Boosted Dark Matter Directionality in Large Liquid Scintillator Detectors

arXiv:2610.31666 · hep-ph, astro-ph.HE · Submitted 2026-10-22 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "Boosted Dark Matter Directionality in Large Liquid Scintillator Detectors".

Jocelyn: Here's the gist of the paper. Reconstructed capture and emission vertices can preserve partial information about the BDM arrival direction because they can be combined to form a displacement vector.

Vera: How it works The study considers the signal produced by boosted dark matter undergoing quasi-elastic scattering off carbon and knocking out a neutron in a large liquid scintillator detector because this allows for the reconstruction of a distinctive delayed gamma ray from neutron capture after ∼ 210 µs, which is paired up with the prompt signal during neutron propagation.

Jocelyn: The analysis focuses on utilizing directional information from the displaced vertex associated with the capture of the final-state neutron, although it finds that directional information does not materially improve sensitivity due to the large nucleon scattering angle.

Vera: From BDM flux to event rates The researchers select a DM model and calculate the BDM flux arriving at Earth from different directions, modeling the Earth attenuation effect and computing the attenuated BDM flux that reaches the detector.

Title and authors: Vera: So we’re looking at what this paper on "Boosted Dark Matter Directionality in Large Liquid Scintillator Detectors" really says is that they can reconstruct these neutron signals—by combining the prompt light from a neutron moving through the detector with a delayed gamma ray from when that neutron finally gets captured—to create something like a displacement vector.

Jocelyn: Right, so they're trying to use those two different signals, the prompt one and the delayed one, to figure out where things came from in space.

Subrahmanyan: And they do this to preserve partial information about the direction of that dark matter signal arriving at Earth. But here's the catch from their analysis—the directional information doesn't actually improve sensitivity much because of how much those neutrons scatter off the stuff inside the detector, which is a large nucleon scattering angle.

Vera: So, they modeled this whole process by first taking a dark matter model and figuring out how much dark matter flux arrives at Earth from different directions, modeling the Earth blocking some of it.

Jocelyn: Then they calculated how that dark matter knocks out these neutrons using this quasi-elastic scattering idea off carbon, determining the rate of those neutrons going in different directions inside the detector.

Subrahmanyan: And before you get to earth, they modeled how Earth itself attenuates that dark matter energy by solving a differential equation for the average attenuated kinetic energy, K-bar-chi, which depends on where you are and which direction it's coming from.

Vera: After the neutron is knocked out, they use Geant4 simulations to model how it travels through the liquid scintillator, looking at secondary light generation and those capture events.

Jocelyn: They define the prompt signal as just the energy from that secondary light—Eprompt equals Esecondary—and they pair that with a delayed gamma ray from when the neutron captures something, which lets them reconstruct this displacement vector.

Subrahmanyan: They also quantify how smeared out that direction information is by calculating this capture angle, theta-c, between where the neutron was captured and where it first came from, which shows the effect of neutron diffusion.

Vera: It turns into a whole process of modeling everything—from secondary light emission using the Birks formula for energy quenching to accounting for final state interactions in neutrons using a nuclear optical potential.

Jocelyn: They then determine the event rates by combining that attenuated dark matter flux with the differential cross section from quasi-elastic scattering to get those counts per bin, dividing the sky into seven hundred sixty-eight patches for integration.

Subrahmanyan: When they calculate sensitivity for a one hundred kiloton per year exposure, they find that incorporating this directional data improves the constraint only mildly because that inherent directionality of the dark matter is already smeared by that nucleon scattering angle.

Vera: They also looked at how this compares to other channels and found that the neutron channel gives stronger constraints than a free-proton channel, and DUNE still looks like it has the strongest projected sensitivity in this framework.

Jocelyn: So even with all this work, they are pointing out that relying only on the direction of the neutron doesn't give you a huge boost to your constraint on dark matter scattering cross sections.

The paper's summary: Vera: So we're looking at what the paper suggests as improvements for this boosted dark matter directionality study, and essentially they're focusing on how to make that directional information actually useful in practice.

Jocelyn: They point out that they can use this reconstructed vertex data to find a specific displacement vector, which keeps some of that directional hint about where the dark matter came from even after all the scattering.

Subrahmanyan: Right, so it's using the combined prompt and delayed signals to get this vector, keeping a little bit of directionality. But they also suggest quantifying this smearing effect by calculating that capture angle, theta-c, between where the neutron was captured and where it first came from.

Vera: That calculation helps show you exactly how much the neutron diffusion smears out that initial direction information, which is a crucial piece of the puzzle for interpreting the results.

Jocelyn: They also talk about modeling vertex resolution using a normal probability density function to generate those reconstructed coordinates around where they think they are, which helps make sense of the uncertainty in those measurements.

Subrahmanyan: And then there's accounting for how neutrons actually propagate through the medium, including final state interactions by modifying that kinetic energy of the neutron using a nuclear optical potential.

Vera: The main implication here is that they are showing exactly what needs to be modeled to get these numbers right, moving from just theoretical scattering rates to simulating the actual physical journey of a particle in that detector material.

Jocelyn: They also look at how they calculate event rates by combining the attenuated dark matter flux and this differential cross section for quasi-elastic scattering to get those counts per bin, dividing the sky into seven hundred sixty-eight patches.

Subrahmanyan: And when they run the sensitivity calculation, they use a joint likelihood across N bins modeled with a Poisson distribution to get those expected event numbers lambda i.

Vera: The result of this whole setup is that incorporating directional information helps you get constraints on the spin-independent dark matter nucleon scattering cross section sigma-n, but it only improves it mildly because the nucleon scattering angle already smears the map substantially.

Jocelyn: They also compare their results with previous work and find that this neutron channel yields stronger constraints than a free-proton channel, which is something we need to keep tracking.

Subrahmanyan: And they mention that at one GeV for the dark matter mass, this sensitivity level is comparable to what Hyper-K could expect from five years of data, while DUNE still projects the strongest sensitivity in this framework.

The paper's improvements: Vera: So we're wrapping up on this paper, "Boosted Dark Matter Directionality in Large Liquid Scintillator Detectors," and what it boils down to is that they tested whether reconstructing vertices can help us get partial directional info about where dark matter came from.

Jocelyn: Yeah, so the core finding is that while you can get a displacement vector, it doesn't give you a huge boost to the final sensitivity because of how much those nucleons scatter during propagation.

Subrahmanyan: From a cosmological view, this confirms what we thought about nucleon scattering; it smooths out the signal too much for directionality to be the primary driver for constraining that dark matter cross section.

Vera: They also made two optimistic assumptions in their study—that dark matter direction doesn't change after every scatter and that the neutron knockout direction isn't affected by final state interactions—which likely overestimated how much directional gain you actually get.

Jocelyn: So, what this means for us is that we have to be careful not to over-interpret these vertex reconstructions without accounting for those smearing effects they talked about so clearly.

Subrahmanyan: It’s a good reminder that the inherent directionality of the dark matter itself is limited by its distribution, and neutron emission directions are further smeared by that scattering angle.

Vera: The paper does show that the neutron channel actually yields stronger constraints than a free-proton channel, which is something we need to keep watching closely.

Jocelyn: And they set a benchmark for what sensitivity might look like with current and future experiments, saying DUNE still looks like it has the strongest projected sensitivity in this framework.

Subrahmanyan: It’s important to remember that while the neutron channel is good, we need better reconstruction techniques for directionality if we want to see a real gain on those constraints.

Vera: So the main implication is that this specific method isn't a silver bullet for directional boosts right now, but it points us toward needing smarter event reconstruction methods in future dark matter searches.

Jocelyn: Exactly, so keep pushing for better vertex reconstruction tools because that's where the real progress will be made in finding those faint dark matter signals.

Subrahmanyan: We need more precise measurements of those scattering angles to truly understand how much information we can pull out from these astrophysical events.

Conclusion: Vera: So we're wrapping up on "Boosted Dark Matter Directionality in Large Liquid Scintillator Detectors," and what they showed is that using neutron capture signals to build a displacement vector can give you a hint about where dark matter came from, even though the scattering smears that information out quite a bit.

Jocelyn: Right, so we're talking about combining the prompt light from the neutron moving through the detector with that delayed gamma ray from when it gets captured to get this displacement vector that keeps some directional information intact.

Subrahmanyan: And they also pointed out that calculating that capture angle, theta-c, shows exactly how much neutron diffusion smears out that initial direction data, which is a crucial piece for interpreting the results.

Vera: They modeled all of this by simulating the entire process from dark matter flux arriving at Earth to the final event rates in different sky patches.

Jocelyn: And when they run those sensitivity calculations for a one hundred kiloton per year exposure, they find that incorporating this directional data only improves the constraint mildly because that nucleon scattering angle already smears the map substantially.

Subrahmanyan: It’s important to remember that while the neutron channel yields stronger constraints than a free-proton channel, it doesn't provide a massive jump in sensitivity right now.

Vera: They also flagged two optimistic assumptions they made about how much directionality is gained, which means we have to be careful not to over-interpret those vertex reconstructions without accounting for the smearing effects.

Jocelyn: So, what this means for us is that we have to be careful not to over-interpret these vertex reconstructions without accounting for those smearing effects they talked about so clearly in the paper.

Vera: They did show that while this specific technique isn't a massive improvement, it still encourages LLSD experiments to keep exploring better ways to reconstruct directionality for dark matter searches.

Jocelyn: Exactly, so they want us to keep looking for new event reconstruction techniques that can cut through that smearing effect we’re seeing here.

Subrahmanyan: We need more precise measurements of those scattering angles to truly understand how much information we can pull out from these astrophysical events.

Samuel S. H. Tse, –Qishan Liu, –Kenny C. Y. Ng, –Koun Choi, §Yu-Feng Li

Department of Physics, The Chinese University of Hong Kong · Institute of High Energy Physics, Beijing China · China Center of Advanced Science and Technology, Beijing 100190 China · Center for Underground Physics, Institute for Basic Science, Daejeon Korea

hep-ph, astro-ph.HE

Submitted: 2026-10-22

Updated: 2026-10-22

Comments: 15 pages, 11 figures, 1 table. To be submitted to Physical Review D

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 79/100

The gist: The gist: Reconstructed capture and emission vertices can preserve partial information about the BDM arrival direction because they can be combined to form a displacement vector.

Key concepts

Reconstructed Capture and Emission Vertices: These are two distinct points in space derived from the signal: one from the initial neutron emission (prompt signal) and another from the delayed gamma ray produced when that neutron is captured. Combining these two points creates a displacement vector, which holds some directional clues about where the dark matter came from.
Nucleon Scattering Angle ($ heta_c$): This angle measures how much the neutron's direction changes between when it was knocked out of the nucleus and when it is captured. This change, caused by nucleon scattering, smears out any precise directional information about the original dark matter arrival direction.
BDM Flux Modeling: The researchers model how dark matter particles arrive at Earth using a cylindrical leaky-box model. They calculate this flux while accounting for attenuation as it passes through the Earth, which is crucial for determining the expected event rates reaching the detector.

Summary

The gist: Reconstructed capture and emission vertices can preserve partial information about the BDM arrival direction because they can be combined to form a displacement vector.

How it works

The study considers the signal produced by boosted dark matter undergoing quasi-elastic scattering off carbon and knocking out a neutron in a large liquid scintillator detector because this allows for the reconstruction of a distinctive delayed gamma ray from neutron capture after ∼ 210 µs, which is paired up with the prompt signal during neutron propagation. The analysis focuses on utilizing directional information from the displaced vertex associated with the capture of the final-state neutron, although it finds that directional information does not materially improve sensitivity due to the large nucleon scattering angle.

From BDM flux to event rates

The researchers select a DM model and calculate the BDM flux arriving at Earth from different directions, modeling the Earth attenuation effect and computing the attenuated BDM flux that reaches the detector. Key steps in this process include:

Modeling the BDM flux using a cylindrical leaky-box model of cosmic rays with a height of 2 kpc and a radius of 10 kpc.
Calculating the BDM–nucleus scattering process that emits a neutron using the quasi-elastic regime, determining the rate of neutrons knocked out in different directions inside the detector.
Modeling Earth attenuation by solving a differential equation for the average attenuated BDM KE, K¯χ(Kχ, z), which depends on detector altitude and direction.

Neutron propagation

After the neutron is knocked out, its propagation through the medium is modeled using Geant4 simulations to study secondary light generation and capture events. The process involves:

**: Modeling secondary light emission from charged particles, calculated using the Birks formula for quenched deposited energy. The prompt signal is defined as Eprompt = Esecondary, which includes only the deposited energy from secondary light and excludes the de-excitation energy of the residual nucleus. 2. Reconstructing vertices by combining the reconstructed vertex for secondary light emission and the delayed gamma ray from neutron capture allows for a displacement vector that can preserve partial directional information. 3. Quantifying smearing effects by calculating the neutron capture angle θc between the neutron capture position vector and the initial emission direction, which helps quantify the smearing effect due to neutron diffusion. Vertex resolution is modeled using a normal probability density function to generate reconstructed coordinates around the true position. 4. Accounting for the propagation of neutrons, including final state interaction (FSI) effects by modifying the final KE of the neutron using a nuclear optical potential. 5. Determining event rates by combining the attenuated BDM flux and the QES differential cross section to obtain dNcount dK¯ ′ n domega′ n. The sky is divided into 768 equal-solid-angle patches for integration. 4. **

Sensitivity

The sensitivity of the knockout neutron signals, with and without directional information, is calculated for a 100 kt·yr exposure. The analysis involves:

**: Integrating Eq. (17) to obtain the number of events in each K¯ ′ n bin for each pixel bin, then combining this with the single-neutron capture retention proportion and transfer function to obtain the expected number of events in each prompt-energy bin. 2. Setting an upper prompt-energy bound to 1 GeV because the retention percentage drops sharply when the KE of the knocked-out neutron exceeds 1 GeV. 3. Using a joint likelihood across N bins modeled with a Poisson distribution to obtain the expected number of events λi. 4. Employing the log-likelihood ratio as the test statistic T S = −2 ln L(NBDM) to obtain the one-sided 95% CL sensitivity limits on the spin-independent DM–nucleon scattering cross section σ¯n. The key result is that incorporating directional information improves the sensitivity only mildly, as the nucleon scattering angle already smears the map substantially. 5. Comparing results with previous work shows that the neutron channel yields stronger constraints than the free-proton channel considered in Ref. [1]. The LLSD at mϕ = 1 GeV is comparable with the level expected for Hyper-K with 5 years of data, while DUNE still provides the strongest projected sensitivity in this framework. The conclusion is that solely using neutron directionality cannot improve the constraint by much because the inherent directionality of the CRBDM is smoothed out by the nucleon scattering angle. 5. **

Discussion

The study concludes that although the CRBDM source is focused toward the GC direction, reconstructed neutron signals are smoothed out by the nucleon scattering angle, meaning sole use of nucleon directionality cannot provide a significant improvement. Two optimistic assumptions made in this work likely overestimate the effect from directionality compared to reality: (1) that BDM direction remains unchanged after each scattering during Earth attenuation, and (2) that the knockout direction of the neutron is not affected by FSI. The inherent directionality of the CRBDM mechanism is limited by the DM distribution itself, and neutron emission directions are further smeared by the scattering angle of the knockout. A larger directional gain is expected for channels whose flux is more sharply concentrated toward the GC, such as DM annihilation into BDM or neutrinos, whose J-factor (∝ ρ 2 χ) provides stronger inherent directionality.

Conclusion

The results indicate that for the same exposure LLSD, the neutron-knockout channel yields stronger constraints than the free-proton channel considered in Ref. [1]. At mϕ = 1 GeV, the sensitivity reaches the level expected for Hyper-K with 5 years of data, while DUNE still provides the strongest projected sensitivity in this framework. However, directional information from the CRBDM mechanism does not yield a significant improvement (e.g., ∼ 3% on σ¯n for mχ = 1 MeV, mϕ = 3 GeV), as the inherent directionality of the CRBDM is smoothed out by the nucleon scattering angle. The paper encourages LLSD experiments to exploit directionality in DM searches and continue exploring avenues for better event reconstruction techniques for directionality. 5.

Acknowledgments

SSHT would like to acknowledge helpful conversations with Chingam Fong, Kelly K. K. Chan, and Cheuk Nam Chong. KCYN and SSHT are supported by Croucher Foundation, RGC grants (24302721, 14305822, 14308023), Joint NSFC/GRC grant (N CUHK456/22), and NSFC grant 12322517. KC is supported by the fund from the Institute for Basic Science (IBS-R016-Y2, IBSR016-R1).

References

[8] Z. Z. Liu et al., Phys. Rev. Lett. 123, 161301 (2019), arXiv:1905.00354 [hep-ex].

[7] Y. Meng et al., PandaX-4T, Phys. Rev. Lett. 127, 261802 (2021), arXiv:2107.13438 [hep-ex].

Improvements for AI systems

Boosted Dark Matter Directionality Analysis: The AI system can accurately reconstruct neutron emission vertices by combining prompt signal and delayed gamma ray signals to obtain a displacement vector, preserving partial directional information about the BDM arrival direction.
Background Rejection via Vertex Reconstruction: The system can distinguish between signal and background events by analyzing the reconstructed capture angle theta c, which helps quantify the smearing effect due to neutron diffusion, as shown in cos theta c = q µz / µ2x + µ2y + µ2z.
Sensitivity Calculation for LLSDs: The AI system can calculate 95% CL sensitivity limits on the spin-independent DM nucleon scattering cross section by integrating the attenuated BDM flux and QES differential cross section, comparing results with and without directional information.
Background Modeling for Neutrino Experiments: The system can model atmospheric neutrino neutral-current quasi-elastic scattering as an indistinguishable background, ensuring that only signal events are considered in the final sensitivity analysis.

Abstract

Light dark matter can be boosted by cosmic-ray-dark matter interactions, which opens the possibility of direct-detection searches with large neutrino detectors. We consider the signal produced by boosted dark matter undergoing quasi-elastic scattering off carbon and knocking out a neutron in a large liquid scintillator detector. A distinctive delayed gamma ray from neutron capture can be reconstructed after about 210 μ s, which is paired up with the prompt signal during neutron propagation. We find that 100 kt times yr of data from the neutron-coincidence channel will be able to probe new parameter space compared to free proton signals found in previous work. We further consider the possibility of utilizing directional information from the displaced vertex associated with the capture of the final-state neutron. However, we find that directional information does not materially improve the sensitivity due to the large nucleon scattering angle.