Physicists like me don’t fully understand what makes up about 83% of the matter of the universe — something we call “dark matter.” But with a tank filled with xenon buried nearly a mile under South Dakota, we’d sooner or later give you the option to measure what dark matter really is.
In the everyday model, dark matter accounts for many of the gravitational attraction within the universe, providing the glue that enables structures like galaxies, including our own Milky Way, to form. Because the solar system orbits across the center of the Milky Way, Earth moves through a dark matter halo, which makes up many of the matter in our galaxy.
I’m a physicist excited about understanding the character of dark matter. One popular guess is that dark matter is a brand new sort of particle, the Weakly Interacting Massive Particle, or WIMP. “WIMP” captures the particle’s essence quite nicely – it has mass, meaning it interacts gravitationally, nevertheless it otherwise interacts very weakly – or rarely – with normal matter. WIMPs within the Milky Way theoretically fly through us on Earth on a regular basis, but because they interact weakly, they only don’t hit anything.
Trying to find WIMPs
Over the past 30 years, scientists have developed an experimental program to attempt to detect the rare interactions between WIMPs and regular atoms. On Earth, nonetheless, we’re consistently surrounded by low, nondangerous levels of radioactivity coming from trace elements – mainly uranium and thorium – within the environment, in addition to cosmic rays from space. The goal in trying to find dark matter is to construct as sensitive a detector as possible, so it may well see the dark matter, and to place it in as quiet a spot as possible, so the dark matter signal will be seen over the background radioactivity.
With results published in July 2023, the LUX-ZEPLIN, or LZ, collaboration has done just that, constructing the biggest dark matter detector up to now and operating it 4,850 feet (1,478 meters) underground within the Sanford Underground Research Facility in Lead, South Dakota.
At the middle of LZ rests 10 metric tons (10,000 kilograms) of liquid xenon. When particles go through the detector, they could collide with xenon atoms, resulting in a flash of sunshine and the discharge of electrons.
In LZ, two massive electrical grids apply an electrical field across the quantity of liquid, which pushes these released electrons to the liquid’s surface. After they breach the surface, they’re pulled into the space above the liquid, which is full of xenon gas, and accelerated by one other electric field to create a second flash of sunshine.
Two large arrays of sunshine sensors collect these two flashes of sunshine, and together they permit researchers to reconstruct the position, energy and sort of interaction that took place.
Reducing radioactivity
All materials on Earth, including those utilized in WIMP detector construction, emit some radiation that would potentially mask dark matter interactions. Scientists subsequently construct dark matter detectors using essentially the most “radiopure” materials – that’s, freed from radioactive contaminants – they will find, each inside and outdoors the detector.
For instance, by working with metal foundries, LZ was in a position to use the cleanest titanium on Earth to construct the central cylinder – or cryostat – that holds the liquid xenon. Using this special titanium reduces the radioactivity in LZ, creating a transparent space to see any dark matter interactions. Moreover, liquid xenon is so dense that it actually acts as a radiation shield, and it is straightforward to purify the xenon of radioactive contaminants that may sneak in.
In LZ, the central xenon detector lives inside two other detectors, called the xenon skin and the outer detector. These supporting layers catch radioactivity on the way in which in or out of the central xenon chamber. Because dark matter interactions are so rare, a dark matter particle will only ever interact one time in your entire apparatus. Thus, if we observe an event with multiple interactions within the xenon or the outer detector, we will assume it’s not being attributable to a WIMP.
The hunt continues
Within the result just published, using 60 days of information, LZ recorded about five events per day within the detector. That is a couple of trillion fewer events than a typical particle detector on the surface would record in a day. By taking a look at the characteristics of those events, researchers can safely say that no interaction thus far has been attributable to dark matter. The result’s, alas, not a discovery of recent physics – but we will set limits on exactly how weakly dark matter must interact, because it stays unseen by LZ.
These limits help to inform physicists what dark matter just isn’t – and LZ does that higher than any experiment on the planet. Meanwhile, there’s hope for what comes next within the seek for dark matter. LZ is collecting more data now, and we expect to take greater than 15 times more data over the subsequent few years. A WIMP interaction may already be in that data set, just waiting to be revealed in the subsequent round of study.
