New Constraints on Cosmic Ray-Boosted Dark Matter from the LUX-ZEPLIN Experiment.

While dual-phase xenon time projection chambers have driven the sensitivity toward weakly interacting massive particles at the GeV/c^{2} to TeV/c^{2} mass scale, the scope for sub-GeV/c^{2} dark matter particles is hindered by a limited nuclear recoil energy detection threshold. One approach to probe for lighter candidates is to consider cases where they have been boosted by collisions with cosmic rays in the Milky Way, such that the additional kinetic energy lifts their induced signatures above the nominal threshold. In this Letter, we report first results of a search for cosmic ray-boosted dark matter (CRDM) with a combined 4.2 metric ton/yr exposure from the LUX-ZEPLIN experiment. We observe no excess above the expected backgrounds and establish world-leading constraints on the spin-independent CRDM-nucleon cross section as small as 3.9×10^{-33} cm^{2} at 90% confidence level for sub-GeV/c^{2} masses.