Research

Dark matter is believed to account for most of the matter in the universe, yet it cannot be explained by the particles of the Standard Model, and its true nature remains unknown.
One of the leading candidates is the WIMP, or weakly interacting massive particle. Searching for such dark matter candidates is important not only for identifying dark matter itself, but also for exploring new physics beyond the Standard Model.
Liquid xenon detectors are particularly powerful for direct dark matter searches because they combine high scintillation yield, scalability to large target masses, low radioactive background event rates, excellent purifiability, and strong self-shielding.

Dark matter searches with XENONnT

XENONnT is one of the world’s leading liquid xenon dark matter experiments, operating underground at the INFN Laboratori Nazionali del Gran Sasso in Italy. It is located deep underground to suppress cosmic rays, and the detector is surrounded by purified water to shield environmental radiation. By reading out both the vacuum-ultraviolet scintillation light and the ionization electrons produced in liquid xenon, the detector reconstructs the position and energy of each interaction and suppresses radioactive background events while searching for dark matter signals.

By drifting the ionization electrons upward and converting them into light in the gas phase, the detector achieves high sensitivity to low-energy events, three-dimensional position reconstruction, and powerful background discrimination. These features make liquid xenon detectors attractive not only for dark matter searches but also for low-energy neutrino observations and other rare-event studies beyond the Standard Model.

In our laboratory, we use data currently being collected by XENONnT to study radioactive background events and detector-response modeling, and to search for dark matter, neutrinoless double-beta decay, and new opportunities in low-energy neutrino astronomy. Trace impurities such as radon, tritium, and neutrons strongly affect the sensitivity, so we pursue both data analysis and detector R&D in parallel.

XENON experiment overview — **Overview of the XENON experiment**The large water shield is on the left, with the liquid xenon detector installed inside it. On the right are the xenon purification and cryogenic systems, the data acquisition system, and the detector control system.

XENONnT water shield — **XENONnT radiation shield**This large shield is filled with about 700 tons of ultrapure water to reduce external radiation. Gadolinium is added to improve neutron-tagging capability, while the liquid xenon detector is installed at the center.

Detector construction and low-radioactivity materials — **Detector construction and ultra-low-radioactivity materials**The liquid xenon detector is assembled carefully in a clean environment, and the materials used for the detector are selected for extremely low radioactivity. Such thorough reduction of background events supports the search for extremely rare signals.

XENONnT photosensors — **Photosensors in XENONnT**These photosensors are installed in the detector to measure the vacuum-ultraviolet light produced in liquid xenon with high sensitivity, forming the basis of the signal measurement.

Ultra-low-background detector R&D toward XLZD

XLZD is a next-generation ultra-large liquid xenon detector under study for operation in the mid-2030s, with a target mass of roughly 60–80 tonnes of liquid xenon, about an order of magnitude larger than current experiments.

Its physics reach is expected to go beyond improved dark matter sensitivity to include neutrinoless double-beta decay and low-energy neutrino observations, opening a broad program of rare-event physics.

To realize this, it is not enough simply to scale up the detector; we also need an even more stringent ultra-low-background environment. In our laboratory, one major theme is the development of a hermetic liquid xenon detector based on synthetic quartz to suppress contamination from radon and tritium. We have established a dedicated system with cryogenics, vacuum, gas purification, controls, and safety instrumentation, demonstrated radon suppression in gaseous xenon, and are now extending the evaluation to liquid xenon.

Another major theme is the development of ultra-low-background photosensors. We pursue lower-radioactivity and higher-performance photomultiplier tubes and silicon photosensors, as well as novel hybrid photosensors that combine both approaches. In collaboration with Hamamatsu Photonics, we have also built a dedicated system for measurements in vacuum ultraviolet light, at low temperature, and under vacuum, optimized for the 175 nm xenon scintillation wavelength.

We are also developing methods to quantify extremely small hydrogen concentrations relevant to tritium backgrounds, as well as assays of ultra-trace uranium and thorium in PTFE reflector materials. These studies are important not only for dark matter searches but also for neutrinoless double-beta decay and future low-energy neutrino astronomy.

**Hermetic liquid xenon detector**A detector concept based on synthetic quartz, designed to suppress contamination from impurities such as radon and tritium.

**Xenon cooling and purification system**An in-house R&D platform with cryogenics, vacuum, purification, control, and safety systems, supporting the evaluation of the hermetic liquid xenon detector.

Inside of VUV cryogenic vacuum setup — **Interior of the VUV low-temperature vacuum setup**This system is used to evaluate novel ultra-low-background photosensors. It supports performance measurements around 175 nm, the xenon scintillation wavelength, under vacuum and at temperatures around −100 °C comparable to liquid xenon operation.

Outside of VUV cryogenic vacuum setup — **Exterior of the VUV low-temperature vacuum setup**The full measurement platform, including the spectrometer, cryocooler, and vacuum system, used for sensor development and evaluation.