Light Dark Matter Detection with Hydrogen-Rich Targets and Low- T_c TES Detectors

Direct detection of nuclear scatterings of sub-GeV dark matter (DM) particles favors low-Z nuclei. Hydrogen nucleus, which has a single proton, provides the best kinematic match. The characteristic nuclear recoil energy is boosted by a factor of a few tens from those for larger nuclei used in traditional Weakly Interacting Massive Particles searches. Furthermore, hydrogen is optimal for detecting spin-dependent nuclear scatterings of sub-GeV DM, where large parameter space still remains unconstrained yet. In this paper, we first introduce several hydrogen-rich targets, which emit two classes of signals under kinetic excitations. One class of the signals is infrared photons, which are from fundamental vibrational and rotational modes of molecules and at several characteristic wavelengths. Another is acoustic phonons and optical phonons that decay into acoustic phonons. We then discuss the technical status and future researches of low- T_c transition-edge sensor (TES) detectors, which measure the infrared photons and acoustic phonons with desirable sensitivities. Utilization of hydrogen-rich targets and ultra-sensitive low- T_c TES detectors for light DM detection requires both theoretical modeling and experimental prototyping.