The primary advantage of moderately superheated bubble chamber detectors is their simultaneous sensitivity to nuclear recoils from weakly interacting massive particle (WIMP) dark matter and insensitivity to electron recoil backgrounds. A comprehensive analysis of PICO gamma calibration data demonstrates for the first time that electron recoils in ${\mathrm{C}}_3{\mathrm{F}}_8$ scale in accordance with a new nucleation mechanism, rather than one driven by a hot spike as previously supposed. Using this semiempirical model, bubble chamber nucleation thresholds may be tuned to be sensitive to lower energy nuclear recoils while maintaining excellent electron recoil rejection. The PICO-40L detector will exploit this model to achieve thermodynamic thresholds as low as 2.8 keV while being dominated by single-scatter events from coherent elastic neutrino-nucleus scattering of solar neutrinos. In one year of operation, PICO-40L can improve existing leading limits from PICO on spin-dependent WIMP-proton coupling by nearly an order of magnitude for WIMP masses greater than $3\mathrm{GeV}{\mathrm{c}}^{-2}$ and will have the ability to surpass all existing non-xenon bounds on spin-independent WIMP-nucleon coupling for WIMP masses from 3 to $40\mathrm{GeV}{\mathrm{c}}^{-2}$.

• Received 1 July 2019
• Corrected 12 June 2020

DOI:https://doi.org/10.1103/PhysRevD.100.082006