The XENON1T Dark Matter Experiment (1708.07051v1)

Abstract: The XENON1T experiment at the Laboratori Nazionali del Gran Sasso (LNGS) is the first WIMP dark matter detector operating with a liquid xenon target mass above the ton-scale. Out of its 3.2t liquid xenon inventory, 2.0t constitute the active target of the dual-phase time projection chamber. The scintillation and ionization signals from particle interactions are detected with low-background photomultipliers. This article describes the XENON1T instrument and its subsystems as well as strategies to achieve an unprecedented low background level. First results on the detector response and the performance of the subsystems are also presented.

• The paper demonstrates unprecedented sensitivity in detecting spin-independent WIMP-nucleon interactions with limits as low as 1.6×10⁻⁴⁷ cm².
• It details the dual-phase TPC design utilizing a 3.2-ton liquid xenon target and 248 low-radioactivity photomultipliers for precise 3D event reconstruction.
• Experimental results reveal record-low electronic recoil backgrounds (70 ± 9 counts) and set the most stringent dark matter exclusion limits to date.

The XENON1T Dark Matter Experiment

The XENON1T experiment, situated at the Laboratori Nazionali del Gran Sasso (LNGS), represents a significant advancement in the search for weakly interacting massive particles (WIMPs), which are compelling candidates for dark matter. The dual-phase time projection chamber (TPC) of XENON1T uses a 3.2-ton liquid xenon (LXe) target to detect WIMPs via their potential scattering off xenon nuclei. With an active target mass of 2.0 tons, this experiment aims at probing spin-independent WIMP-nucleon interactions with an unprecedented sensitivity, targeting cross-sections as low as $1.6 \times 10^{-47} \text{ cm}^2$ for a WIMP mass of 50 GeV/$c^2$.

Instrumentation and Methodology

The TPC of XENON1T is designed to maximize both light collection and charge extraction efficiency through detailed electrostatic and radiopurity considerations. The dual-phase TPC allows for simultaneous detection of scintillation (S1) and ionization (S2) signals, providing a mechanism for 3-dimensional positioning of particle interactions using a combination of S2 signal patterns and S1-S2 delay times. The experiment employs 248 Hamamatsu R11410-21 photomultipliers for S1 and S2 signal readouts, emphasizing low radioactivity and high quantum efficiency to enhance detection capabilities.

To minimize backgrounds, the instrument is housed within a large water tank that functions both as passive shielding and as an active muon veto via Cherenkov radiation detection. The XENON collaboration has implemented stringent material selection processes and developed a cryogenic distillation system to reduce krypton contamination to $\text{nat}$Kr/Xe levels below 0.2 ppt, thereby addressing non-negligible sources of electronic recoil backgrounds.

First Experimental Results

During its first science run, XENON1T reached an electronic recoil background rate of $70 \pm 9$ (t y keV)$^{-1}$. This corresponds to the lowest achieved background level in a dark matter experiment to date, aligning closely with pre-experiment predictions and confirming a radon background of approximately 10 µBq/kg. Analysis of the science run yielded the most stringent constraints on spin-independent WIMP-nucleon cross-sections for WIMP masses above 10 GeV/$c^2$, with the lowest exclusion limit at $7.7 \times 10^{-47} \text{ cm}^2$.

Future Directions and Implications

The configuration and successful commissioning of XENON1T pave the way for future advancements in direct dark matter searches. The planned XENONnT upgrade aims to enhance sensitivity by an order of magnitude beyond XENON1T, utilizing a larger LXe target while leveraging the existing infrastructure and insights gained from current operations. Furthermore, the experiment's results contribute to refining models of dark matter particle interactions, consistent with a world-wide effort to test WIMP hypotheses within the framework of beyond the Standard Model (BSM) physics.

With advancements in background reduction and increased sensitivity, the XENON1T experiment sets a new benchmark for dark matter detection. Continued investigation in this domain is expected to either detect a WIMP signal or constrain viable dark matter parameter spaces further, potentially guiding future theoretical and experimental pursuits in understanding the nature of dark matter.