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 ($\text{nuc}$), $\nu \text{nuc}\to \chi \text{nuc}$ 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 $m_{\varphi }$ and coupling $y_{\chi ,q}$. For example, with $m_{\chi }\sim 1$ GeV, and $\sqrt{y_{\chi }{y}_q}/m_{\varphi }=2\times 10^{-2}$ GeV${}^{-1}$ can satisfy such an excess of events while remaining allowed by other existing constraints as well.

Comments: 4 pages, 2figures