Astrophysical observations, from galactic to cosmological scale, hint at a significant amount of invisible matter in the universe. The nature of the constituent particle of this invisible, or dark, matter is unlike any of the particles described by the Standard Model of particle physics. Since almost three-quarters of the total matter content of the universe is estimated to be dark, the search for its constituent particle is thrilling, especially since it calls for an extension or modification to our current understanding of particle physics.
While the weakly interacting massive particle (WIMP) is the favorite candidate for dark matter particles, model-free theoretical descriptions of its interaction demand a broad experimental search. Hence, various collaborative efforts to directly detect interactions between these hypothesized WIMPs and the atomic nucleons (protons and neutrons) are under way with a variety of technologies and target nuclei. Direct detection of this particle involves identifying the small amount of energy deposited in the detector material as a WIMP collides (undergoes a nuclear recoil) with a nucleus in the detector material.
Since these highly sensitive nuclear recoil detectors aim to identify a rare WIMP interaction, the detection technology needs to be practically free of any backgrounds. Although they find refuge from the abundant cosmic rays in underground laboratories, these detectors are nevertheless prone to backgrounds from neutrons, radioactive decays that emit helium nuclei (alpha decays) and electron scatterings. Enclosed within a large mass of water, which provides shielding from the locally produced neutrons in the underground laboratory, various detector technologies use specialized techniques to distinguish the alpha decays and electron scatterings from the expected WIMP-nuclear recoil signals.
Superheated detectors, such as bubble chambers, offer a unique advantage as they are insensitive to electron scatterings even when the detector sensitivity is tuned to low-energy nuclear recoils (on the order of few thousand electronvolts, or keV). Due to this reason among others, the PICO collaboration has rejuvenated and augmented the bubble chamber technology for WIMP detection. A combination of optical, acoustic and pressure sensors is used to identify the candidate nuclear recoils in the bulk of the active mass of the detector. So far, the collaboration has operated a 2-liter (PICO-2L) and a 30-liter (PICO-60) bubble chamber detector in the Sudbury Neutrino Observatory underground laboratory (SNOLAB) in Sudbury, Ontario, Canada.
When the PICO group deployed their first bubble chamber detector in the beginning of 2014 with 2 liters of superheated fluorocarbon fluid in SNOLAB, it performed as well as expected, except for an unknown background signal that was observed during this experimental run. This signal was consistent with neither the known backgrounds nor WIMP signal and had characteristics that hinted at particulate contamination as the cause. The discovery of micron sized quartz and stainless steel particulates in the detector’s active fluid fortified this hypothesis. The first WIMP search with the PICO-2L, even with the unknown background, provided the world’s leading sensitivity to WIMP-proton spin-dependent interactions.
A profound effort was made to mitigate the particulate contamination, especially that of natural quartz. The 2-liter detector was redeployed in early 2015 with an ultraclean bubble chamber jar, along with various technical improvements and advancements. The modifications were such that the initial state of the bubble chamber was significantly cleaner, and the mechanisms of particulate generation during the experimental run were suppressed.
The effects were apparent right away. The observed events had a remarkably different spatial distribution from that of the first run. The data of the WIMP search during this second run showed an absence of the unknown background signal, and only one candidate nuclear recoil was observed, consistent with the expected neutron background. Since contamination control was the key to these results, much of the credit for this success is attributed to the work accomplished at Fermilab in the development of cleaning and fluid handling techniques.
The results from the second PICO-2L run are highlighted in the journal Physical Review D as an editor’s suggestion. These results present the strongest exclusion limits on the spin-dependent dark matter scatterings for WIMP masses less than 50 GeV/c2. For higher WIMP masses, the recent results from the PICO-60 experiment, also published in Physical Review D, provide the leading exclusion limits.
Now, with the absence of the unknown background and a low background signal, the collaboration is focusing on the larger bubble chamber detectors in their search for WIMP interactions. PICO-60 detector has been redeployed with its own ultraclean bubble chamber vessel in SNOLAB with 40 liters of active mass. Currently, in its initial tests and calibrations stage, it is well on the path to initiate a new WIMP search run this summer.
Chanpreet Amole is a Ph.D. candidate at Queen’s University in Canada.