Nuclear Recoils from Invisible Neutron-Pair Annihilation and the LZ event

Abstract: Two bound neutrons can annihilate into a single invisible scalar carrying baryon number two, producing a monochromatic nuclear recoil. Pair removal connects the $0^+$ ground states of even-even nuclei and, for several detector isotopes, leaves a stable daughter where single-neutron removal would leave a radioactive one. Below the first daughter excitation, the leading transition produces neither nuclear de-excitation nor subsequent daughter decay. We identify the corresponding mass windows in argon and xenon, including argon recoils up to $108\,\mathrm{keV}$, and obtain an approximate partial-lifetime bound of $8.5 \times 10<sup>{26}\,\mathrm{yr}$ from DEAP-3600 data. Recoil lines in different isotopes reconstruct a common invisible-particle mass. The emitted scalar can itself be dark matter, making these searches a probe of ordinary matter converting into a dark sector.