Searching for dark matter with bubble chambers

The COUPP Collaboration (recently renamed PICO, after merging with the Canadian PICASSO experiment) uses bubble chambers to search for dark matter particles. Unlike the bubble chambers of the 1960s and 1970s, PICO chambers are operated in only mildly superheated conditions, rendering them insensitive to the minimum ionizing particles that typically constitute the largest background in dark matter searches. The bubble chamber allows for rejection of other backgrounds as well - for example, alpha decays can be discriminated from signal events because they sound louder. The result is a detector with minimal backgrounds that can be scaled to large masses. I will introduce the field of direct detection of dark matter and discuss the status of PICO, including results from the COUPP-4 detector and a progress report on COUPP-60 and PICO-2L, two chambers recently commissioned in the deep underground site of SNOLAB.