Strong Constraints on Higgsino Dark Matter from Solar Capture

The search for dark matter, the invisible substance that makes up most of the universe's mass, has long focused on a specific type of particle known as a WIMP, or Weakly Interacting Massive Particle. For decades, physicists have hunted for these particles, which are theorized to be heavy and to interact with normal matter only through gravity and the weak nuclear force. Among the many candidates proposed, one stands out for its simplicity and theoretical elegance: the Higgsino. This particle arises naturally in extensions of our current understanding of physics, specifically in theories that suggest a hidden symmetry in nature. If Higgsinos exist, they would be the perfect dark matter candidate, yet they have proven incredibly difficult to find. Their elusive nature stems from a subtle internal structure that allows them to evade detection by the most sensitive instruments on Earth, leaving scientists to wonder if they are hiding in plain sight or if they simply do not exist.

To solve this mystery, a team of physicists turned their gaze away from the Earth and toward the Sun. In a recent study, researchers Maxim Pospelov and Harikrishnan Ramani proposed that the Sun acts as a massive, natural trap for these particles, accelerating them to speeds far beyond what any terrestrial experiment could ever achieve. By analyzing how these particles would behave inside our star, the team discovered a powerful new way to test for their existence. Their work suggests that if Higgsinos are the dark matter filling our galaxy, they should be colliding with the Sun's core so frequently that they would produce a detectable signal of high-energy neutrinos. When they compared their predictions with ten years of data from the IceCube neutrino observatory at the South Pole, they found no such signal. This absence of evidence allowed them to rule out a specific range of properties for the Higgsino, effectively closing the door on a popular explanation for a recent, puzzling event recorded by a different experiment.

The story begins with the unique properties of the Higgsino. Unlike other dark matter candidates, the Higgsino is thought to exist in a "quasi-Dirac" state. This means it is composed of two parts that are almost, but not quite, identical. The tiny difference in their masses, known as the mass splitting, is the key to whether we can detect them. If this difference is very small, the particle can interact with normal matter easily. However, if the difference is larger, the particle becomes "inelastic," meaning it requires a significant kick of energy to interact at all. On Earth, dark matter particles move relatively slowly, typically at speeds of a few hundred kilometers per second. For a Higgsino with a mass splitting larger than a certain threshold, the energy provided by these slow-moving particles is insufficient to trigger a collision with the heavy atoms used in underground detectors. This creates a blind spot for Earth-based experiments, allowing these particles to slip right through the sensors without a trace.

The Sun, however, offers a different environment entirely. As dark matter particles fall toward the Sun, the star's immense gravity accelerates them to incredible speeds. By the time they reach the core, they are traveling at over 1,300 kilometers per second. This extra speed provides the necessary energy to overcome the mass splitting barrier, allowing the Higgsinos to scatter off the heavy elements found in the solar core, such as iron and uranium. When a Higgsino collides with a nucleus in the Sun and loses enough energy, it becomes gravitationally trapped. It cannot escape the Sun's pull and remains stuck in orbit within the star. Over time, these captured particles accumulate in the center, where they eventually collide with one another and annihilate. This annihilation process releases a flood of high-energy neutrinos, ghostly particles that can travel unimpeded through the Sun and the rest of the universe.

The researchers used this mechanism to calculate how many neutrinos should be arriving at Earth if Higgsinos with a specific mass of 1.08 tera-electronvolts were indeed the dark matter. They treated the mass splitting as a variable, testing a wide range of possibilities. They then compared their theoretical predictions against the actual data collected by the IceCube experiment, which has been monitoring the sky for high-energy neutrinos for a decade. The results were stark: IceCube has seen no excess of neutrinos coming from the direction of the Sun. This non-detection places a strict limit on the mass splitting. The team found that if the splitting were smaller than 566 kilo-electronvolts, the Sun would have captured enough Higgsinos to produce a neutrino signal that IceCube would have definitely seen. Since the signal is absent, the mass splitting must be larger than this value.

This finding has immediate and significant consequences for the interpretation of a recent event recorded by the Large Underground Xenon (LZ) experiment. The LZ detector, located deep underground in South Dakota, recently reported a single event that looked like a collision between a dark matter particle and a xenon nucleus. Some physicists proposed that this event could be explained by an inelastic Higgsino scattering, but only if the mass splitting fell within a very narrow window between 340 and 360 kilo-electronvolts. The new analysis from the Sun completely rules out this possibility. The mass splitting required to explain the LZ event is far too small; if such particles existed, they would have been captured by the Sun and annihilated, creating a neutrino flux thousands of times stronger than what IceCube observes. The Sun's silence, therefore, proves that the LZ event cannot be caused by this specific type of Higgsino dark matter.

The study also explored what happens after the initial capture. Once trapped, the dark matter particles do not stay still; they continue to bounce around the solar core, losing energy through further collisions with heavier elements like uranium. Eventually, they slow down enough to sink toward the very center of the Sun. The researchers calculated that even with the most conservative estimates for how these particles interact, the limit on the mass splitting remains robust. They found that the constraint holds true whether the particles interact through standard forces or through more complex, loop-induced processes that are harder to predict. The limit of 566 kilo-electronvolts stands as a solid boundary, excluding a wide range of theoretical models that had previously seemed viable.

In the end, this work demonstrates the unique power of using the Sun as a laboratory. While Earth-based detectors are limited by the relatively low speeds of dark matter in our galaxy, the Sun acts as a natural accelerator, pushing these particles to speeds that unlock interactions otherwise impossible to observe. By combining the physics of solar capture with the observational power of neutrino telescopes, the researchers have provided a stringent test for one of the most compelling dark matter candidates. The absence of the expected neutrino signal does not just suggest that Higgsinos might be heavier or behave differently than hoped; it definitively excludes a specific, well-motivated scenario that had been used to explain experimental anomalies. The Sun, in its quiet, massive way, has spoken, and its message is that the Higgsino dark matter responsible for the recent LZ event simply cannot be there.