[Paper Review] The XENONnT Dark Matter Experiment

XENON Collaboration, Aprile, E.+193 more|arXiv (Cornell University)|Feb 16, 2024

Dark Matter and Cosmic Phenomena3 citations

TL;DR

The XENONnT experiment is a next-generation dark matter search using a liquid xenon time projection chamber, achieving a record-low electronic recoil background of 15.8 ± 1.3 events/(tonne·year·keVee). It demonstrates improved background suppression and stable gain calibration, leading to a 90% confidence upper limit on the WIMP-nucleon scattering cross section of 2.6×10⁻⁴⁷ cm² for a 28 GeV/c² WIMP mass.

ABSTRACT

The multi-staged XENON program at INFN Laboratori Nazionali del Gran Sasso aims to detect dark matter with two-phase liquid xenon time projection chambers of increasing size and sensitivity. The XENONnT experiment is the latest detector in the program, planned to be an upgrade of its predecessor XENON1T. It features an active target of 5.9 tonnes of cryogenic liquid xenon (8.5 tonnes total mass in cryostat). The experiment is expected to extend the sensitivity to WIMP dark matter by more than an order of magnitude compared to XENON1T, thanks to the larger active mass and the significantly reduced background, improved by novel systems such as a radon removal plant and a neutron veto. This article describes the XENONnT experiment and its sub-systems in detail and reports on the detector performance during the first science run.

Motivation & Objective

• To reduce electronic recoil background in liquid xenon dark matter detectors to enhance sensitivity to weakly interacting massive particles (WIMPs).
• To achieve sub-1% stability in light and charge gain calibration for precise energy reconstruction.
• To implement advanced background suppression techniques, including radon reduction and neutron veto with gadolinium-doped water.
• To conduct a high-sensitivity dark matter search with improved detector performance over XENON1T.
• To validate the performance of the XENONnT detector through systematic monitoring of light and charge yields across multiple monoenergetic calibration lines.

Proposed method

• Utilizes a liquid xenon time projection chamber (TPC) with dual-phase detection of scintillation (S1) and ionization (S2) signals.
• Employs a gain calibration method using monoenergetic gamma and electron sources (e.g., 37Ar, 83mKr, 131mXe, 129mXe) to determine S1 and S2 gains.
• Applies the equation $ L_y = \frac{1}{g_1} \left( \frac{cS1}{g_1} + \frac{cS2}{g_2} \right) $, with $ W = 13.7 $ eV, to relate light and charge yields to energy deposition.
• Monitors S1 and S2 yields regularly to ensure gain stability within 1.0% (light) and 1.9% (charge) over time.
• Uses a neutron veto system with gadolinium-doped water (500 ppm Gd-sulphate) to suppress radiogenic neutron backgrounds.
• Applies a background suppression strategy reducing ²²²Rn to below 1 μBq/kg, significantly lowering cosmogenic and radiogenic backgrounds.

Experimental results

Research questions

• RQ1 What is the achievable electronic recoil background level in a liquid xenon TPC after advanced background suppression?
• RQ2 How stable are the light and charge gains over time, and what is the systematic uncertainty in energy reconstruction?
• RQ3 Can the XENONnT detector achieve a factor of 5 reduction in electronic recoil background compared to XENON1T?
• RQ4 What is the resulting sensitivity to WIMP-nucleon scattering cross sections after one year of data taking?
• RQ5 How effective is the gadolinium-doped water tank in suppressing neutron-induced backgrounds?

Key findings

• XENONnT achieved a record-low electronic recoil background of (15.8 ± 1.3) events/(tonne·year·keVee), representing a factor of 5 reduction from XENON1T.
• Light and charge gains were stable within 1.0% and 1.9%, respectively, over the science run, with a 1–2% bias in line energy reconstruction included in systematic uncertainty.
• The detector achieved a minimum upper limit on the WIMP-nucleon scattering cross section of 2.6×10⁻⁴⁷ cm² at 90% confidence level for a 28 GeV/c² WIMP mass.
• The ²²²Rn background was reduced to below 1 μBq/kg, significantly improving detector purity and long-term stability.
• The neutron veto system with 500 ppm Gd-sulphate doped water effectively suppressed radiogenic neutron backgrounds.
• Calibration data from multiple monoenergetic lines (2.8 keV to 236.2 keV) were well described by the gain calibration model, confirming consistent detector response.