Detecting Sub-GeV Dark Matter with Superconducting Nanowires

We propose the use of superconducting nanowires as both target and sensor for direct detection of sub-GeV dark matter. With excellent sensitivity to small energy deposits on electrons and demonstrated low dark counts, such devices could be used to probe electron recoils from dark matter scattering and absorption processes. We demonstrate the feasibility of this idea using measurements of an existing fabricated tungsten-silicide nanowire prototype with 0.8-eV energy threshold and 4.3 ng with 10 000 s of exposure, which showed no dark counts. The results from this device already place meaningful bounds on dark matter-electron interactions, including the strongest terrestrial bounds on sub-eV dark photon absorption to date. Future expected fabrication on larger scales and with lower thresholds should enable probing of new territory in the direct detection landscape, establishing the complementarity of this approach to other existing proposals.

Figure 1

(a) Schematic depiction of the operating principle of SNSPDs: (i) The detector is biased at a current close to the critical value. (ii) When the energy is absorbed by the nanowire, the electrons depart from equilibrium and diffuse out of the formed hot spot. A resistive region formed across the nanowire then leads to a measurable voltage pulse in the readout. (b) The SEM image of the prototype WSi device after fabrication. The active area is $400\times 400\mu {\mathrm{m}}^2$. Nanowires are consistently connected to two contact pads.

Figure 2

The photon counts as a function of the absolute bias current, exhibited by the prototype WSi device tested in a fiber-coupled package at 300 mK.

Figure 3

Expected reach for DM-electron scattering via a light (left) and heavy (right) mediator as a function of DM mass. The solid black curve labeled “Bound WSi” indicates the new bound placed by our prototype device with 4.3 ng exposed for 10 000 s. Other solid curves indicate our background-free 95% C.L. projected reach for either NbN or WSi targets, with various exposures and thresholds. Also shown are the existing constraints from Xenon10 [2] (shaded gray), SuperCDMS [36] (shaded red), and SENSEI [37] (shaded purple), as well as the projected reach for a kg yr exposure of a silicon target [38] (dotted green) and superconducting bulk aluminum with a 10 meV threshold [9, 10] (dotted gray). For clarity, $177\mu \mathrm{g}$ corresponds to a $10\times 10{\mathrm{cm}}^2$ area of NbN at 4 nm thickness and a 50% fill factor, and a 248 (124) meV threshold corresponds to a $5(10)\mu \mathrm{m}$ wavelength.

Figure 4

Expected reach for absorption of relic dark photons. The solid blue curve labeled Bound WSi indicates the new bound placed by our prototype device in this Letter, which showed no dark counts in 4.3 ng over $10^4\mathrm{s}$. The projected reach for a NbN target SNSPD with $177\mu \mathrm{g}$ yr (corresponding to a $10\times 10{\mathrm{cm}}^2$ area with 4 nm thickness and a 50% fill factor) and kg yr exposures is shown (solid magenta curves); NbN reach into lower masses than depicted should be possible and can be estimated from lower energy data should it become available. Also shown is our projected reach for a kg yr exposure of a WSi SNSPD (solid black). The reach for a kg yr exposure of aluminum superconductors [11], semiconductors such as germanium and silicon [44], Dirac materials [12], molecules [45], and polar crystals such as GaAs and sapphire [17] are given as well (dotted curves). Constraints from stellar emission [42, 43] are indicated (shaded orange), along with terrestrial constraints from SuperCDMS [36] (shaded purple), DAMIC [41] (shaded green), and SENSEI [37] (shaded turquoise). Unless otherwise stated, projected reach refers to background-free kg yr exposure.