Higgsino Dark Matter Interpretation of the LZ High-Recoil Event in the GNMSSM with TeV-Scale Gauginos

For decades, the search for dark matter has been a hunt for a ghost. Scientists know this invisible substance makes up most of the matter in the universe, holding galaxies together with its gravity, yet it refuses to interact with light or ordinary matter in any way we can easily detect. The leading theory suggests these particles are "weakly interacting massive particles," or WIMPs, which should occasionally bump into the nuclei of atoms in deep underground detectors, creating a tiny flash of energy. However, for years, the most sensitive detectors have found nothing but silence, ruling out many of the simplest ideas about what dark matter could be. Recently, a major experiment called LUX-ZEPLIN, buried deep beneath a mine in South Dakota, reported a single, puzzling event. A xenon atom in the detector seemed to recoil with an energy of about 248 kilo-electron volts, a value far higher than the gentle taps expected from standard dark matter collisions. This single blip, occurring where almost no background noise was expected, sparked a new question: could this be a sign of a more complex kind of dark matter that behaves differently than the simple models predicted?

A team of physicists led by researchers at Zhengzhou University has taken this anomaly and built a new theoretical framework to explain it, moving beyond the standard models that have dominated the field. They focused on a specific type of dark matter candidate known as a Higgsino, a particle that arises naturally in theories extending the Standard Model of physics. In the simplest version of these theories, a Higgsino dark matter particle would need to be incredibly heavy, around 1.1 trillion electron volts, and would require other heavy particles to be millions of times heavier than anything we can currently create in particle accelerators. This extreme separation of scales makes the theory feel unnatural and difficult to accept. The researchers proposed that by adding a single new ingredient to the theory—a "singlet" particle that does not interact with the known forces—they could create a scenario where the Higgsino behaves exactly as the LUX-ZEPLIN event suggests, without needing those impossible masses.

The core of their work involves a process called inelastic scattering. In a standard collision, a dark matter particle hits an atom and bounces off, transferring a small amount of energy. In this new scenario, the dark matter particle hits the atom and simultaneously transforms into a slightly heavier version of itself. This transformation requires a specific amount of energy, like a toll that must be paid to cross a bridge. Because the dark matter particle must pay this toll, it needs to be moving very fast to have enough energy to make the switch. This requirement filters out the slow-moving dark matter that usually fills the galaxy, leaving only the fastest particles to cause a collision. When these fast particles do collide, they transfer a large amount of energy, creating the high-energy recoil seen in the detector. The researchers calculated that for this to produce the specific 248 keV signal, the mass difference between the two versions of the dark matter particle must be very small, roughly 330 to 350 thousand electron volts.

To make this work, the team utilized a model called the General Next-to-Minimal Supersymmetric Standard Model. In this model, the Higgsino mixes with the new singlet particle. This mixing is the key to solving the puzzle. In older theories, creating such a tiny mass difference between the two dark matter states required the other heavy particles in the theory to be impossibly massive, creating a rigid and unnatural structure. In this new model, the mixing with the singlet provides a second way to generate that mass difference. This new contribution can cancel out or balance the old one, allowing the mass difference to be just right while keeping the other heavy particles at a much more reasonable scale of a few trillion electron volts. This removes the need for the extreme, unnatural hierarchy of masses that plagued previous attempts to explain the event.

The researchers tested this idea by creating six specific examples, or "benchmark points," of how the universe could look under these rules. In each example, they adjusted the masses and interaction strengths of the particles to see if they could reproduce the observed event while also satisfying all other known laws of physics. They found that in all six cases, the model successfully produced the correct amount of dark matter in the universe today, matching the precise measurements made by cosmologists. They also checked that these models would not have been ruled out by other experiments looking for dark matter, or by searches for new particles at the Large Hadron Collider. The results were consistent across the board. The models predicted that the dark matter particles would have masses ranging from about 660 to 1,100 billion electron volts, a range that is much more flexible than the fixed 1.1 trillion electron volts required by the older theories.

Perhaps most importantly, the new model changes how the dark matter interacts with the sun. In the older, rigid theories, the same properties that made the dark matter interact with the detector also meant it would be captured by the sun's gravity and annihilate, producing high-energy neutrinos that should have been detected by the IceCube telescope. The lack of such neutrinos was a major problem for the old theory. In this new model, the mixing with the singlet particle weakens the interaction between the dark matter and the detector, and also changes the mass of the dark matter. This combination significantly reduces the rate at which the dark matter is captured by the sun, meaning the model can survive the IceCube constraints that would have otherwise ruled it out. The researchers calculated that their best-fitting scenario matches the LUX-ZEPLIN data very closely, with a statistical measure of agreement that is better than almost any other point they tested.

This work does not claim to have proven that dark matter is a Higgsino or that the LUX-ZEPLIN event was definitely a dark matter collision. It is a theoretical demonstration that such an event is possible within a specific, well-motivated extension of physics. The researchers have shown that by introducing a single new type of particle, the universe can accommodate a high-energy recoil event without requiring the extreme and unnatural conditions of previous models. They have provided a concrete path forward, suggesting that if future experiments confirm this signal, the answer may lie in a version of supersymmetry where the heavy particles are not millions of times heavier than the light ones, but are instead within a more accessible range. The study offers a clear, self-consistent picture of how the anomaly could arise, turning a single, mysterious blip into a potential window into a more complex and flexible reality.