Novel Search for absorption of ultra-light fermionic dark matter on electron with cosmic ray boost

The search for the absorption of fermionic dark matter (DM) on electron is hindered by a partial or complete loss of sensitivity for the mass of DM ($m_{\chi }$) below $\mathcal{O}(10)$ keV. We introduce a novel search using a small but high-energy portion of DM boosted by cosmic rays, and the loss of sensitivity is circumvented because the subsequent absorption within the detector imparts to the target electron not only $m_{\chi }$ but also substantial $T_{\chi }$. The one-sided 90% confidence level constraints on (${\sigma }_e{v}_{\chi }$) are derived with the XENONnT public dataset for $m_{\chi }$ $\in$ (1$\times$10${}^{-5}$–1) keV which has not been explored yet. Our results have placed constraints on DM at the lowest $m_{\chi }$ currently achievable among the DMDD experiments utilizing ionizing signals from DM interactions within the detectors.