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.