In the search for dark matter, detectors using liquid xenon have proven to be at the forefront. Currently, experiments such as XENONnT, LZ or PandaX are pushing the boundaries in the search for WIMP dark matter candidate to unprecedented levels. This progress has been made possible by combining reductions in background components with significant technical improvements in detector technology.

The detectors used in these experiments are time projection chambers (TPCs) immersed in liquid xenon, with a layer of gaseous xenon on top. Currently, these experiments operate on the scale of tens of tons of xenon. To further advance the search for dark matter, a new generation of detectors must be developed. The DARWIN collaboration was established with the goal of constructing a 2.6-m high TPC capable of operating 50 tons of xenon. This detector will achieve unprecedented sensitivity, ultimately limited by the irreducible background of neutrino interactions. Interestingly, neutrinos themselves also represent a compelling physics channel for DARWIN. However, building such a detector requires addressing several challenges, not only in terms of background reduction but also in the structural design of the detector itself.

Xenoscope is a vertical prototype of this new generation detector. Funded by an ERC advanced grant, the research for the Xenoscope project started in October 2017 with the goal of specifying the required input for the technical design of a 50 tones detector to be realised by the DARWIN collaboration.

The initial phase of the project consisted in the construction of a 50 cm TPC fully immersed in liquid xenon. This purity monitor collected the signals produced by electron populations generated by means of a flashing xenon lamp onto a photocathode plate located at the bottom. The operation of the purity monitor was successfully completed, and the results of the research have been published here.

The modular design of the detector makes it easier to expand. In a second stage, the purity monitor was upgraded to a 2.6 m long dual-phase TPC. At this stage, there are two electric fields: one in the drift region of the electrons, in the LXe phase, followed by an extraction field in the gas phase. Several upgrades to the system were performed, such as the installation of the high-voltage system, liquid-gas level control, and a top SiPM array to measure the secondary electroluminescence signal induced by the electrons in the gas phase. All these components have been successfully tested in a commissioning run, and the results can be seen in this publication.

The design and construction of the Xenoscope - DARWIN Demonstrator has been made in close collaboration with our mechanical workshop, and it is currently located in the assembly hall of the physics institute of the University of Zurich.

The Xenoscope DARWIN demonstrator is complemented by the full-diameter ULTIMATE DARWIN demonstrator, in construction at the University of Freiburg.

To make the task of building the next dark matter detector easier, three of the leading collaborations in the field have joined forces: XENON, LZ, and DARWIN formed the XLZD collaboration in 2022. Combining all the R&D expertise of DARWIN with the experience of more than 20 years in building these detectors, XLZD represents the perfect framework to achieve this goal. As a result, Xenoscope, which was initially conceived as the demonstrator for DARWIN, is now the vertical demonstrator for the future detector to be built by XLZD.

For more information about this project, you can visit the page of the XLZD project at UZH and the webpage of the collaboration.