Galactic Endothermic Production and Exothermic Detection of Excited Dark Matter: Implications for LUX-ZEPLIN

Abstract

The LUX-ZEPLIN experiment reported a candidate nuclear recoil at $248\mathrm{keV}$. This work proposes that endothermic self-scattering ${\chi }_1+{\chi }_1\to {\chi }_2+{\chi }_2$ in the Galaxy can produce an excited dark matter population, whose exothermic scattering ${\chi }_2+\mathrm{Xe}\to {\chi }_1+\mathrm{Xe}$ in xenon can account for the LZ candidate event. In a benchmark with a light scalar mediator, the conversion cross section is large enough for high-velocity Galactic dark matter particles to generate an excited fraction at the percent level. For dark matter masses near the TeV scale and mass splittings $\delta \gtrsim 500\mathrm{keV}$, the xenon recoil spectrum covers the candidate region and can yield an $\mathcal{O}(1)$ event count. The ${\chi }_2$ population produced in the Galaxy is colder than a relic population from the early Universe and yields a different nuclear recoil spectrum.

arXiv comment14 pages, 3 figures