|A calibration event from PICO-2L shows boiling from multiple recoiling atomic nuclei. PICO-2L is designed to see a recoiling nucleus a from dark matter interaction.|
We are not yet seeing dark matter, but we could.
Dark matter is all around us. There is six times more dark matter in our universe than there is the ordinary matter that we experience every day. Like neutrinos, dark matter can pass right through us and the Earth without being noticed. Moreover, dark matter does not interact with light, so we cannot see it, except perhaps in a bubble chamber.
Bubble chambers, including Fermilab’s 15-foot bubble chamber, were among the largest and most important particle detectors of the 1960s and 1970s investigating the physics of the weak force. In operation, bubble chambers are filled with liquid held above its boiling point temperature at the bubble chamber’s expansion pressure. The liquid boils if a nucleation site, such as a dust grain or a surface scratch, is available. In their absence, ionizing radiation (that is, particle tracks) can create bubbles. The exploding line of bubbles formed along a track lets physicists “see” the particle and photograph it.
The PICO collaboration, of which Fermilab is a member (formed from the merger of PICASSO and COUPP), has revisited bubble chamber technology in order to look for dark matter particles. While dark matter particles pass through the Earth, they may very occasionally bounce off an atomic nucleus. PICO bubble chambers can see these nuclear recoils. They do not see most other types of ionizing radiation that can emulate dark matter in other detector technologies. PICO can also see bubbles from alpha radiation, but these are distinguishable as they sound different from those made by dark matter. As a bubble rapidly grows, PICO uses ultrasonic acoustic sensors to measure the sound of this small explosion and to reject the louder bubbles created by alpha radiation.
The PICO-2L bubble chamber operated in 2013 and 2014 at SNOLAB, 6,800 feet underground in a Canadian nickel mine. Using 2 liters of perfluoropropane, C3F8, PICO-2L has set the world’s strongest proton spin-dependent dark matter search limits. Fluorine provides the most sensitive target nucleus to detect a spin-dependent dark matter interaction, which means future large bubble chambers may see, and hear, dark matter interactions that other detectors can not.
—Alan Robinson, University of Chicago