The search for dark matter remains one of the central challenges in particle physics. The LUX-ZEPLIN (LZ) experiment, using a liquid xenon detector, accumulated an exposure of 2.84 tonne-years and for the first time explored the high-energy nuclear recoil region up to 270 keV. In this region, a single event with an energy of 248 keV was recorded, which could not be explained by any of the considered background processes. This work stands alongside studies that search for dark matter manifestations through rare events in detectors, but it differs by proposing a specific theoretical model, the higgsino, to explain an already observed event rather than just setting limits.
The authors used a supersymmetric theoretical model in which dark matter is represented by the higgsino, the superpartner of the Higgs boson. In this model, the two neutral higgsino states are nearly mass-degenerate, and their splitting δ determines the kinematics of inelastic scattering on xenon nuclei. The higgsino-nucleon interaction cross section is fixed by the electroweak interaction and is not a free parameter. To compare with LZ data, the authors recast the published confidence intervals into a vector interaction normalization and estimated the impact of a higgsino mass of 1.1 TeV on the expected event rate using the WimPyDD code.
For a higgsino mass of 1 TeV and a mass splitting δ = 350 keV, the predicted inelastic scattering cross section σ ≈ 1.86×10⁻³⁹ cm² falls within the two-sided 90% confidence interval published by LZ. When considering a more realistic mass of 1.1 TeV, which is implied by the thermal relic density, the estimated interval shifts downward by about 20%, and the higgsino prediction falls slightly below the lower bound. However, the authors note that their estimate does not account for changes in the recoil spectrum shape, and a precise conclusion requires a full likelihood analysis. With a slight increase in the mass splitting above 350 keV, the prediction again falls within the interval.
This work shows that the higgsino is a viable candidate to explain the LZ event, linking the observed signal to a specific and predictive model. Unlike neighboring studies that either set constraints on generic parameters or explore other candidates, here the interaction cross section is fixed, making the interpretation more rigorous. If the event is indeed caused by a higgsino, it would be the first direct hint of supersymmetry and would pin down the dark matter mass.
The topic could advance through a more precise analysis of LZ data that incorporates the recoil spectrum shape for a mass of 1.1 TeV. Additional data with larger exposure will help determine whether the event is a statistical fluctuation or a real signal. Searches for the higgsino in other experiments, such as argon-based detectors or future collider facilities, are also possible.
The results will influence the interpretation of direct dark matter search data and the choice of strategies for future experiments. They also stimulate theoretical studies of supersymmetric models with heavy gauginos.
The next step should be a full likelihood analysis of LZ data for a higgsino mass of 1.1 TeV, taking into account the spectral shape. Independent checks in other detectors and a refined calculation of the relic density for the higgsino are also important.
This work is directly connected to the problem of the nature of dark matter, which has remained unsolved for several decades. It also touches on the question of whether supersymmetry exists at high energies, a key open problem in physics beyond the Standard Model.
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What the work does
The authors interpret a nuclear recoil event with an energy of 248 keV in the LUX-ZEPLIN detector as a possible inelastic scattering of the Higgsino, the supersymmetric partner of the Higgs boson. For a Higgsino mass of about 1 TeV and a mass splitting of δ ≈ 350 keV, the predicted cross section falls within the LZ confidence interval. They note that at a mass of 1.1 TeV the prediction falls below the interval, but they emphasize the need for a full likelihood analysis.
Where its strength is
The strength of the work lies in using a specific, predictive model with a fixed interaction cross section, which allows linking the observed event to a particular dark matter candidate and its parameters.
Where it could go next
Worth trying
Why this may matter
If the LZ event is indeed caused by the Higgsino, it would be the first hint of supersymmetry at high energies and would allow determining the dark matter mass, thereby connecting collider and astrophysical observations.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.58, against a usual 0.64. It looks like few have drilled nearby.
The energy of the event, 248 keV, is about a million times smaller than the energy carried by protons in the Large Hadron Collider, yet such low-energy nuclear recoils could reveal the presence of heavy dark matter particles.