The LUX‑ZEPLIN (LZ) experiment reported a single nuclear‑recoil candidate with recoil energy around 248 keV. This paper asks whether that event can be explained if dark matter comes in two nearly equal states, a light ground state and a slightly heavier excited state, and the local halo contains some fraction of the excited particles. The authors show that when the excited state survives to today, scattering that releases energy (exothermic scattering) can dominate and change how the event should be interpreted.

The particle picture is simple at a high level. A single species of dark matter can split into two states, χ1 (lighter) and χ2 (heavier), that differ by a small mass splitting δ. If a χ1 particle scatters and becomes χ2, the process is endothermic: it requires extra kinetic energy and tends to favor high‑velocity dark matter. If a χ2 particle scatters and becomes χ1, the process is exothermic: the mass difference is released to the nucleus and even slow particles can produce large recoils. Endothermic scattering therefore has a velocity threshold and a kinematic cutoff, while exothermic scattering does not and can produce a sharp peak in recoil energy.

Because LZ reported only public projections of their data and not the full unbinned likelihood, the authors rebuilt the experiment’s statistical analysis from public figures. They used a binned likelihood based on LZ’s published slices and their detector response model, and they took care to use a nuclear response model that matches LZ’s treatment near the observed recoil. They validated their reconstruction against LZ’s published intervals. They also used toy Monte Carlo simulations to build test statistics, because the signal region contains very few events and standard asymptotic formulas for significance are not reliable.

Applying this inference to a halo that contains a fraction f of excited‑state dark matter, the authors find that even a tiny excited‑state fraction can change the result. For f roughly between 10^−5 and 10^−2 (depending on the mass splitting) the exothermic contribution already dominates. If half the local dark matter is in the excited state (f = 1/2), the dark‑matter–nucleon cross section required to explain the single event falls by factors between about 80 and 1.6×10^4 for mass splittings δ in the range 250–350 keV. They also note a concrete implication: a benchmark point that would explain the event assuming only χ1 would lie about a factor of 30 above the experimental upper limit if half the halo were χ2.