⚛️ phenomenology

Invisible decay of solar neutrinos at dark matter experiments

This paper presents the first constraints on invisible solar-neutrino decay using coherent elastic neutrino-nucleus scattering data from current dark matter experiments (XENONnT, PandaX-4T, and LUX-ZEPLIN) and projects that future xenon-based detectors could significantly improve these bounds and extend sensitivity to low-energy neutrinos via electronic-recoil channels.

Original authors: Martina Beccaria, Veronica Beligotti, Valentina De Romeri, Giulia Pagliaroli, Dimitrios K. Papoulias, Federica Pompa, Christoph A. Ternes

Published 2026-07-28

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Original authors: Martina Beccaria, Veronica Beligotti, Valentina De Romeri, Giulia Pagliaroli, Dimitrios K. Papoulias, Federica Pompa, Christoph A. Ternes

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

The Ghostly Journey of Solar Neutrinos

Imagine the Sun as a giant, glowing factory that never stops working. Every second, it spits out trillions of tiny, ghost-like particles called neutrinos. These particles are the ultimate escape artists: they have almost no mass, no electric charge, and they barely interact with anything. They can zip through the entire Earth without hitting a single atom, passing through your body, through the ground, and out the other side as if you weren't even there. For decades, scientists have been trying to catch these ghosts to learn secrets about the universe, like how stars burn and why the laws of physics work the way they do.

Usually, we think of these particles as indestructible travelers that fly straight from the Sun to our detectors. But what if they aren't indestructible? What if, on their long journey across space, some of them simply vanish? In the world of particle physics, this is called "invisible decay." It's like a runner in a marathon who suddenly dissolves into thin air before crossing the finish line. If this happens, it would mean our understanding of these particles is incomplete, potentially hinting at new, hidden forces or particles that we haven't discovered yet. Scientists are eager to catch a glimpse of this vanishing act because it could rewrite the rulebook of physics.

The Paper's Story: Catching Ghosts in the Dark

In this paper, a team of researchers decided to look for these disappearing solar neutrinos in a very unexpected place: inside giant tanks of liquid xenon designed to hunt for dark matter. You might wonder, "Why look for neutrinos in a dark matter experiment?" Well, these massive detectors are so incredibly sensitive that they can now feel the tiny "bumps" caused by neutrinos hitting the xenon atoms. It's like having a microphone so sensitive it can hear a whisper in a hurricane.

The researchers focused on two specific ways these neutrinos interact with the detector. The first is like a heavy bowling ball hitting a pin (called Coherent Elastic Neutrino-Nucleus Scattering, or CEνNS). This happens when high-energy neutrinos from the Sun's core hit the heavy xenon nuclei. The second is like a ping-pong ball hitting a tennis ball (called Elastic Scattering, or EνES), which happens when lower-energy neutrinos bounce off the electrons in the xenon.

Using data from three of the world's most advanced dark matter experiments—XENONnT, PandaX-4T, and LUX-ZEPLIN—the team analyzed the "bumps" they recorded. They asked a simple question: "Do we see fewer neutrinos than we expect, and could that be because they decayed on the way?"

What They Found:
The team found that the current data doesn't show a massive disappearance, but it does set a new, very strict limit on how fast these neutrinos could be vanishing. They calculated that if the second type of solar neutrino (called $\nu_2$ ) is decaying, it must be incredibly slow. Specifically, they found that the lifetime of this neutrino must be long enough that the decay parameter $\alpha_2$ is less than $1.2 \times 10^{-11}$ eV $^2$ . This is a big deal because it's the first time this specific type of decay has been constrained using these dark matter detectors, and the result is already just as strong as the best limits we had from dedicated solar neutrino experiments like the Sudbury Neutrino Observatory (SNO).

Looking to the Future:
The paper also ran simulations for a future, even bigger detector called XLZD. They found that if this new machine comes online, it could do two amazing things:

With the "bowling ball" method (CEνNS): If they can improve their knowledge of how many neutrinos the Sun produces, this detector could tighten the rules on the $\nu_2$ neutrino by about ten times, reaching a limit around $10^{-12}$ eV $^2$ .
With the "ping-pong ball" method (EνES): This is the real game-changer. By looking at the lower-energy neutrinos, the future detector could constrain the decay of the first type of neutrino ( $\nu_1$ ) to a limit of $7.9 \times 10^{-15}$ eV $^2$ and the second type ( $\nu_2$ ) to $3.0 \times 10^{-14}$ eV $^2$ . This would be 10 to 100 times more sensitive than any current experiment, effectively opening a new window to see if these solar ghosts are truly vanishing.

The Bottom Line:
The authors aren't claiming they've caught neutrinos decaying yet. Instead, they've built a much tighter cage around the possibility. They've shown that if these neutrinos are disappearing, they are doing so at a rate so slow it's almost unimaginable. But the most exciting part is the roadmap they've drawn for the future: with bigger detectors and better data, we might finally be able to catch these cosmic ghosts in the act of vanishing, or prove once and for all that they are the eternal travelers we thought they were.

Technical Summary: Invisible Decay of Solar Neutrinos at Dark Matter Experiments

Problem Statement
Neutrino decay is a potential non-standard phenomenon where neutrinos decay into lighter states. If the decay products are invisible (e.g., sterile neutrinos or other light particles), the flux of active neutrinos arriving at a detector is suppressed by a factor dependent on the baseline ( $L$ ), energy ( $E_\nu$ ), and the decay parameter $\alpha_i = m_i/\tau_i$ . Solar neutrinos offer an ideal probe for this phenomenon due to their long propagation distance ( $L \approx 1.5 \times 10^8$ km) and relatively low energies, which maximize the $L/E_\nu$ ratio. While previous constraints on invisible decay parameters ( $\alpha_1, \alpha_2, \alpha_3$ ) have been derived from atmospheric, accelerator, reactor, and supernova neutrino data, as well as dedicated solar neutrino experiments like the Sudbury Neutrino Observatory (SNO), there has been no prior constraint derived specifically from the recently observed Coherent Elastic Neutrino-Nucleus Scattering (CE $\nu$ NS) of solar neutrinos in dark matter direct detection experiments.

Methodology
The authors perform a combined analysis of nuclear-recoil data from three current xenon-based dark matter experiments: XENONnT, PandaX-4T, and LUX-ZEPLIN (LZ). These experiments have recently entered the "neutrino fog" regime, where solar neutrinos induce observable nuclear recoils via CE $\nu$ NS.

Physical Model: The study models invisible neutrino decay by modifying the standard neutrino flavor evolution Hamiltonian with a decay term. The survival probability of a neutrino mass eigenstate $\nu_i$ is suppressed by $D_i(E_\nu, L) = \exp(-\alpha_i L/E_\nu)$ .
- High-Energy Regime ( $E_\nu \gtrsim 7$ MeV): Dominated by $^8$ B solar neutrinos. Due to the Mikheyev–Smirnov–Wolfenstein (MSW) effect and incoherent propagation, these neutrinos arrive at Earth predominantly as the $\nu_2$ mass eigenstate. Consequently, the CE $\nu$ NS channel is primarily sensitive to the decay parameter $\alpha_2$ .
- Low-Energy Regime ( $E_\nu \lesssim 0.4$ MeV): Dominated by $pp$ solar neutrinos. In the vacuum regime, these arrive as a mix of $\nu_1$ and $\nu_2$ , allowing sensitivity to both $\alpha_1$ and $\alpha_2$ .
Data Analysis:
- Current Experiments: The authors analyze nuclear-recoil data from XENONnT (3 science runs), PandaX-4T (S2-only data), and LZ (S2 bins). They construct a $\chi^2$ statistic comparing predicted event rates (including oscillation and decay effects) against observed data, accounting for systematic uncertainties in solar flux normalization (specifically the $^8$ B flux) and detector backgrounds.
- Future Projections: The study projects the sensitivity of the future multi-ton Xenon Observatory (XLZD). This includes:
  - CE $\nu$ NS: Scaling the LZ analysis to 200 and 1000 t $\cdot$ yr exposures under optimistic and pessimistic background scenarios.
  - Electron Recoil (E $\nu$ ES): Analyzing the potential to detect low-energy $pp$ neutrinos via elastic scattering off electrons. This channel is currently subdominant in existing data but becomes detectable in future high-exposure scenarios.

Key Contributions

First CE $\nu$ NS Constraints: This work presents the first constraints on invisible solar-neutrino decay derived from CE $\nu$ NS data in dark matter detectors.
Combined Analysis: It provides a joint constraint on $\alpha_2$ using data from XENONnT, PandaX-4T, and LZ.
Future Sensitivity Assessment: It quantifies the potential of next-generation xenon detectors (XLZD) to improve these bounds, specifically distinguishing between the capabilities of nuclear-recoil (CE $\nu$ NS) and electron-recoil (E $\nu$ ES) channels.
Systematic Limitation Identification: The analysis explicitly demonstrates that for CE $\nu$ NS, the sensitivity to $\alpha_2$ is currently limited by the theoretical uncertainty of the $^8$ B solar neutrino flux rather than statistical exposure.

Results

Current Bounds: The combined analysis of current nuclear-recoil data yields a 90% confidence level (CL) bound of $\alpha_2 < 1.2 \times 10^{-11} \text{ eV}^2$ $α_{2} < 1.2 \times 1 0^{- 11} eV^{2}$ .
- This result is comparable in strength to the bound derived from the combined analysis of SNO data ( $\alpha_2 < 8.1 \times 10^{-12} \text{ eV}^2$ ), despite using a completely different detection channel and a fraction of the total exposure.
- XENONnT provides the strongest individual constraint among current experiments, while PandaX-4T data alone yields a weaker bound due to the reduced dataset used in this specific analysis.
Future CE $\nu$ NS Sensitivity: For XLZD, increasing exposure alone (e.g., to 1000 t $\cdot$ yr) does not significantly improve the bound if the $^8$ B flux uncertainty remains at 13.13%. However, if the flux uncertainty is reduced to 5%, the sensitivity improves by roughly a factor of 2, reaching the $O(10^{-12}) \text{ eV}^2$ level.
Future E $\nu$ ES Sensitivity: The electron-recoil channel offers significantly stronger constraints. By detecting $pp$ neutrinos, XLZD could constrain:
- $\alpha_1 < 7.9 \times 10^{-15} \text{ eV}^2$
- $\alpha_2 < 3.0 \times 10^{-14} \text{ eV}^2$
  These projections represent an improvement of 1 to 2 orders of magnitude over existing dedicated solar neutrino bounds.

Significance and Claims
The paper claims that the observation of solar CE $\nu$ NS in dark matter experiments has opened a new, competitive avenue for testing invisible neutrino decay. The current constraints derived from these experiments are already comparable to those from the Sudbury Neutrino Observatory, validating dark matter detectors as precision instruments for neutrino physics.

The authors emphasize that while CE $\nu$ NS is a powerful probe for $\alpha_2$ , the electron-recoil channel (E $\nu$ ES) in future large-scale detectors will be the most sensitive probe for invisible decay, capable of constraining both $\alpha_1$ and $\alpha_2$ with unprecedented precision. They note that these results are complementary to cosmological constraints, as solar neutrino observations directly probe the survival probability of mass eigenstates over the Sun-Earth baseline without relying on assumptions about decay products or their interactions in the early universe. The work concludes that as next-generation dark matter detectors come online, they will play a pivotal role in either discovering or placing stringent limits on non-standard neutrino decay.

The Ghostly Journey of Solar Neutrinos

The Paper's Story: Catching Ghosts in the Dark

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