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 ($\nu$) up-scattering, thus producing a massive beyond standard model (BSM) particle $\chi$. 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 ($\nuc$), $\nu \nuc\to \chi \nuc$ can be naturally realized in sterile neutrino models, though we keep our analysis generic without specifying $\chi$. We identify the region of parameter space that can produce such an isolated event assuming a scalar mediator with mass $m_\phi$ and coupling $y_{\chi,q}$. For example, with $m_\chi\sim1$ GeV, and $\sqrt{y_\chi y_q}/m_\phi=2 \times 10^{-2}$ GeV$^{-1}$ can satisfy such an excess of events while remaining allowed by other existing constraints as well.