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

## TL;DR

This paper develops a physical model to understand and improve acoustic discrimination between alpha particles and nuclear recoils in bubble chambers used for dark matter detection, enhancing background rejection.

## Contribution

It introduces a novel physical model of bubble growth dynamics that explains acoustic signals for the first time in PICO bubble chambers.

## Key findings

- The model accurately predicts the acoustic energy distribution for alpha and recoil events.
- Simulation results match experimental data, validating the model.
- Improved discrimination enhances dark matter search sensitivity.

## Abstract

The PICO experiment uses bubble chambers filled with superheated C$_3$F$_8$ for spin-dependent WIMP dark matter searches. One of the main sources of background in these detectors is alpha particles from decays of environmental $^{222}\mathrm{Rn}$, 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.