⚛️ phenomenology

Dark Matter implications from the LZ, PandaX-4T and XENONnT Data

Using a unified framework to analyze combined data from LZ, PandaX-4T, and XENONnT, this study suggests that velocity-dependent or inelastic dark matter interactions could explain high-energy nuclear-recoil-like events with up to 3.5σ significance, though these findings are highly sensitive to uncertainties in $^{124}$ Xe double electron capture backgrounds.

Original authors: Haipeng An, Fei Gao, Jia Liu, Minghao Liu, Haoming Nie, Changlong Xu

Published 2026-07-13

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Haipeng An, Fei Gao, Jia Liu, Minghao Liu, Haoming Nie, Changlong Xu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe is a giant, invisible ocean made of "Dark Matter," a mysterious substance that makes up about one-fourth of everything in existence. For decades, scientists have been trying to catch a single drop of this ocean using massive tanks of liquid xenon, which act like super-sensitive fishing nets. The most famous nets are the LZ, PandaX-4T, and XENONnT experiments.

Usually, these nets are designed to catch "WIMPs" (Weakly Interacting Massive Particles), the standard suspects for Dark Matter. The standard theory says these particles should bounce off xenon atoms gently, creating a tiny, predictable splash that fades away quickly. But recently, all three nets started seeing something weird: a bunch of big, energetic splashes happening at the same time, way above the quiet background noise. It's like fishing in a calm lake and suddenly seeing a school of sharks jumping out of the water when you were only expecting minnows.

The Mystery of the "Ghost" Signal
The scientists in this paper asked: "What is causing these high-energy splashes?" They ran a massive combined analysis, looking at data equivalent to 8.8 tonne-years of exposure (that's a lot of fishing time!). They found that the standard "gentle bounce" model of Dark Matter simply cannot explain these big splashes. The math just doesn't add up; the standard model predicts the splashes should get smaller and smaller as they get more energetic, but the data shows them staying strong.

Two New Suspects
Instead of the usual suspects, the authors suggest two new types of Dark Matter that could explain the chaos:

The Speed-Dependent Ghosts: Imagine a ghost that only gets angry and hits hard if you run past it fast. These are Dark Matter particles that interact differently depending on how fast they are moving. The paper suggests specific mathematical rules (called operators $Q^{(7)}_3$ and $Q^{(7)}_4$ ) that describe this behavior. When they tested these rules, the "ghosts" fit the data pretty well, reaching a statistical "significance" of about 2.3 to 2.6 sigma. In the world of particle physics, that's a strong hint, but not a shout of victory yet.
The Shape-Shifting Ghosts: Imagine a ghost that can change its weight slightly when it bumps into something. Sometimes it gets a tiny bit heavier (endothermic), and sometimes a tiny bit lighter (exothermic). This "mass splitting" (denoted as $\delta$ ) changes how the collision happens. The paper found that if the Dark Matter shifts its mass by about 120–165 keV (when getting heavier) or -200 keV (when getting lighter), it perfectly explains the high-energy splashes. These models reached an even higher significance of up to 3.5 sigma. That's a very exciting signal, but still not a confirmed discovery.

The "Fake" Clue: The Xenon Glitch
However, there is a huge catch. The paper points out a very tricky background noise that could be faking these signals. Inside the xenon tanks, a rare isotope called $^{124}\text{Xe}$ sometimes undergoes a "double electron capture" (DEC). Think of this as a tiny atomic hiccup. Usually, this hiccup is quiet, but sometimes it might produce a "charge yield" (a signal strength) that is lower than expected, making it look like a Dark Matter splash instead of an atomic glitch.

The paper ran simulations to see how much this "hiccup" matters.

If we assume the hiccup is quiet (Case I), the Dark Matter signal looks very real (up to 3.5 sigma).
If we let the "hiccup" signal strength float and change freely (Case II), the signal drops to about 1.8 sigma.
If we are super conservative and assume we know nothing about the hiccup (Case III), the signal vanishes almost completely, dropping to 1.1 sigma or even zero.

This means the "evidence" for Dark Matter is currently hanging on a very thin thread: our understanding of how that specific atomic hiccup behaves. The authors explicitly state that the current data is consistent with "background-only models," meaning it's possible there is no Dark Matter at all, just a misunderstood atomic glitch.

What's Next?
The paper doesn't claim to have solved the mystery. Instead, it offers a way to solve it. The authors predict that if these "Shape-Shifting" or "Speed-Dependent" Dark Matter particles are real, they should create even more splashes at very high energies (above 100–300 keV). The good news? The atomic "hiccup" background is expected to be negligible at those super-high energies.

So, the next step for the LZ, PandaX-4T, and XENONnT teams is to look at the data from those higher energy ranges. If they see more splashes there, it's likely Dark Matter. If the splashes stop, it was probably just the atomic hiccup all along. Until then, the universe remains a little more mysterious, with the answer hiding somewhere between a new particle and a misunderstood glitch.

Technical Summary: Dark Matter Implications from the LZ, PandaX-4T, and XENONnT Data

Problem Statement
Recent data from liquid xenon time projection chamber (LXe TPC) experiments—specifically LZ, PandaX-4T, and XENONnT—have reported an excess of nuclear-recoil (NR)-like events at energies above 20 keV. While the standard elastic spin-independent (SI) Weakly Interacting Massive Particle (WIMP) scattering model predicts an exponentially falling recoil spectrum that cannot reproduce these high-energy features, the collaborations have offered varying interpretations. The LZ collaboration attributes these events to the two-neutrino double electron capture (2 $\nu$ ECEC or DEC) of $^{124}$ Xe, specifically involving sub-dominant L-shell capture channels that produce low charge yields, causing background leakage into the NR region. Conversely, XENONnT and PandaX-4T have not fully incorporated this specific charge-yield suppression mechanism into their primary background models. The central problem addressed is whether these high-energy NR-like events can be explained by non-standard dark matter (DM) interactions or if they are artifacts of poorly constrained $^{124}$ Xe DEC backgrounds.

Methodology
The authors perform the first combined profile-likelihood analysis of high-energy NR-like events across the LZ, PandaX-4T, and XENONnT experiments, utilizing a total exposure of approximately 8.8 tonne $\times$ year. The analysis relies on the DIAMX framework, a unified statistical inference tool built on publicly available data and likelihood models to reproduce official collaboration results.

Data Selection: The analysis incorporates six datasets: LZ (WS2022, WS2024), PandaX-4T (Run 0, Run 1), and XENONnT (SR0, SR1). Events are selected based on corrected S1 and S2 signals in the 2D plane, with specific cuts applied to mitigate low-energy mismodeling (e.g., $cS2 > 400$ PE for XENONnT and $cS1 > 30$ PE for PandaX-4T).
Dark Matter Models: Two broad classes of DM-nucleon interactions are investigated:
1. Velocity-Dependent Interactions: Described within the Dark Matter Effective Field Theory (DMEFT) framework. The authors systematically scan dimension-seven operators, identifying $Q^{(7)}_3$ and $Q^{(7)}_4$ as the only operators capable of reproducing the observed spectral features due to their $q$ and $q^2$ scaling, which suppresses low-energy recoils.
2. Inelastic Scattering: Both endothermic ( $\delta > 0$ ) and exothermic ( $\delta < 0$ ) scenarios are considered. These models introduce a mass splitting $\delta$ between the DM ground state and an excited state, kinematically suppressing low-energy events while populating the high-energy tail.
Background Modeling: The analysis explicitly addresses the uncertainty in $^{124}$ $^{124}$ Xe DEC charge yields. Three scenarios are tested:
- Case I (Baseline): Fixed charge yields based on LZ measurements ( $Q_{LL}/Q_\beta = 0.88$ ) for LZ/PandaX-4T and standard yields for XENONnT.
- Case II: The $Q_{LL}$ ratio is treated as a free parameter for LZ and PandaX-4T.
- Case III: All unmeasured charge yield ratios are treated as unconstrained nuisance parameters.

Key Results

DM Interpretation: The standard SI elastic scattering model fails to describe the data. In contrast, the velocity-dependent operators ( $Q^{(7)}_3, Q^{(7)}_4$ $Q_{3}^{(7)}, Q_{4}^{(7)}$ ) and inelastic scattering models provide a significantly better fit.
- Under the baseline assumption (Case I), the local statistical significances reach up to 3.5 $\sigma$ for inelastic models (endothermic with $\delta \approx 150$ keV and exothermic with $\delta \approx -200$ keV) and approximately 2.6 $\sigma$ for the velocity-dependent operators.
- The best-fit parameters for inelastic models suggest mass splittings in the range of 120–165 keV (endothermic) and $|\delta| > 135$ keV (exothermic).
Systematic Uncertainty: The significance of the DM signal is highly sensitive to the treatment of $^{124}$ $^{124}$ Xe DEC charge yields.
- In the most conservative scenario (Case III), where charge yields are floated, the local significance for all DM benchmarks drops below 1.1 $\sigma$ .
- This indicates that current uncertainties in the DEC charge yield dominate the quantitative assessment of the signal, even though the qualitative preference for high-energy-enhancing spectra remains.
Predictions for Future Data: The authors predict event rates at higher recoil energies (up to 300 keV) outside the current WIMP search region of interest (ROI).
- Velocity-dependent models predict a large excess of events at high masses ( $m_{DM} \gtrsim 110$ GeV), which contradicts current null results in the high-energy tails of LZ and PandaX-4T data.
- Inelastic models predict a modest yield of a few tens of events in the high-energy range, providing a distinct, testable signature.

Significance and Claims
The paper claims to provide the first combined profile-likelihood analysis of these specific datasets, demonstrating that non-standard DM interactions (velocity-dependent or inelastic) can reproduce the observed high-energy NR-like events with local significances up to 3.5 $\sigma$ . However, the authors maintain a modest stance regarding the definitive nature of this signal. They emphasize that the apparent preference for DM is strongly contingent on the poorly constrained $^{124}$ Xe DEC charge yields.

The primary conclusion is that while specific DM models fit the current data well, the DEC background remains the dominant systematic uncertainty. The paper asserts that future analyses extending to higher recoil energies ( $\gtrsim 100-300$ keV) are crucial. Since the $^{124}$ Xe DEC background is expected to be negligible in this high-energy range, such data will serve as a powerful test to distinguish between a genuine dark matter interpretation and background modeling deficiencies. The work does not claim a discovery but rather identifies a tension in current data that requires refined background characterization and extended energy range searches to resolve.

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