Dark Matter Search Results from the Complete Exposure of the PICO-60 C$_3$F$_8$ Bubble Chamber (1902.04031v1)

Abstract: Final results are reported from operation of the PICO-60 C$_3$F$_8$ dark matter detector, a bubble chamber filled with 52 kg of C$_3$F$_8$ located in the SNOLAB underground laboratory. The chamber was operated at thermodynamic thresholds as low as 1.2 keV without loss of stability. A new blind 1404-kg-day exposure at 2.45 keV threshold was acquired with approximately the same expected total background rate as the previous 1167-kg-day exposure at 3.3 keV. This increased exposure is enabled in part by a new optical tracking analysis to better identify events near detector walls, permitting a larger fiducial volume. These results set the most stringent direct-detection constraint to date on the WIMP-proton spin-dependent cross section at 2.5 $\times$ 10$^{-41}$ cm$^2$ for a 25 GeV WIMP, and improve on previous PICO results for 3-5 GeV WIMPs by an order of magnitude.

• The paper presents the most stringent direct-detection constraint on the WIMP-proton cross-section, achieving 2.5×10⁻⁴¹ cm² for a 25 GeV WIMP.
• The paper employs a superheated C₃F₈ bubble chamber with 1404 kg-day exposure at a 2.45 keV threshold using advanced acoustic and optical calibration techniques.
• The paper demonstrates enhanced fiducial volume determination via improved bubble tracking, significantly advancing direct dark matter detection methodologies.

Summary of Dark Matter Search Results from the PICO-60 C$_3$F$_8$ Bubble Chamber

The research presented in this paper comprises significant results from the PICO-60 dark matter detector, which utilizes a C$_3$F$_8$ bubble chamber situated at the SNOLAB underground laboratory. The experiment seeks to directly detect weakly interacting massive particles (WIMPs), a leading candidate for dark matter. The operation of the bubble chamber at various thermodynamic thresholds offers a platform for operating a superheated liquid detector to achieve substantial exposure while mitigating background noise.

Key Findings

The paper reports on a comprehensive exposure of 1404 kg-days at a \SI{2.45}{\keV} threshold, improving on previous exposures both in terms of threshold reduction and efficiency. Critically, this exposure resulted in the most stringent direct-detection constraint on the WIMP-proton spin-dependent cross-section reported to date, namely $2.5 \times 10^{-41}$ cm$^2$ for a \SI{25}{\GeV} WIMP.

Furthermore, advances in fiducial volume determination facilitated the expansion of the detector's sensitive region due to improved bubble position tracking technologies and a sophisticated optical analysis for discerning events proximal to the chamber walls. The position determination algorithm enhancements allowed the identification of bubble trajectories, aiding the increase in confident rejection of non-bulk event candidates.

Methodology and Results

The PICO-60 detector uses C$_3$F$_8$ to leverage its favorable nuclear recoil properties and intrinsic rejection capabilities against electron recoils from gamma and beta interactions. The PICO experiment uniquely employs acoustic detection to discriminate alpha decays. The acoustic parameter (AP) was optimized using calibration data, and further fidelity was achieved by a newly implemented tracking algorithm.

The experiment successfully operated under low-pressure conditions, maintaining system stability at thresholds below \SI{1.2}{\keV}, albeit with increased sensitivity to background electrons. The likelihood of bubble formation for nuclear recoils was gauged using extensive neutron calibrations, including new datasets at lower energies for improved calibration precision.

Implications

The detection results underscore the capabilities of superheated liquid detectors in dark matter searches, especially relating to improving constraints on WIMP models, specifically for low-mass WIMPs. This highlights the potential of future enhancements in both target mass and detector sensitivity to further probe the parameter space of WIMP interactions.

Future Considerations

The evolution of machine learning approaches, as referenced for future PICO detectors, points toward upcoming improvements in real-time event classification accuracy—potentially enhancing the discrimination capability against acoustic signature overlaps in WIMP search datasets.

The development of more nuanced models of gamma response and the application of advanced calibration methodologies constitute critical next steps for increasing event classification efficacy and reaching more profound background suppression levels.

The current and planned enhancements for detectors such as the PICO-60 are expected to continue pushing the boundary conditions of direct WIMP detection parameter spaces, offering a deepened understanding of dark matter characteristics. The convergence of theoretical models—integrating nuclear form factors and effective fields—further guides the interpretation and reach of observational data gathered from these high-fidelity experiments.