Anomalous Background and Calibration of PICO Bubble Chamber Dark Matter Detectors

Abstract

Cosmological evidence strongly suggests that 85% of the matter in the universe is composed of an unknown beyond the standard model material referred to as dark matter. Among many experiments attempting to detect the rare interactions theorized to occur between dark matter and Standard Model particles is the PICO experiment, which searches primarily for spin-dependent WIMP-proton interactions with Fluorine in superheated freon-filled bubble chambers. Despite mitigation efforts, PICO detectors are subject to nuisance particle backgrounds which must be well understood in order to extract a potentially underlying WIMP signal. PICO-40L is currently constructed 2km underground at SNOLAB. Unblinded background data was taken with this detector between November 2023 and February 2024, during which it was occasionally put in a static and insensitive 'idle' operational mode for maintenance. A class of unidentified background events, characterized by their low acoustic parameter (AP), is apparent in the unblinded background data. A log-likelihood analysis shows that the event rate of the low-AP events slightly increases over the few hundred expanded hours after the detector resumes operation following an idle period, with >95% certainty for 8 of 9 idle time thresholds tested. Kolmogorov-Smirnov testing also finds a statistically significant difference in the cumulative distributions of the low-AP events and high-AP alpha events, with >95% certainty for 7 of 9 idle time thresholds tested. The results may indicate a drift in the Seitz threshold of PICO-40L over the time scale of hundreds of hours. The analysis does not produce a means of discriminating the low-AP events from a WIMP signal using the current data. PICO-500 is the up-scaled successor of PICO-40L, and is currently under construction at SNOLAB. The detailed design of the physical structure of the calibration system of PICO-500 is developed and prototyped. A new ultrasonic-sensor based calibration source tracking system capable of operating in a narrow (1.682" inner diameter) pipe environment is also designed, prototyped and tested. It demonstrates the ability to reliably report the position of the calibration source with 1.2" resolution, which exceeds the precision necessary for the comparison of calibration data with simulation results.