  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 to a light dark matter particle. 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 Particle (WIMP)
searches. Furthermore, hydrogen is optimal not only for spin-independent
nuclear scatterings of sub-GeV DM, but also for spin-dependent nuclear
scatterings, where large parameter space remains unconstrained. In this paper,
we first introduce hydrogen-rich crystals, which include water ice, acetylene,
anthracene, trans-stilbene, and naphthalene. These crystals emit two classes of
signals under kinetic excitations. One class of the signals is infrared
photons, which are from optically active fundamental vibrational modes of
molecules and are at corresponding characteristic wavelengths. The other is
acoustic phonons, and optical phonons that decay into acoustic phonons. We then
discuss the technical status and future researches of low-Tc Transition-Edge
Sensor (TES) detectors, which measure single infrared photons and a small flux
of acoustic phonons with desirable sensitivities. With theoretical modeling to
select the hydrogen-rich crystals for the optimized science reach, development
of ultra-sensitive low-Tc TES detectors for readout, and experimentally
characterizing transport properties of photons and phonons in the selected
hydrogen-rich crystals, a direct detection experiment can be built for
measuring the large unexplored parameter space of light DM particles.
