By Ann Miao Wang
Several KIPAC members, myself included, are running an experiment almost a mile underground in South Dakota. We are looking for evidence of the invisible: dark matter!
Many astrophysical observations indirectly point to the existence of dark matter, which is predicted to make up nearly 85% of the matter in the Universe. Such a detection could point to the existence of a whole new zoo of theorized particles that have never been seen.
The LUX-ZEPLIN experiment
The LUX-ZEPLIN (LZ) experiment, located in the Sanford Underground Research Facility (SURF), has been collecting data since Fall 2021 to search for a dark matter particle called a Weakly Interacting Massive Particle (WIMP). WIMPs are theorized particles that are 10 to 1000 times more massive than a proton, and their existence is also predicted by other fundamental theories such as supersymmetry. The heart of the LZ experiment consists of a 10-tonne bath of liquid xenon to look for evidence of WIMPs, which scatter off the xenon atoms.
When any particle scatters in the xenon (not only dark matter particles), two light signals are produced: a prompt signal, called the “S1,” and, up to a millisecond later, a second delayed signal, called the “S2.” Both of these signals are used to reconstruct the time and the position of the particle that scattered. These signals are detected with arrays of sensors called photomultiplier tubes, which convert these signals into an electric current.
Members at KIPAC are responsible for a wide array of jobs to ensure that we don’t miss any of these potential signals from dark matter. We help ensure that the detector is running smoothly and that data moves from SURF to data centers, where it is processed for analysis. KIPAC also had a critical role in constructing key parts of the detector and building a prototype of the experiment! Currently, my day-to-day consists of working with other scientists across the world to analyze these signals to see if there is any evidence of dark matter.
How to find a dark matter particle
You can think of the LZ detector as being very sensitive to all particles, not just dark matter particles. One of the major challenges for this search is that other particles can travel through the xenon and produce similar signals.
So how would we know if we’re seeing dark matter?
First, the detector is constructed using materials that are extremely clean (contribute very little to background noise), to ensure that as few ordinary particles like electrons and photons bump into the xenon atoms as small as possible. We even cleaned the xenon itself. One radioactive contaminant, krypton, was filtered out using a charcoal column system designed and operated by members of KIPAC. The xenon bath is also surrounded by an outer detector system that can identify and capture neutrons that could scatter in the xenon and mimic dark matter particles.
Second, the detector location 4850 feet under the ground reduces the background from cosmic rays, which come from space and constantly stream through the Earth. SURF is in what was formerly known as the Homestake gold mine and is the deepest lab in the United States.
Finally, dark matter particles scattering in the detector mainly interact with the xenon nuclei, while electrons and photons interact with the xenon electron cloud. These two interaction types produce different S1 and S2 signal sizes, which is a powerful clue to help us differentiate scatters from ordinary matter.
World-leading sensitivity
In August 2024, the LZ collaboration announced its findings from analyzing the results of its largest dataset to date. The events which looked like particle scatters after analysis are shown in Figure 2, plotted to show the S1 (prompt) and S2 (delayed) signals.
The top plot shows the filtered dataset as black points. Each point shows where a particle likely scattered in the xenon and produced S1 and S2 signals. The purple areas indicate where typical WIMP dark matter events would lie, while the gray areas show where backgrounds from particles such as electrons or photons would lie. A closer look shows that almost all of the black data points are in the gray shaded regions. There is one data point near the bottom of the plot, which is likely to have come from an instrumental background source .
But just looking at the plot isn’t good enough. To better understand how consistent the data are with background-only sources, statistical analysis showed that this data likely contain no WIMP dark matter events and are consistent with ordinary matter.
However, despite the fact that we didn’t see any evidence of dark matter, the result showed that the LZ experiment has world-leading sensitivity to WIMPs!
The search continues - XLZD
The LZ experiment is still collecting data, and we look forward to using this added information to improve our sensitivity to dark matter events. In the meantime, physicists are already working on ideas for how to build a more sensitive experiment that is even better at looking for rare dark matter signals.
A new collaboration of physicists, which includes scientists at KIPAC, was formed this year to design and build a proposed experiment called XLZD. This experiment will be an order of magnitude more sensitive to dark matter particles and aims to have about 10 times more xenon than LZ. It will also be sensitive to neutrinos from various sources, including from the sun and a possible supernova!
Members at KIPAC are investigating better ways to remove radioactive contaminants and reduce the instrumental background, which become more challenging with the larger detector size. XLZD is still in the planning phase, but building the experiment will pave the way to continue the hunt for dark matter. Either with LZ or XLZD, dark matter could be detected in the near future!
More information:
Edited by Toby Satterthwaite, Lori Ann White, Jack Dinsmore, and Xinnan Du
