The paper asks whether a single high-energy nuclear recoil recently reported by the LUX‑ZEPLIN (LZ) experiment can probe a specific way dark matter could have been produced in the early Universe. The authors study a model where the dark matter particle is mostly a “singlet” state that can convert into a slightly heavier “doublet” state. That conversion, not ordinary annihilation, can set the present-day dark matter abundance—a mechanism known as coscattering or conversion-driven freeze-out.

The researchers build a simple particle physics setup: two nearly equal-mass dark fermions (a singlet and a doublet) with a small mass gap of order 100 keV, and a light vector mediator often called a dark photon. The dark photon mixes a little with the ordinary photon, so it can link the dark sector to atomic nuclei. In the early Universe, the rate at which singlet particles convert into the doublet controls when the dark matter abundance freezes out. Today, the same off-diagonal interaction allows the light state to scatter off nuclei into the heavier state, producing an endothermic (energy-absorbing) nuclear recoil that tends to be higher energy.

A key tension is that the small mixing needed for coscattering usually makes direct detection rates tiny. The authors point out that a light dark photon changes this picture. It can enhance the inelastic, endothermic upscattering χ1 + N → χ2 + N and make the recoil rate visible to LZ even when mixing is small. Using a benchmark mediator mass of about 250 MeV, they find the LZ-sensitive region can cover almost the full parameter space compatible with getting the right relic density through conversion, including the regime dominated by coscattering. For heavier mediators, LZ’s reach shifts toward larger singlet–doublet mixing and overlaps less with the coscattering region.

Combining the requirement of reproducing the observed dark matter density and producing a detectable high-energy recoil picks out a narrow band of dark matter masses in the specific model studied. For the mediator masses they considered, viable dark matter masses lie roughly between 750 GeV and 1.6 TeV. This means the same interaction parameters that controlled the particle’s abundance in the early Universe could be tested today by the LZ recoil, linking cosmology and an Earth-based measurement.