Heavy Higgsino Interpretation of the LZ Event

The universe is filled with invisible matter that we cannot see, touch, or smell, yet its gravity holds galaxies together. Scientists call this dark matter, and while we know it exists, we have never directly caught a single particle of it. For decades, the leading theory has been that this dark matter consists of heavy, slow-moving particles that occasionally bump into normal atoms, creating tiny flashes of light or heat. To find these elusive collisions, researchers have built massive detectors deep underground, shielding them from cosmic rays and other background noise. One of the most sensitive of these is the LUX-ZEPLIN experiment, located in a former gold mine in South Dakota, which uses a tank of liquid xenon to watch for the faintest signs of a dark matter particle striking a xenon nucleus.

Recently, this detector reported a single, intriguing event: a nucleus recoiled with an energy of about 248 kilo-electronvolts. This is a specific amount of energy, and in the quiet environment of the detector, it stood out. The question facing the scientific community is what caused this bump. One exciting possibility is that it was a collision with a specific type of theoretical particle called a higgsino. Higgsinos are predicted by a popular extension of the standard model of particle physics and are expected to interact with normal matter through a force carrier called the Z boson. If these particles exist, they should leave a distinct signature, but the details of their mass and how they interact are still a mystery.

A researcher at the Massachusetts Institute of Technology has taken a fresh look at this single event to see if it fits the profile of a heavy higgsino. Their analysis suggests that if this event was indeed caused by a higgsino, it cannot be the kind of particle that was produced in the standard way the universe cooled down after the Big Bang. Instead, the data points toward a particle that is vastly heavier than previously thought possible for this type of dark matter, weighing in between 105,000 and 1,060,000 times the mass of a proton. This finding resolves a conflict between the detector's single event and other observations, but it requires rewriting the story of how the early universe evolved.

The researcher began by testing the most straightforward idea: that the event came from a higgsino with a mass of about 1,100 times that of a proton, a value that would naturally arise if these particles were created in the standard thermal process of the early universe. They found that this specific mass creates a problem. A particle of this weight would not only cause the single event seen at 248 kilo-electronvolts, but it would also be expected to cause several other events with much higher energies. The detector, however, saw nothing in that high-energy range. Furthermore, particles of this mass would be captured by the Sun's gravity, sink to its core, and annihilate to produce high-energy neutrinos. Observations from the IceCube detector in Antarctica have already ruled out this scenario, as they have not seen the expected flood of neutrinos from the Sun.

To solve this puzzle, the researcher performed a detailed statistical fit, adjusting the mass of the higgsino and the energy gap between its different states to see what combination would explain the single event while leaving the high-energy range empty. They discovered that the data is best explained if the higgsino is extremely heavy, with a mass between 105,000 and 1,060,000 times the mass of a proton. In this scenario, the particle is so massive that it moves very slowly relative to the xenon nuclei in the detector. This slowness means it can only transfer a specific, limited amount of energy, creating the 248 kilo-electronvolt bump without generating the higher-energy events that would have been seen if the particle were lighter.

This heavy mass also solves the problem with the Sun. Because the particle is so heavy, the kinetic energy lost in a single collision with atoms inside the Sun becomes insignificant. This makes it very rare for the higgsino to lose enough energy to get trapped by the Sun's gravity. Consequently, they do not accumulate in large numbers, do not annihilate frequently, and do not produce the neutrinos that IceCube would have detected. This allows the heavy higgsino to exist without contradicting the solar observations. However, this solution comes with a new challenge: if such heavy particles were created in the standard way the universe cooled, there would be far too many of them today, far exceeding the amount of dark matter we observe.

To address this overabundance, the author proposes a modification to the history of the early universe. They suggest that after the Big Bang, the universe went through a period where a different type of energy dominated, rather than the usual radiation. During this time, a heavy particle decayed and released a burst of heat, effectively diluting the number of higgsinos that had already formed. This process would reduce their numbers to the correct level we see today. The researcher constructed a simple model to show how this could happen, linking the mass of the higgsino to a mechanism that also explains why the strong nuclear force behaves the way it does and why the universe has a specific property called the strong CP problem.

The study concludes that while the single event at the LZ detector is tantalizing, it cannot be explained by the standard, lighter version of the higgsino. The data is consistent only with a much heavier particle, provided the early universe underwent a specific, non-standard evolution to keep their numbers in check. The author notes that their model is a proof of concept, demonstrating that such a scenario is physically possible, but it relies on assumptions about the early universe that have not yet been proven. If future experiments confirm the existence of these heavy particles or find evidence for the early universe conditions they propose, it would open a new chapter in our understanding of the cosmos. Until then, the single event remains a mystery, but one that points toward a universe far more complex and varied than previously imagined.