The XENON100 Dark Matter Experiment

Abstract: The XENON100 dark matter experiment uses liquid xenon (LXe) in a time projection chamber (TPC) to search for Xe nuclear recoils resulting from the scattering of dark matter Weakly Interacting Massive Particles (WIMPs). In this paper we present a detailed description of the detector design and present performance results, as established during the commissioning phase and during the first science runs. The active target of XENON100 contains 62 kg of LXe, surrounded by an LXe veto of 99 kg, both instrumented with photomultiplier tubes (PMTs) operating inside the liquid or in Xe gas. The LXe target and veto are contained in a low-radioactivity stainless steel vessel, embedded in a passive radiation shield. The experiment is installed underground at the Laboratori Nazionali del Gran Sasso (LNGS), Italy and has recently published results from a 100 live-days dark matter search. The ultimate design goal of XENON100 is to achieve a spin-independent WIMP-nucleon scattering cross section sensitivity of \sigma = 2x10^-45 cm^2 for a 100 GeV/c^2 WIMP.

• The paper demonstrates a novel liquid xenon TPC design that combines scintillation and ionization detection with a 99.5% rejection efficiency for electronic recoils.
• It implements rigorous xenon purity control, low-radioactivity materials, and an LXe veto shield to effectively minimize background interference.
• The study achieves a sensitivity of 2×10⁻⁴⁵ cm² for 100 GeV/c² WIMPs, establishing the most stringent limits on dark matter interaction rates.

An Overview of the XENON100 Dark Matter Experiment

The study reported in "The XENON100 Dark Matter Experiment" presents a comprehensive investigation into dark matter detection using a liquid xenon (LXe) time projection chamber (TPC). It primarily focuses on searching for nuclear recoils from Weakly Interacting Massive Particles (WIMPs), a leading dark matter candidate, through direct detection methods. This paper details the design, operational principles, and initial performance results of the XENON100 experiment, installed underground at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy.

Detector Design and Operational Insights

XENON100 employs a target core of 62 kg of LXe, surrounded by a 99 kg LXe veto shield for background suppression, within a radiation-shielded stainless steel vessel. Key to the experiment is its ability to detect both the scintillation and ionization signals resulting from particle interactions within the LXe, facilitated by a two-phase TPC configuration. Photomultiplier tubes (PMTs) are employed to detect the xenon scintillation light, hence enabling both spatial event localization—critical for identifying nuclear recoils—and background discrimination with an impressive rejection efficiency of 99.5% for electronic recoils.

The paper reveals significant technical undertakings, including precise control of the xenon purity and LXe level, the use of materials with low intrinsic radioactivity, and the implementation of a sophisticated cryogenic and purification system designed for stringent control of contaminating krypton, a known radioactive background source in xenon.

Results and Numerical Strengths

The experiment's initial results over 100 live-days of data reveal a background rate 100 times lower than its predecessor, XENON10. This achievement is attributed to the novel LXe veto design, improved materials selection, and enhanced shielding. XENON100 aims for a sensitivity to spin-independent WIMP-nucleon cross sections as low as $2 \times 10^{-45}$ cm$^2$ for a 100 GeV/c$^2$ WIMP. These results currently hold the record for the most stringent upper limits on WIMP-nucleon scattering for a wide range of WIMP masses, highlighting its preeminence in the field of dark matter research.

Implications and Future Directions

The implications of this work are multifaceted. On a practical level, XENON100 not only pushes technological boundaries within particle detection but also establishes protocols and architecture that could inform future endeavors such as the forthcoming XENON1T project. Theoretically, the weak interaction cross section sensitivities attained approach those predicted by some supersymmetric models, thus placing significant constraints on dark matter parameter space and compelling revisions of theoretical models should anticipated signals continue to elude detection.

Future developments in the XENON project are expected to build upon XENON100's success, focusing on detector scaling to more massive targets with improved purity and lower thresholds, which remain crucial for validating or constraining the presence of dark matter particles as physics models evolve. The continuance of such investigations promises to elucidate further insights into the fundamental constituents of the universe, continuing to challenge and refine the current paradigms of particle physics and cosmology.