Searching for dark matter with superheated liquid detectors
One of the most prominent questions in the fields of particle physics and cosmology is the nature of dark matter which comprises 85% of the total mass of the universe. The PICASSO and PICO experiments are both direct detection experiments situated at SNOLAB that use the superheated liquid or bubble chamber technique to search for dark matter. The PICASSO collaboration pioneered the use of this technique for dark matter searches, and moreover, discovered an important background suppression feature: the acoustic alpha-neutron discrimination. The last PICASSO result was published in 2017 and still holds to this day the best spin-dependent cross-section limit of 7 x 10-2 pb (90% C.L.) for weakly interacting dark matter candidates (WIMPs) with a mass of 4 GeV/c2 [1]. Since the merger of PICASSO and COUPP into PICO, PICO holds the world best limit on WIMP cross sections with the most stringent spin-dependent limit of 2.5 x 10-5 pb (90% C.L) at 25 GeV/c2 set by the recent PICO60 detector result [2]. The PICO collaboration is currently building a new detector called PICO40L with a significantly improved design which will allow to substantially decrease the neutron background by a factor of  50, and pave the way forward for the next stage, PICO500, which will contain approximately 500L of superheated liquid. The present work presents first the observational and theoretical framework of dark matter searches, i.e., its proof of existence (Chap. 2), the most probable particle candidates (Chap. 3), as well as its expected recoil spectra and count rates in typical dark matter detectors (Chap. 4). It will be followed by a description of the superheated liquid technique (Chap. 5), by the description of the PICASSO and PICO detectors (Chap. 6 & 7), of their calibrations and common backgrounds (Chap. 8). In Chap. 9 & 10, the final PICASSO result are presented together with the most recent PICO dark matter limits. A GEANT4 simulation of the PICO40L neutron background will then be described in detail (Chap. 11), along with a discussion of the physic reach of PICO within the context of the effective field theory description of dark matter (Chap. 12). Finally, this thesis concludes with the current and future research and development program of the PICO collaboration, such as the future PICO500 detector, and the exciting possibility of using C2H2F4 as an active target (Chap. 13).
Dark matter PICO PICASSO WIMPs GEANT4 SNOLAB Simulation