Testing the inelastic dark matter interpretation of the LZ event with heavy-element experiments

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

Vera: We're looking at the paper: "Testing the inelastic dark matter interpretation of the LZ event with heavy-element experiments".

Jocelyn: The gist:

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So, we're looking at this paper called "Testing the inelastic dark matter interpretation of the LZ event with heavy-element experiments". The main idea here is that the signal from that two hundred forty-eight keV event might not be what we think it is if we only consider standard elastic scattering <ref:2610.21041#pg1>.

Jocelyn: Right. Basically, they're suggesting this two hundred forty-eight keV event points toward a model where dark matter isn't just bouncing off nuclei elastically, but something more complex is happening, like inelastic dark matter <ref:2610.21041#pg1>.

Subrahmanyan: The authors focus on the Higgsino as one such possibility, which arises in the minimal supersymmetric standard model or MSSM fifty-eight fifty-nine <ref:2610.21041#pg2>. They're talking about a transition between two nearly degenerate states separated by a mass splitting delta.

Vera: That mass splitting is crucial because it explains why this event might happen, and they note that constraints from solar capture and annihilation exclude a certain range for that splitting, specifically excluding delta less than five hundred sixty-six keV for a thermal Higgsino seventy <ref:2610.21041#pg2,566 keV for a thermal Higgsino 70>.

Jocelyn: And then they bring in the problem of the high-velocity tail of the dark matter distribution. The paper says this tail is where our Standard Halo Model is least trustworthy seventy-two seventy-three <ref:2610.21041#pg2,is least trustworthy 72, 73>.

Subrahmanyan: Exactly. They point out that because of the Large Magellanic Cloud's approach roughly fifty million years ago, this high-velocity tail could be enhanced

seventy-four–seventy-six: , which makes it relevant for direct detection of inelastic dark matter, as pointed out by some of us in Refs <ref:2610.21041#pg2,pointed out by some of us in Refs>. fifty-eight seventy-seven <ref:2610.21041#pg2>.

Vera: The paper asks if we can reconcile the LZ event with both the solar neutrino bound and the lack of other high-energy recoils by having a high-velocity tail that has less dark matter flux than some simulations predict <ref:2610.21041#pg2>.

Jocelyn: It also questions whether this event could come from a different inelastic dark matter model besides the Higgsino, even if the Higgsino fits some of the parameters <ref:2610.21041#pg3>.

Subrahmanyan: The paper lays out how the dependence on that velocity distribution is contained in a time-averaged lab-frame halo integral, which they call eta sub n min vmin(ER) = one √ 2m a ER m u a ER + delta <ref:2610.21041#pg3>.

Vera: And they show some simulations of the LMC effect, comparing the Standard Halo Model to models where the tail flux is at different levels, like one percent or ten percent <ref:2610.21041#pg3>.

Jocelyn: They also discuss how if we look at specific parameters, for instance m chi equal to one point one TeV, the LZ event contours show compatibility with the Higgsino cross section for certain splitting values like delta sub shm around three hundred forty to three hundred sixty keV <ref:2610.21041#pg3>.

Subrahmanyan: But they caution that once you take into account the full efficiency window in figure three b, the lack of events at higher recoil energy shifts those contour regions away from the Higgsino line <ref:2610.21041#pg3>.

Vera: They also mention that smaller splitting values, delta less than two hundred keV, are incompatible because they would predict more events at low recoil energy, but no events below two hundred forty-eight keV were observed <ref:2610.21041#pg3>.

Jocelyn: So, what's the big picture here? This paper is pushing for complementary experiments because while the LZ event fits some models, it doesn't fully pin down everything yet.

Subrahmanyan: The authors argue that heavy-element paleodetectors could cover most of the parameter space relevant to this LZ event <ref:2610.21041#pg3>.

Vera: And they say these detectors use different elements, which would help determine the dark matter mass and splitting, and their sensitivity to the history of the dark matter distribution tests if it's really coming from the LMC <ref:2610.21041#pg3>.

Jocelyn: There's also a possibility of luminous detection with heavy-element supplements if there's a radiative decay for that excited dark matter state, specifically the Higgsino, which would give us a photon with energy equal to delta <ref:2610.21041#pg3>.

Subrahmanyan: If that radiative decay is confirmed, it would provide strong evidence for the Higgsino interpretation because its radiative decay rate is calculable fifty-eight <ref:2610.21041#pg2>.

Vera: The authors conclude that two heavy-element experiments are well suited for this purpose, and they aim to show how correlated signals in different experiments could test this inelastic dark matter interpretation <ref:2610.21041#pg3>.

Jocelyn: The title of the paper, "Testing the inelastic dark matter interpretation of the LZ event with heavy-element experiments", really captures what they're doing: using these new experimental avenues to check that specific theory.

Subrahmanyan: It really boils down to needing these complementary measurements because it's incredibly difficult to break the degeneracy between parameters like mass and mass splitting, but a decay length matching the Higgsino would help avoid that issue <ref:2610.21041#pg3>.

Conclusion: Vera: So, we've been looking at how that two hundred forty-eight keV event could be explained by dark matter scattering, and this paper is all about testing if that interpretation is right using these new experiments called heavy-element experiments <ref:2610.21041#pg3>.

Jocelyn: Exactly. The authors are suggesting that to really nail down what's going on with this signal, we need to look at different types of detectors, not just the ones that caught the original event.

Subrahmanyan: They focus on whether the underlying model is inelastic dark matter, which means it involves a transition between two different energy states in the dark matter particle <ref:2610.21041#pg3>.

Vera: Right. And they use this to try and untangle some confusing parameter relationships, like separating the mass of the dark matter from that mass splitting delta <ref:2610.21041#pg3>.

Jocelyn: They suggest these heavy-element detectors could actually cover most of the parameter space that's relevant to this LZ event, which is a big deal because it means more room to explore possibilities.

Subrahmanyan: It's important because they point out that these detectors have a unique sensitivity to how the dark matter velocity distribution has changed over time, like if it came from the Large Magellanic Cloud <ref:2610.21041#pg3>.

Vera: And they also look at luminous detection, where if that excited dark matter state decays into a photon with energy equal to delta, we could actually see a direct signal of the inelastic nature <ref:2610.21041#pg3>.

Jocelyn: So the big implication is that we're looking at correlated signals across these different kinds of experiments, which should help us figure out if this whole inelastic dark matter thing is actually what's happening.

Subrahmanyan: It really does, because it helps break that difficulty we talked about earlier between parameters like mass and splitting <ref:2610.21041#pg3>.

Vera: We've shown how these two types of experiments fit together, and the authors lay out a clear path forward for testing this specific theory of dark matter.