Boosted Dark Matter Directionality in Large Liquid Scintillator Detectors

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.

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: 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.