Probing sub-GeV dark matter through dark matter-electron scattering in the superheated $C_2H_2F_4$ target of the InDEx experiment
The Indian Dark Matter Search Experiment (InDEx) operates superheated droplet detectors with tetrafluoroethane ($\mathrm{C_2H_2F_4}$) as the active target at the Jaduguda Underground Science Laboratory (JUSL), and has so far probed dark matter (DM) through nuclear recoils at operating temperatures at which the target is insensitive to electron recoils. In this work, we consider six operating temperatures, $T=45$--$70\,^\circ\mathrm{C}$ in steps of $5\,^\circ\mathrm{C}$; the corresponding Seitz thresholds range from $0.61\,\mathrm{keV}$ down to $26.8\,\mathrm{eV}$. The ionization response of the $\mathrm{C_2H_2F_4}$ target is computed in the isolated-atom approximation from Roothaan--Hartree--Fock wavefunctions. The projected $90\%$ confidence-level upper limits on the DM--electron scattering cross section are derived for a dark-photon-type mediator in both the heavy- and light-mediator limits, for a benchmark exposure of $10^3\,\mathrm{kg\,days}$ under the zero-background assumption. For the lowest threshold, $E_{\rm th}=26.8\,\mathrm{eV}$ at $T=70\,^\circ\mathrm{C}$, the search probes DM masses down to $\sim 12\,\mathrm{MeV}$, with the $90\%$ confidence-level limit reaching $2.7\times10^{-41}\,\mathrm{cm}^2$ near $m_\chi\simeq100\,\mathrm{MeV}$ for the contact interaction and $3.0\times10^{-37}\,\mathrm{cm}^2$ near $m_\chi\simeq83\,\mathrm{MeV}$ for the long-range interaction. The electron channel thus extends the reach of InDEx to sub-GeV DM masses, down to $\mathcal{O}(10\,\mathrm{MeV})$, providing a complementary probe of low-mass dark matter alongside its nuclear-recoil program and enhancing sensitivity to DM candidates coupling predominantly to leptons.