Speaker
Description
The LUX-ZEPLIN (LZ) experiment has reported an event consistent with a $248$ keV nuclear recoil, with no accompanying lower-energy recoil events. In this talk, I present a framework in which the LZ observation has a neutrino origin. A scenario in which atmospheric neutrinos directly upscatter to a heavier neutral state would also produce a large number of neutral-current events, with much larger nuclear recoil energies, in large neutrino experiments. I show that this constraint can be evaded through a two-step process, $\nu\to N_1\to N_2$, in which atmospheric neutrinos within a narrow energy window first produce a nearly monoenergetic state $N_1$, which subsequently upscatters to a heavier state $N_2$ in LZ. For $N_2$ masses near $250$ MeV, scattering on xenon can be kinematically allowed while remaining forbidden on oxygen, carbon, and other relevant targets. We realize the required narrow $N_1$ spectrum through a parametric resonance induced by a dark-matter background, followed by $N_1\to N_2$ upscattering mediated by a vector boson in a gauged $U(1)_B$ model. The latter interaction can be much stronger than the weak interaction, allowing the neutrino floor to be lifted to an $\mathcal{O}(1)$ event rate at LZ.