Solar capture and annihilation of fermionic and scalar dark matter motivated by the LUX-ZEPLIN high-recoil candidate

The search for dark matter is one of the most profound investigations in modern physics, driven by the overwhelming evidence that the universe contains far more mass than we can see. Astronomers have long observed that stars in galaxies move too fast to be held together by the gravity of visible matter alone, and that light from distant objects bends in ways that require invisible mass to explain. This invisible substance, known as dark matter, is thought to make up about eighty-five percent of all matter in the cosmos, yet it does not emit, absorb, or reflect light, making it impossible to detect with traditional telescopes. Instead, scientists rely on indirect methods, looking for the subtle gravitational tugs dark matter exerts on visible objects or searching for rare collisions between dark matter particles and the atoms in detectors deep underground.

One of the most promising strategies involves looking for these collisions in massive tanks of liquid xenon, where a dark matter particle striking a xenon nucleus would create a tiny flash of light and a small electrical signal. Recently, the LUX-ZEPLIN experiment, a leading detector of this type, reported a candidate event that could be a dark matter collision. This single event, occurring at a specific energy level, has sparked intense interest because it suggests that dark matter might interact with ordinary matter in a way that requires the dark particle to gain a small amount of energy to trigger the collision. This scenario, known as endothermic scattering, implies that dark matter particles come in two forms: a lighter, stable version and a slightly heavier, excited version. To create the signal, a dark matter particle must absorb energy from its collision to jump from the light state to the heavy one. However, this single event does not tell us what the dark matter particle actually is. It could be a type of fermion, a class of particles that includes electrons and protons, or it could be a scalar, a different kind of particle that behaves more like a field than a solid object. Distinguishing between these two possibilities is crucial for understanding the fundamental nature of the universe, but on Earth, the signals they produce in a xenon detector are nearly identical.

To solve this puzzle, a team of researchers turned their attention to the Sun. They asked a simple but profound question: if two different types of dark matter produce the exact same signal in a xenon detector on Earth, can the Sun tell them apart? The Sun acts as a massive gravitational trap, pulling in dark matter particles from the galaxy. When these particles collide with the nuclei of atoms inside the Sun, they can lose enough speed to become trapped in orbit around the solar core. Once trapped, they continue to bounce around, losing more energy with each collision until they settle into a dense, hot cloud at the center. Eventually, these particles meet and annihilate each other, releasing high-energy neutrinos that can be detected by telescopes on Earth. The researchers realized that while the initial capture of these particles might look the same for both fermions and scalars, what happens next inside the Sun could be very different.

The team constructed a detailed simulation to compare two specific models of dark matter: a split fermionic doublet and an inert scalar doublet. They carefully tuned the properties of these two models so that they would produce the exact same number of collisions in the LUX-ZEPLIN detector, effectively making them indistinguishable in a terrestrial experiment. They then tracked the journey of these particles as they fell into the Sun. The key difference lay in how long the excited state of the particle lasted before it decayed back to its ground state. For the fermionic model, the excited state is extremely short-lived, decaying almost instantly. For the scalar model, the excited state is much longer-lived, surviving for days. This difference in timing changes the entire story of what happens inside the Sun.

In the fermionic scenario, the particle decays so quickly that it essentially behaves like a single type of particle that has already lost its extra energy. It cools down efficiently and settles into the solar core, where it annihilates with its partner. In the scalar scenario, the particle hangs around in its excited state for a much longer time. During this extended period, it is more likely to bounce off other atoms and gain speed, potentially escaping the Sun's gravity before it ever has a chance to cool down and settle in the core. The researchers found that this subtle difference in lifetime leads to a measurable difference in the final outcome. While both models successfully capture dark matter and reach a state of equilibrium where capture balances annihilation, the scalar model retains slightly fewer particles in the core. Specifically, the rate at which scalar dark matter annihilates is about five to eight percent lower than that of the fermionic dark matter, even though they started with the same capture rate.

This result is significant because it shows that the Sun can act as a filter, revealing differences between particle models that are hidden in terrestrial detectors. The researchers did not find a way to completely rule out either model, nor did they prove that one is the correct answer to the dark matter mystery. Instead, they demonstrated that the two models, while indistinguishable in a xenon tank, follow different evolutionary paths inside a star. The scalar particles, due to their longer-lived excited state, are slightly more prone to escaping the Sun, leading to a modest reduction in the total number of annihilation events. This difference is small, but it is consistent across a wide range of assumptions about the density of dark matter in our galaxy and the internal structure of the Sun.

The study also looked at the nature of the signals these annihilations would produce. When dark matter particles annihilate, they often produce pairs of W or Z bosons, which are heavy particles that carry the weak nuclear force. These bosons then decay into other particles, including neutrinos. The researchers found that the fermionic and scalar models produce these bosons with different characteristics, such as different polarization states, which would result in slightly different energy spectra for the resulting neutrinos. When they compared their predicted neutrino signals to existing data from the IceCube Neutrino Observatory, they found that the predicted signals for both models were stronger than the current limits set by IceCube for a specific type of dark matter. However, this comparison was made at the source level, meaning it did not account for all the complexities of how neutrinos travel through the Sun or how the detector responds to them. Therefore, this finding does not yet constitute a definitive exclusion of either model, but it highlights that the two models would produce distinct neutrino signatures that future, more detailed analyses could potentially distinguish.

Ultimately, this work provides a new tool for probing the nature of dark matter. It shows that even when two theories predict the same signal in a direct detection experiment, the astrophysical environment of the Sun can expose their underlying differences. The researchers did not discover what dark matter is, but they did show how the Sun can help us tell the difference between two very similar candidates. By combining the data from underground detectors with the physics of stellar evolution, scientists can begin to narrow down the vast landscape of possible dark matter theories. The study confirms that the Sun is not just a passive observer in the search for dark matter, but an active laboratory where the subtle dynamics of particle physics play out on a grand scale, offering a complementary perspective that terrestrial experiments alone cannot provide.