This simulation employs the Geant4 toolkit, adapted to model the XENONnT detector's response to both signal and background event types. It accounts for various detector effects such as light collection efficiency, electron extraction efficiency, and signal processing, ensuring accurate background modeling and realistic sensitivity projections.

The expected background rates from several sources are estimated, including intrinsic detector radioactivity, radon contamination, $^{85}$Kr, and solar neutrinos. Achieving a radon activity goal of \SI{1}{\micro\becquerel/\kilogram} and a krypton concentration of 0.1 ppt is pivotal for maintaining a low electronic recoil background at approximately 12.3 (keV.t.y)^{-1. The neutron background from materials and cosmogenic sources is anticipated to contribute a nuclear recoil background rate of about $(2.2\pm 0.5)\times 10^{-3}$ (keV.t.y)^{-1 in a four-tonne fiducial volume.

Sensitivity to WIMP Interactions

Utilizing a sophisticated statistical model, the projected sensitivity of XENONnT to spin-independent and spin-dependent WIMP-nucleon couplings is thoroughly examined. For a 50 GeV/c$^2$ WIMP, the sensitivity could achieve cross-sections as low as $1.4\times10^{-48}$ cm$^2$, benefiting from the significant reduction in background rates and enhanced exposure of 20 tonne-years. This projection surpasses the XENON1T limits by more than an order of magnitude.

Additionally, the paper projects XENONnT’s sensitivity to spin-dependent WIMP-neutron interactions at $2.2\times10^{-43}$ cm$^2$ and spin-dependent WIMP-proton interactions at $6.0\times10^{-42}$ cm$^2$, which represents notable progress in these challenging detection channels.

Implications and Future Directions

The implications of this paper are substantial for the field of dark matter research. By extending the sensitivity limits significantly beyond current boundaries, XENONnT positions itself as a leading experiment in probing the WIMP parameter space more comprehensively. The methodology exemplified by XENONnT provides a template for future dark matter detection strategies, highlighting the importance of significant mass increases and enhanced background rejection.

Future developments in this domain may involve scaling the experiment's mass to even larger values or innovating further in background rejection techniques. Assuming successful implementation and operationalization, XENONnT will likely lead to pivotal theoretical advancements, potentially necessitating new models or adaptations to the prevailing dark matter paradigms, particularly if significant detections are achieved or further null results are obtained.

Overall, the XENONnT experiment embodies a significant stride forward in both scale and sophistication, promising rich insights into dark matter properties and the possible detection of WIMPs. This research not only enhances our comprehension of dark matter but also deepens the understanding of the structure and composition of the universe.