Solar Spin-Dependent Dark Matter-Neutron Cross Section Constraints: The Lost Case
This paper presents the first computation of solar capture and evaporation rates for spin-dependent dark matter-neutron scattering, demonstrating that the resulting constraints on annihilation channels can surpass direct detection limits and, in models with long-lived mediators, probe below the neutrino fog.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
For decades, physicists have been hunting for a ghost. This ghost is dark matter, a substance that makes up most of the universe's mass but refuses to interact with light or ordinary matter in any way we can easily see. We know it is there because its gravity holds galaxies together, yet it remains invisible to our telescopes. To find it, scientists have built massive detectors deep underground, waiting for a dark matter particle to bump into an atom in their equipment. But there is another way to look for this invisible stuff: by watching the Sun. The Sun acts as a giant, natural trap. As the galaxy spins, dark matter particles drift through space, and some of them happen to crash into the Sun's core. If they hit hard enough, they lose speed and get trapped by the Sun's gravity, sinking toward the center. Over billions of years, these trapped particles can pile up, collide with one another, and vanish in a burst of energy that might be detectable from Earth.
For a long time, researchers focused on how dark matter might hit protons or electrons inside the Sun, or how it might scatter off neutrons in a way that doesn't depend on the spin of the particles. However, there was a missing piece in this puzzle. Scientists had largely ignored a specific scenario where dark matter interacts with the spin of neutrons inside the Sun. This is a crucial interaction because, in experiments on Earth, it is one of the most promising ways to catch dark matter. The reason this solar case was overlooked is that the Sun is mostly made of hydrogen and helium, which do not have the right kind of spinning neutrons to catch this specific type of dark matter. The only atoms in the Sun that could catch it are rare, odd-neutron isotopes like helium-3, carbon-13, and oxygen-17. These are present in the Sun only in trace amounts, like a few grains of sand in a vast desert. Because they are so scarce, many scientists assumed the Sun was too weak a detector to say anything useful about this interaction.
A team of researchers has now challenged that assumption, showing that the Sun is not a lost cause for this search. They performed a detailed calculation of how dark matter would scatter off these rare, odd-neutron atoms within the Sun's core. They accounted for how the Sun captures these particles, how they might bounce around and gain energy, and how they could eventually escape back into space if they get too hot. Their work revealed that while the capture rate is indeed much lower than for other types of interactions—roughly 10,000 to 100,000 times weaker than interactions with protons—it is not zero. In fact, for dark matter particles with masses between 100 million electron volts and a few billion electron volts, the Sun still manages to trap enough of them to produce a signal.
The researchers then looked at what happens when these trapped particles annihilate. If they collide and destroy each other, they should produce high-energy neutrinos or gamma rays. The team compared their predictions against data from some of the world's most powerful telescopes, including neutrino detectors like Super-Kamiokande and IceCube, and gamma-ray observatories like Fermi-LAT and HAWC. They found that for certain types of dark matter, the limits set by these solar observations are actually stronger than the best limits from underground detectors on Earth. Specifically, for dark matter that annihilates into neutrinos or into long-lived particles that later decay into gamma rays, the Sun can rule out interaction strengths that terrestrial experiments cannot yet reach.
Perhaps most significantly, this study opens a window into a mass range that has been difficult to explore. For dark matter particles lighter than about 2 billion electron volts, the Sun's own heat can boil them away, a process called evaporation. However, the researchers found that for the specific interaction they studied, the Sun can still hold onto these lighter particles if the interaction is strong enough. This allows them to set constraints on dark matter masses as low as 100 million electron volts, a region where many underground experiments lose their sensitivity. Furthermore, for some scenarios, the solar constraints dip below a theoretical limit known as the "neutrino fog," a background of neutrinos from the Sun that makes it incredibly difficult for Earth-based detectors to distinguish a dark matter signal from natural noise.
The study concludes that the Sun remains a powerful tool for hunting dark matter, even for the elusive spin-dependent neutron interaction that was previously thought to be out of reach. By carefully modeling the rare atoms inside the Sun and comparing them with decades of observational data, the researchers have shown that this channel is far from a lost case. Their work suggests that with current and future telescopes, we can probe dark matter in ways that complement and sometimes surpass the capabilities of our best underground laboratories. The Sun, it turns out, is still whispering secrets about the dark universe, and we are finally learning how to listen.
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