Modeling emission of acoustic energy during bubble expansion in PICO bubble chambers

The PICO experiment uses bubble chambers filled with superheated C₃ F₈ for spin-dependent weakly interacting massive particle (WIMP) dark matter searches. One of the main sources of background in these detectors is alpha particles from decays of environmental ²²²Rn, which nucleate bubbles that are visually indistinguishable from WIMP candidate events. Alpha-induced bubbles can be discriminated acoustically, because the signal from alpha events is consistently larger in magnitude than that from nuclear-recoil-/WIMP-like events. By studying the dynamics of bubbles nucleated by these two types of ionizing radiation from the first stages of their growth, we present a physical model for the acoustic discrimination for the first time. The distribution of acoustic energies that we generate for a simulated sample of bubble nucleations by alpha particles and nuclear recoils is compared directly to the experimental data.