Higgsino dark matter compatible with the LUX-ZEPLIN high-energy nuclear-recoil event and IceCube constraints
This paper demonstrates that Higgsino dark matter with masses ranging from approximately 2 TeV to hundreds of TeV and mass splittings between 350 and 525 keV can simultaneously explain the 248 keV nuclear-recoil event observed by LUX-ZEPLIN while remaining consistent with IceCube's null results on dark matter annihilation in the Sun, particularly when accounting for uncertainties in solar cooling mechanisms and potential enhancements from the Large Magellanic Cloud.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Dark matter is the invisible scaffolding that holds the universe together, making up roughly 85 percent of all mass, yet no one has ever directly seen a single particle of it. For decades, physicists have hunted for these particles using massive detectors buried deep underground, hoping to catch a rare collision between a dark matter particle and an atom in the detector. The leading theory suggests these particles are heavy and interact weakly, earning them the name Weakly Interacting Massive Particles, or WIMPs. Recently, a sophisticated experiment called LUX-ZEPLIN, located deep in a South Dakota mine, reported a single, puzzling event: a xenon atom recoiling with a specific amount of energy that standard background noise could not easily explain. This tiny spark of data has reignited the search for a specific type of theoretical particle known as a Higgsino, a heavy partner to the Higgs boson, which might be the missing piece of the cosmic puzzle.
The researchers behind this new study, Katherine Freese and Dionysios P. Theodosopoulos, set out to see if this mysterious event could indeed be caused by Higgsino dark matter. They focused on a unique property of these particles: they exist in two slightly different mass states, separated by a tiny energy gap. When a Higgsino hits an atom in a detector, it can jump from the lighter state to the heavier one, transferring a significant kick to the atom. This "inelastic" collision is exactly what the LUX-ZEPLIN experiment observed, with a recoil energy of 248 keV. However, explaining this single event is only half the battle. The scientists had to ensure their theory did not contradict other powerful observations, specifically from the IceCube Neutrino Observatory at the South Pole, which monitors the Sun for signs of dark matter. If dark matter particles are captured by the Sun's gravity, they should sink to the core, collide, and annihilate, producing a flood of neutrinos. The fact that IceCube has not seen this flood places strict limits on how many dark matter particles can be hiding in our solar system.
The team performed a detailed statistical analysis to map out which combinations of Higgsino mass and energy gap could explain the LUX-ZEPLIN event while remaining invisible to IceCube. They discovered that the most famous candidate, a Higgsino with a mass of 1.1 TeV produced naturally in the early universe, is in serious trouble. When they accounted for the fact that the Large Magellanic Cloud, a neighboring galaxy, might be boosting the speed of dark matter particles in our neighborhood, this standard candidate barely survived the constraints. It required very specific, almost impossible conditions to avoid detection by IceCube. However, the story changes dramatically when the scientists looked at heavier Higgsinos. They found that particles with masses ranging from about 2 TeV up to hundreds of TeV offer a much more comfortable fit. These heavier candidates can produce the observed LUX-ZEPLIN event without generating enough neutrinos to trigger an alarm at IceCube, provided the energy gap between the two particle states falls between roughly 350 and 525 keV.
A crucial part of their work involved understanding how dark matter behaves inside the Sun. Once captured, these particles must lose energy to sink to the center, a process called cooling. The researchers carefully modeled different ways this cooling could happen, realizing that previous studies had made overly optimistic assumptions about how efficiently these particles would settle. By accounting for the fact that cooling might be less efficient than thought, they found that the limits from IceCube are actually more flexible than previously believed. This flexibility allows for a wide range of heavier Higgsinos to exist without violating any known laws of physics or observational data. While the standard 1.1 TeV particle is likely ruled out, the door remains wide open for a heavier, non-standard version of dark matter to be the explanation for the recent discovery.
The study also addressed a second potential hurdle: the absence of other high-energy events in the LUX-ZEPLIN data. The experiment looked at a range of energies higher than the single event and found nothing. The researchers tested whether their proposed Higgsinos would have created a flood of these extra events. They found that for the heavier Higgsinos, the rate of collisions drops off naturally as the mass increases, meaning the single event can exist without a crowd of unwanted neighbors. This makes the heavier candidates even more attractive. The authors conclude that while the simplest version of the theory is struggling, a broader family of heavy Higgsinos remains a very strong candidate for solving the mystery of dark matter, waiting to be confirmed by future data.
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