New ideas for tonne-scale bubble chambers and a search for superheavy dark matter with PICO-60

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Broerman, Benjamin
Dark matter , Bubble chambers , PICO
The PICO dark matter search experiments using bubble chambers have been successful in probing the potential WIMP-proton scattering cross section. The largest of these experiments (PICO-60 and PICO-40) contain approximately 50~kg of the superheated liquid Freon C$_3$F$_8$ in a quartz vessel. Future larger-scale bubble chambers will face technical challenges in their construction and operation. To overcome the construction challenges, the feasibility of alternative chamber materials to quartz in the form of acrylic and stainless steel, which are easier to construct in large sizes, is investigated in small-scale test chambers. The first C$_3$F$_8$~and C$_4$F$_{10}$~mixture in a bubble chamber, which could alleviate the demands on the thermal performance in large detectors, is further demonstrated confirming acoustic discrimination between nuclear recoils and alpha particles. The gamma rejection in this mixture and pure C$_4$F$_{10}$~is additionally measured for the first time. The acoustic response of bulk nucleations is found to decrease exponentially with increasing pressure while increasing exponentially with increasing temperature. This model will inform the future operating conditions of large bubble chambers which need to reach nuclear recoil thresholds above 10~keV to avoid backgrounds from the coherent elastic neutrino nucleus scattering of solar neutrinos. Finally, existing PICO-60 data is reanalyzed in a search for superheavy dark matter through its unique multi-scatter signature producing novel limits on carbon and fluorine. For fluorine, spin dependent scattering is probed up to $4\times10^{16}$ GeV$/c^2$.
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