Probing low-mass WIMP candidates of dark matter with tetrafluoroethane
  superheated liquid detectors
  Probing low mass (sub-GeV -- few GeV) Weakly Interacting Massive Particle
(WIMP) candidates of dark matter through WIMP-induced nuclear recoils in direct
detection experiments requires use of detector materials consisting of low mass
target nuclei and low threshold energy. Here we explore the potential of
superheated liquid detectors (SLD) with a hydrogen containing liquid, namely,
tetrafluoroethane (C$_{2}$H$_{2}$F$_{4}$) (b.p.~$-26.3^\circ\,$C), as the
target material for probing low mass WIMPs. It is found that few-keV level
recoil energy thresholds possible for bubble nucleation by WIMP-induced
$^{12}$C and $^{19}$F recoils in C$_{2}$H$_{2}$F$_{4}$ SLDs operated at
atmospheric pressure and gamma-ray insensitive temperatures of $T\, \lesssim\,
35^\circ\,$C have the potential to allow WIMPs in the few-GeV mass range to be
probed at a WIMP-nucleon spin-independent cross section sensitivity levels (90%
C.L.) better than $4.6\times 10^{-5}\,$pb at WIMP masses down to $\sim$ 4 GeV
with a total exposure of $\sim$ 1000 kg.day, provided that the ``thermodynamic
efficiency" $\eta_{\rm T}$ that determines the bubble nucleation thresholds for
the recoiling nuclei in C$_{2}$H$_{2}$F$_{4}$ is $\sim$ 50% or higher.
Sensitivity to sub-GeV WIMP masses generally requires the detector to be
sensitive to the WIMP-induced $^1$H recoils, which in turn requires the
detector to be operated at temperatures $T\gtrsim 50^\circ\,$C and $\eta_{\rm
T}$ close to 100%. At such relatively high temperatures (at atmospheric
pressure), however, the detector would be sensitive to background gamma rays.
