The XLZD Design Book: Towards the Next-Generation Liquid Xenon Observatory for Dark Matter and Neutrino Physics

TL;DR

XLZD proposes a scalable, next-generation liquid xenon observatory that leverages mature LXe-TPC technology to push dark matter searches into the neutrino fog regime, pursue neutrinoless double beta decay in 136Xe, and study astrophysical neutrinos with high precision. The design combines a 60–80 t active Xe target, comprehensive background suppression (radon, krypton, surface contaminants), advanced veto systems, and a robust xenon handling/purification and cryogenics program, supported by an extensive calibration, electronics, and software framework. By integrating proven technology from LZ and XENONnT with strategic xenon acquisition, risk-mitigating interim configurations, and a global collaboration, XLZD aims for 3σ discovery potential at cross sections around $3\times10^{-49}$ cm$^{2}$ for 40 GeV/$c^2$ WIMPs and a 3σ $0\nu\beta\beta$ sensitivity up to a half-life of $5.7\times10^{27}$ years, while also enabling solar and galactic supernova neutrino observations. The project anticipates substantial scientific impact across particle, nuclear, and astrophysics and seeks to complement other detectors and collider experiments in a coordinated, multi-isotope, multi-messenger search for new physics.

Abstract

This report describes the experimental strategy and technologies for XLZD, the next-generation xenon observatory sensitive to dark matter and neutrino physics. In the baseline design, the detector will have an active liquid xenon target of 60 tonnes, which could be increased to 80 tonnes if the market conditions for xenon are favorable. It is based on the mature liquid xenon time projection chamber technology used in current-generation experiments, LZ and XENONnT. The report discusses the baseline design and opportunities for further optimization of the individual detector components. The experiment envisaged here has the capability to explore parameter space for Weakly Interacting Massive Particle (WIMP) dark matter down to the neutrino fog, with a 3$σ$ evidence potential for WIMP-nucleon cross sections as low as $3\times10^{-49}\rm\,cm^2$ (at 40 GeV/c$^2$ WIMP mass). The observatory will also have leading sensitivity to a wide range of alternative dark matter models. It is projected to have a 3$σ$ observation potential of neutrinoless double beta decay of $^{136}$Xe at a half-life of up to $5.7\times 10^{27}$ years. Additionally, it is sensitive to astrophysical neutrinos from the sun and galactic supernovae.

Figures (14)

• Figure 1: The science channels of the proposed LXe observatory for rare events span many areas and are of interest to particle physics, nuclear physics, astrophysics, solar physics, and cosmology.
• Figure 2: Left: Projected $90$% C.L. upper limits on the spin-independent WIMP-nucleon cross section for 200 and 1000 tonne-year (t$\cdot$y) exposures of the XLZD detector, along with current upper limits LZ_2024_DM_resultsaprile2025wimpdarkmattersearchPandaX-4T:2024dmAkerib:2015rjg. The blue shaded regions illustrate the neutrino fog as defined inOHare:2021utq. Right: The dashed contours indicate the median 3$\sigma$ detection limit for 200 and 1000 t$\cdot$y exposure. Example evidence contours for 20 and 80 GeV WIMPs are shown with confidence intervals of 1, 2, and 3$\sigma$ (yellow, orange, and red, respectively). These illustrate that extending exposure from 200 t$\cdot$y to 1000 t$\cdot$y significantly improves our ability to constrain dark matter properties after initial detection. The figure displays several well-motivated dark matter candidates within XLZD's reach: Electroweak multiplet DM Bloch:2024 is an example of a minimal dark matter model still largely unexplored, while Higgsino Ellis_2023 and Bino DM ATLAS2024 candidates arise from supersymmetry. In green, we highlight that XLZD is highly complementary to collider experiments, with the shaded region showing Bino DM exclusion limits from ATLAS ATLAS2024 and the vertical green line indicating the maximum mass testable with ATLAS electroweak searches for this DM model.
• Figure 3: Left: Energy spectra of a hypothetical 5e27yr $^{136}$Xe $0\nu\beta\beta$ signal (yellow) and the dominant backgrounds to this search: $\gamma$-rays from materials before (grey) and after (black) rejection by the veto systems (see text), $2\nu\beta\beta$-decays of $^{136}$Xe (blue), $^{222}$Rn induced $\beta$-decays of $^{214}$Bi (green), $\nu$-$e^-$ scattering of $^8$B neutrinos (purple), and $\beta$-decays of $^{137}$Xe (red) for examples of different host laboratories. Right: Projected evidence sensitivity at $3\sigma$ significance to the $0\nu\beta\beta$ decay of $^{136}$Xe as a function of exposure time for the two final target mass scenarios: 60t and 80t, and an interim 40t configuration further discussed in Section \ref{['sec:project']}. For each target mass, the band represents the range of detector performance parameters and background assumptions between the nominal (lower limit) and optimistic (upper limit) scenarios, as discussed in the text. The right axis shows the sensitivity in effective Majorana mass $m_{\beta\beta},$ assuming a maximum (minimum) nuclear matrix element $M_{136\rm{Xe}}^{0\nu}$ of 4.77 (1.11) Agostini:2023. The projected sensitivity of other proposed $^{136}$Xe $0\nu\beta\beta$ experiments is shown for comparison Agostini:2023nEXO:2021ujkNEXT:2020amj.
• Figure 4: The $\nu_e$ survival probability versus neutrino energy, assuming the high-$Z$ Solar Standard Model (SSM). The blue dots represent the solar measurements from Borexino (above an energy threshold of 190 keV) Agostini:2018ulyAgostini_2020. The green (purple) point shows a measurement of $^7$Be ($^8$B) from KamLAND (SNO) KamLAND:2011fldAharmim:2011vm. The red point indicates that XLZD, with 600 t$\cdot$y exposure, could enhance the precision of the $\nu_e$ survival probability in the low energy region to $\sim5\%$, using solar pp neutrino events. The point is set at the mean neutrino energy and the range bars in x indicate the full energy range (5.1 keV to 420 keV) accessible with XLZD for this measurement. The grey band represents the 1$\sigma$ prediction of the MSW-LMA solution Capozzi:2017.
• Figure 5: Operating principle of the dual-phase (liquid/gas) xenon time projection chamber. Particle interactions in the active liquid xenon target produce prompt (S1) and delayed (S2) optical signals. These are detected by two arrays of VUV-sensitive photosensors located at the top and bottom of the TPC.