The PICO-40L detector

For decades, astronomers have known that the universe is filled with something invisible. We cannot see it, and it does not shine, yet its gravity holds galaxies together and shapes the cosmos. This invisible substance is called dark matter. While we know it exists, we do not know what it is made of. One leading theory suggests it consists of massive particles that rarely interact with normal matter, known as Weakly Interacting Massive Particles, or WIMPs. If these particles exist, they should occasionally bump into the nuclei of atoms in a detector on Earth, creating a tiny flash of energy. The challenge is that these collisions are incredibly rare and produce signals so faint that they are easily drowned out by natural background noise, such as radiation from rocks or cosmic rays. To find them, scientists must build detectors deep underground, shielded by miles of rock, and use materials so pure that they do not emit their own radiation.

A team of researchers known as the PICO Collaboration has built a new detector to hunt for these elusive particles, and they have published a detailed report on its design and early performance. The detector, called PICO-40L, is a large bubble chamber located two kilometers underground in a mine in Sudbury, Canada. Unlike traditional detectors that use solid crystals or liquid tanks, this machine uses a special liquid called octafluoropropane, which is kept in a state where it is ready to boil but held back by pressure. When a particle hits an atom in this liquid, it deposits a tiny amount of energy, causing a small bubble to form instantly. The machine is designed to be so sensitive that it can detect the tiny energy deposit from a dark matter collision, while ignoring the much larger energy deposits from other types of radiation that do not create bubbles.

The PICO-40L detector represents a significant evolution in how these bubble chambers are built. Previous versions of the experiment used a design where the target liquid sat on top of a buffer fluid, separated by a single container. This older setup created a boundary between two different liquids, which could sometimes generate false signals. The new PICO-40L uses a completely different arrangement, which the team calls the "Right-Side-Up" design. In this configuration, the target liquid is held inside a double-walled glass vessel that sits at the bottom of the chamber, with the pressure system located underneath it rather than on top. This change removes the interface between two fluids and moves the mechanical parts that could create noise to a colder, inactive zone. The result is a cleaner environment where the detector is less likely to be tricked by background noise.

To keep the detector working correctly, the team had to solve a difficult thermal problem. The liquid inside the chamber must be kept at two different temperatures at the same time. The main volume where the dark matter search happens must be kept warm enough to be ready to form bubbles, typically around thirteen degrees Celsius. However, the bottom part of the vessel, where the pipes and mechanical seals are located, must be kept much colder, around minus twenty-two degrees Celsius. If the bottom gets too warm, the liquid there could start to boil on its own, creating bubbles that look like dark matter signals but are actually just thermal noise. The researchers built a complex system of cooling coils and heating plates to maintain these two distinct temperature zones, ensuring that bubbles only form in the active area where the cameras can see them.

The detector is equipped with a sophisticated system to watch for these bubbles. Four high-speed cameras are mounted around the chamber, taking pictures of the liquid from different angles. When a bubble forms, the cameras capture its growth, allowing scientists to calculate exactly where it happened and how big it got. Alongside the cameras, the machine uses twelve tiny microphones, known as piezoelectric transducers, pressed against the outside of the glass vessel. These microphones listen for the sound of the bubble forming. Different types of particles create different sounds; for example, alpha particles, which are a common source of background noise, make a distinct acoustic signature compared to the quiet pop of a dark matter candidate. By combining the visual data from the cameras with the sound data from the microphones, the team can tell the difference between a real signal and a false alarm.

The researchers tested the detector with two main runs. First, they ran a calibration test using a neutron source to simulate the kind of collisions they hope to find. Later, they ran a long commissioning period with no external source, just waiting to see what happened naturally. During these tests, the detector operated stably, successfully distinguishing between single bubbles in the liquid and unwanted events like bubbles forming on the walls of the vessel or multiple bubbles forming at once. The team found that the new design effectively suppressed the background noise that had plagued earlier versions. They calculated that the detector should see very few false signals from natural radioactivity or cosmic rays, with the main remaining background expected to come from a rare interaction involving neutrinos, a subatomic particle that is nearly impossible to shield against.

While the detector has not yet found a dark matter particle, the paper confirms that the new design works as intended. The PICO-40L successfully demonstrated that the Right-Side-Up geometry can operate reliably, keeping the target fluid superheated in the center while keeping the plumbing cold and quiet. This success paves the way for the next generation of the experiment, a much larger detector called PICO-500, which is currently under construction. By proving that this new design can handle the extreme sensitivity required for dark matter hunting, the PICO Collaboration has cleared a major hurdle in the search for the invisible substance that makes up most of the matter in our universe.