Atmospheric neutrino up-scattering explanation of LZ 2026 excess

Abstract: The recent observation of an isolated nuclear recoil at $248\pm 23\pm 23$ keV energy by LUX-ZEPLIN (LZ) experiment has motivated the community to look for a new physics explanation, as the Standard model background estimation fails to accomodate that. Most of the existing literature hitherto considers a galactic halo dark matter with a heavier partner. In this work, we traverse the alternate route of atmospheric neutrino ($ν$) up-scattering, thus producing a massive beyond standard model (BSM) particle $χ$. The kinematic requirement of such a scattering poses a cut off in the lower recoil energies providing an explanation of the unique isolated event at such a higher recoil energy. Such up-scattering with the nucleons ($\nuc$), $ν\nuc\to χ\nuc$ can be naturally realized in sterile neutrino models, though we keep our analysis generic without specifying $χ$. We identify the region of parameter space that can produce such an isolated event assuming a scalar mediator with mass $m_φ$ and coupling $y_{χ,q}$. For example, with $m_χ\sim1$ GeV, and $\sqrt{y_χy_q}/m_φ=2 \times 10<sup>{-2}$ GeV$<sup>{-1}$ can satisfy such an excess of events while remaining allowed by other existing constraints as well.