Reply to "Revisiting bounds on neutrino dark matter interaction at spikes"

Deep in the cosmos, invisible particles known as neutrinos stream through the universe, rarely interacting with the matter they pass. To understand the hidden rules of the universe, physicists often look at how these ghostly particles behave when they encounter something dense, such as the regions around supermassive black holes at the centers of active galaxies. These galaxies are powered by black holes that are surrounded by a dense concentration of dark matter, the mysterious substance that makes up most of the universe's mass but does not emit light. Scientists have long wondered if neutrinos might bump into this dark matter as they travel, and if such collisions could reveal the nature of the dark matter itself. By observing neutrinos that arrive at Earth from these distant cosmic engines, researchers can test theories about how these particles interact with the invisible stuff that fills the space around them.

A team of researchers recently revisited a specific set of calculations they had previously made regarding these interactions. They had used observations from a massive detector in Antarctica, which captures neutrinos from two famous active galaxies, to set limits on how strongly neutrinos might scatter off dark matter. Their earlier work suggested that if dark matter forms a very dense spike around the black hole, the interaction would be strong enough to leave a clear mark on the neutrinos we detect. However, another group of scientists recently pointed out several potential flaws in those calculations, arguing that the original limits might be too strict or based on unrealistic assumptions. The authors of the new paper, James M. Cline and Matteo Puel, have now responded to these criticisms, clarifying where they agree with the new concerns and firmly correcting where they believe the critics have misunderstood the physics.

The authors acknowledge that some of the criticisms are valid, but they emphasize that these points were already addressed in their original studies. They agree that if the probability of a collision between a neutrino and dark matter stays the same regardless of the neutrino's energy, the limits they set are robust. However, they concede that if this probability were to drop significantly as the energy increases, their constraints would become much weaker. They clarify that such a drop in interaction strength is not a common feature in standard physics models. In most realistic scenarios involving dark matter, the interaction probability either stays constant or grows with energy, rather than shrinking. The only way for the interaction to decrease with energy is in very specific and unusual theoretical setups that require dark matter to be a particular type of particle with a specific kind of connection to the force carrier that mediates the interaction. The authors argue that these special cases are not the general rule and should not be used to dismiss the broader constraints.

A significant portion of the response focuses on correcting a misunderstanding about how the particles scatter. The critics had suggested that the mathematical formula used to describe the collision included a term that causes the interaction to weaken at high energies, similar to what happens when two identical particles bounce off each other. The authors point out that this is a mistake because the dark matter particles are not identical to the neutrinos. Since they are different types of particles, the specific mathematical effect that causes the weakening does not exist in this scenario. Therefore, the interaction remains strong and constant at high energies, which supports the original, tighter limits on how much neutrinos can interact with dark matter.

The discussion also touches on the structure of the dark matter around the black hole. Critics had worried that the intense heat and collisions near the black hole would create a flat region where dark matter cannot form a dense spike, which would weaken the signal. The authors counter this by noting that if the dark matter is asymmetric—meaning there is more dark matter than anti-dark matter—this flat region does not form, and the dense spike remains intact. Even if the dark matter is symmetric, they explain that the interaction between the neutrinos and the dark matter creates an effect that changes how the neutrinos behave, effectively suppressing the oscillations that would otherwise allow them to escape detection. This means that even in symmetric scenarios, the dense environment still leaves a detectable imprint on the neutrinos.

Ultimately, this paper serves as a rigorous defense of the original constraints on neutrino-dark matter interactions. The authors confirm that while certain edge cases might weaken the limits, the standard models of particle physics predict a constant or rising interaction strength, which keeps the original constraints valid. They have shown that the criticisms regarding the mathematical description of the scattering and the stability of the dark matter spike do not invalidate their findings. By clarifying these points, the researchers reinforce the idea that observations of neutrinos from active galaxies provide a powerful and reliable way to probe the nature of dark matter, even in the extreme environments surrounding supermassive black holes.