Heavy Higgsino Dark Matter With Low Reheating in View of the LZ High-Recoil Event

The universe is filled with a mysterious substance called dark matter, an invisible substance that does not emit light but exerts a gravitational pull strong enough to hold galaxies together. While scientists have mapped its effects across the cosmos, they have yet to identify the specific particle that makes it up. One leading candidate is a heavy version of a particle known as the higgsino, a theoretical partner to the Higgs boson that arises in models of supersymmetry. For decades, researchers have searched for these particles using massive detectors buried deep underground, hoping to catch a rare collision between a dark matter particle and an atomic nucleus. Recently, the LUX-ZEPLIN experiment reported a single, high-energy event that could potentially be such a collision, sparking intense interest in whether a heavy higgsino could explain this signal without contradicting other astronomical observations.

A new study by Constantininos Pallis explores how a very heavy higgsino, with a mass around 0.1 PeV, could account for the dark matter in our universe while remaining consistent with this recent experimental signal. The challenge is that if these particles were created in the standard way after the Big Bang, there would be far too many of them left over today, far exceeding the amount of dark matter we actually observe. To solve this, the author proposes a scenario where the early universe reheated to a much lower temperature than previously thought. In this model, the universe was dominated for a time by a heavy, unstable particle called a modulus, which decayed slowly. This decay process not only set the temperature of the universe but also produced the dark matter particles in a controlled, non-standard way, reducing their final abundance to the precise level observed today.

The study suggests that this specific heavy higgsino can explain the single event seen by the LUX-ZEPLIN detector while avoiding conflicts with other data, such as limits from neutrino telescopes that monitor the Sun. If the dark matter particles were too light or too abundant, they would have been captured by the Sun and annihilated, producing a flood of high-energy neutrinos that telescopes like IceCube would have detected. By pushing the mass of the higgsino up to the PeV scale and lowering the reheating temperature of the universe to below 595 GeV, the model successfully suppresses these unwanted signals. The author demonstrates that this delicate balance is achievable if the modulus particle decays into the dark matter particles with a specific frequency, a process that naturally arises in certain theories of gravity and particle physics.

To make this work, the paper identifies the modulus particle with a specific component of a field responsible for breaking supersymmetry, a theoretical framework that pairs every known particle with a heavier partner. This connection is significant because the same mechanism that generates the mass of the modulus also helps create the mass parameter for the higgsino, linking the origin of dark matter to the fundamental structure of the theory. The calculations show that for the model to fit the data, the universe must have undergone a period of low reheating, where the temperature never rose high enough to produce the excess dark matter that would otherwise ruin the fit. This low-temperature phase is driven by the slow decay of the modulus, which acts as a cosmic thermostat, ensuring that the final amount of dark matter matches the observations.

The research outlines several specific scenarios, or benchmark points, that satisfy all the necessary conditions. In some cases, the dark matter is produced through thermal processes where particles interact and reach equilibrium, while in others, it is produced directly from the decay of the modulus without ever reaching equilibrium. The study finds that the most viable solutions involve a non-zero rate of direct production from the modulus decay, which allows the reheating temperature to be even lower. This lower temperature is crucial because it prevents the overproduction of dark matter and keeps the model consistent with the constraints from the Sun and other cosmic observations. The author notes that while the model requires specific values for the masses and decay rates, these values are not arbitrary but are derived from a coherent theoretical framework that connects the early universe to the properties of the Higgs boson.

Ultimately, the paper presents a self-consistent picture where a heavy higgsino serves as the dark matter, explaining a recent experimental hint without violating other physical laws. It suggests that the universe's history was more complex than the standard model assumes, involving a prolonged period dominated by a decaying particle that set the stage for the matter we see today. By carefully tuning the temperature of the early universe and the way particles were produced, the model reconciles the heavy mass of the higgsino with the observed abundance of dark matter. This work does not prove that the heavy higgsino is the answer, but it shows that such a particle is a viable candidate if the early universe followed this specific, low-temperature path. The findings offer a concrete target for future experiments, suggesting that if the LUX-ZEPLIN event is indeed a dark matter detection, the universe's thermal history must have been significantly cooler than previously believed.