Threshold effects of inelastic neutrino-dark-matter scattering in AGN spikes

Deep within the centers of some galaxies, supermassive black holes act as cosmic engines, accelerating particles to energies far beyond what any machine on Earth can create. These engines shoot out high-energy neutrinos, ghostly particles that rarely interact with anything, allowing them to travel across the universe and reach our detectors. Around these black holes, astronomers suspect that dark matter—the invisible substance that makes up most of the universe's mass—might pile up into incredibly dense clouds. If these clouds exist, the neutrinos passing through them should bump into the dark matter particles. By studying how these neutrinos change as they travel, scientists hope to learn what dark matter is made of and how it behaves.

A team of researchers has now explored a specific way this interaction could happen, focusing on a scenario where a neutrino hits a dark matter particle and knocks it into a heavier, excited state. Unlike previous ideas where the collision might be a gentle bounce that barely changes the neutrino's speed, this new scenario requires the neutrino to lose a specific, minimum amount of energy to make the jump. The researchers used data from the IceCube Neutrino Observatory in Antarctica, which has detected a steady stream of these high-energy particles coming from a galaxy called NGC 1068. They built a detailed computer model to simulate what would happen if these neutrinos traveled through a dense spike of dark matter surrounding the galaxy's central black hole, accounting for the fact that the dark matter particles would not just bounce off but would change their internal state.

The study reveals that if this type of collision occurs, it leaves a very distinct fingerprint on the neutrino signal. Because every collision forces the neutrino to lose at least a certain amount of energy, the number of neutrinos arriving with energies just above that limit would drop sharply. At the same time, the neutrinos that lost energy would pile up just below that limit, creating a noticeable gap and a buildup in the energy spectrum. This is different from other types of collisions where the energy loss is so small and variable that the signal gets washed out. The researchers calculated exactly how this gap and pile-up would look for different sizes of the energy loss and different widths of the interaction. They found that even if the dark matter particles are moving, which slightly blurs the effect, the signature remains clear enough to be identified.

When the team compared their predictions with the actual data from IceCube, they found no evidence of this sharp gap or pile-up. The neutrinos arriving from NGC 1068 follow a smooth pattern that matches the standard expectation of no interaction with dark matter. This absence of the predicted feature allows the scientists to set strict limits on how often these collisions could be happening. They determined that the probability of a neutrino interacting with dark matter in this way must be very low, specifically ruling out scenarios where the interaction is strong enough to significantly alter the flow of particles. Their analysis showed that for a narrow range of energy loss, the interaction strength must be less than a certain value, and for a broader range, the limit is even tighter.

The researchers also looked at data from two other cosmic sources, a galaxy named CGCG 420-015 and a blazar called TXS 0506+056, to see if combining the information would reveal a signal that was too faint to see in any single source. While the data from these other sources showed a very slight, statistically insignificant hint that might favor the presence of such a feature, the combined analysis did not confirm it. Instead, the joint data reinforced the conclusion that if these collisions are happening, they are happening very rarely. The study effectively narrows the search for dark matter by showing that this specific type of inelastic scattering is not the dominant force shaping the neutrino signals we see today.

Ultimately, the work provides a powerful new tool for testing theories about dark matter. By demonstrating that a specific energy threshold creates a unique and observable distortion in the neutrino stream, the researchers have given experimentalists a clear target to look for. Even though they did not find the signal, the fact that they could calculate exactly where to look and how strong the signal would be is a significant step forward. It means that future observations with more sensitive detectors could either find this dark matter interaction or rule it out completely, helping to solve the mystery of what dark matter is. The study confirms that the universe is not hiding this particular type of interaction in the data we have so far, pushing the boundaries of our understanding of the invisible matter that surrounds the most powerful objects in the cosmos.