Detectability of Light Dark Matter with Superfluid Helium

We show that a two-excitation process in superfluid helium, combined with sensitivity to meV energy depositions, can probe dark matter down to the $\sim \mathrm{keV}$ warm dark matter mass limit. This mass reach is 3 orders of magnitude below what can be probed with ordinary nuclear recoils in helium at the same energy resolution. For dark matter lighter than $\sim 100\mathrm{keV}$, the kinematics of the process requires the two athermal excitations to have nearly equal and opposite momentum, potentially providing a built-in coincidence mechanism for controlling backgrounds.

Figure 1

The two-excitation process we consider and the corresponding kinematics. The dashed lines denote excitations, while solid lines denote dark matter.

Figure 2

95% confidence level sensitivity expected with a 1 kg-year exposure of superfluid helium. We show both two-excitation processes in the superfluid (labeled 2X) as well as ordinary nuclear recoils (labeled NR), with 1 meV energy resolution in the detectors. The results are computed analytically via the formula Eq. (11) (dashed), as well as tabulated from Ref. [40] (solid); we have stopped these curves once the scattering begins to probe kinematic regions beyond that tabulated in Ref. [40]. Also shown are benchmarks based on couplings that are consistent with current limits. For the massive mediator, we assume ${\alpha }_X={10}^{-5}$ for all three curves, while for the light mediator we set ${\alpha }_X={10}^{-19}$.