Boosted dark particles and the LZ nuclear recoil event

Abstract

The LZ experiment reported one anomalous event with nuclear recoil energy of 248 keV. Galactic halo dark matter (DM) cannot produce such a large energy unless heavier than 74 GeV, and heavy DM with spin-independent interactions would have produced unseen recoils at lower energy. Inelastic DM has been proposed as a way out. We consider, instead, an elastic collision of a dark sector particle with a momentum of at least 123~MeV and mass in the range 1-74 GeV as deuteron break-up in SNO disfavors sub-GeV masses. The absence of accompanying lower-energy recoils favours interactions whose rate grows with the momentum transfer, such as a pseudoscalar-pseudoscalar nucleon operator. This implies no leading-order signal in argon, an annual modulation below 1 %, and recoils in spinful light nuclei where inelastic halo DM gives nothing.