The stars at a galaxy’s edge rotate at speeds much faster than is possible if they were held in orbit only by the gravity from visible matter. This phenomenon suggests that some invisible source of gravity must also be at work—matter that does not reflect, emit, or otherwise interact with visible light, but still has mass. Since it cannot be seen, this theoretical substance is given the name “dark matter.”
According to cosmological understanding, approximately five times as much dark matter exists in the universe compared to visible matter. Dark matter plays a crucial role in the formation of the known universe, shaping its structure and appearance into what is observed today. “Dark matter is the anchor for the visible universe,” says Jingke Xu, a staff physicist and principal investigator for Lawrence Livermore’s LUX-ZEPLIN (LZ) dark matter search project. “To understand why we are here today and to understand the evolution of the universe, we need to look for dark matter and characterize it—or, if we really cannot find it, we need to consider revising our theory about the universe’s evolution.”
The global effort to detect and characterize dark matter has produced several compelling candidates—including weakly interacting massive particles (WIMPs), massive compact halo objects (MACHOs), axions, and sterile neutrinos—yet only small numbers of MACHOs (too few to account for all the dark matter mass that exists) have been definitively confirmed. Among these possibilities, WIMPs have emerged as the leading candidate when scientists initially proposed them in the 1980s. These tiny particles, which have relatively large mass and interact only through gravity and the weak nuclear force, offer a convenient and elegant solution to the dark matter puzzle: If even a single type of WIMP were added to the known family of particles, the Big Bang could have produced it in the abundance needed to explain today’s observations. Despite this theoretical appeal and decades of intensive research, scientists have yet to detect WIMPs, leaving the mystery unresolved. Lawrence Livermore has a storied approach in one avenue in the WIMP search—liquid xenon.
Liquid Xenon Detectors
The global search for WIMPs has taken many forms, from detection on Earth to observing the centers of galaxies—where dark matter is thought to be highly concentrated—for byproducts of dark matter interactions. Among these different methods, using a liquid xenon detector on Earth offers appealing advantages that have led to a several-decades-long endeavor into WIMP detection. Liquid xenon is straightforward to continually circulate and purify, so it can be used to build large, monolithic detectors. That property, along with its density, also makes it self-shielding; its outer regions absorb background radiation and leave a pure center for WIMP detection.
Liquid xenon’s most valuable feature, though, is its ability to scintillate brilliantly, meaning that energy deposited in the liquid generates a burst of light. Even a very small energy transfer, such as that expected from a dark matter particle interaction, leads to the production of many photons that can be detected with single-photon-sensitive photomultiplier tubes, allowing researchers to “see” dark matter’s interaction with the xenon. A longstanding design for a liquid xenon detector is a time projection chamber (TPC), in which an interaction can be seen twice—first via the immediate scintillation of photons as the affected xenon atoms de-excite, and again when electrons ionized from the atoms drift up an electric field into a gas volume above the liquid xenon. A higher-strength electric field in the gas accelerates the electrons rapidly, producing a second burst of scintillation.
Together, the two scintillations and the time between them can produce the exact horizontal (XY) and vertical (Z) coordinates of an interaction event inside the detector, which can help to identify background events. “The amount of light seen by different photomultiplier tubes can be used to reconstruct the position of the event,” says Livermore scientist Teal Pershing, who is the TPC and high-voltage grid coordinator for the LZ experiment. “The XY position is reconstructed with the top tubes, and the time difference between the two scintillations can be used to reconstruct the depth. As a result, we can reconstruct the full energy and position of particles inside the TPC.” Simulated data sets of the region and distribution for an actual WIMP signature inform the team about how such an event will appear, and the detector’s calibration validates the simulations. Livermore physicist Kareem Kazkaz developed simulation toolkits for the Large Underground Xenon (LUX) and LZ experiments, and Livermore staff scientist Rachel Mannino has overseen planning LZ detector calibrations.
Although a TPC can identify either a recoiling nucleus or electron when an incoming particle scatters off a xenon atom, WIMP scatters are predicted to produce mostly nuclear recoils, offering a focused search for scientists. “When performing a WIMP-based nuclear recoil search, we can specifically select regions where nuclear recoil-like signatures are expected and remove many fundamental backgrounds that come from radiation in the materials of the detector,” says Pershing. “Gamma rays often cause electron recoils, and we use the capabilities of the detector to identify and reject them.”
Livermore has conducted research on liquid xenon detection since the early 2000s as a founding member of the XENON10 experiment that Columbia University led in Italy. With Adam Bernstein leading the Livermore portion, this experiment—involving a detector with a total volume of 15 kilograms (kg) of pure liquid xenon—first demonstrated that xenon could be a successful medium for dark matter search, but not without leaving room for improvement.
The Bigger, The Better
After XENON10’s promising start, liquid xenon–based WIMP detection was poised to advance. Subsequent projects, including LUX (with which Livermore was involved), and XENON100, featured larger detectors for improved sensitivity, as increasing the volume of liquid xenon in a detector increases the likelihood of a WIMP interaction. Larger size also increases a detector’s self-shielding capabilities. Liquid xenon is about three times as dense as water, and in larger detectors, the outer layers of xenon absorb more background radiation, creating a better-shielded central region for WIMP detection. This pure center is known as the fiducial volume.
LUX was a dual-phase TPC specifically designed to detect the nuclear recoil signal produced from the collision of a WIMP with a liquid xenon atom (see S&TR, December 2012, Positively Scintillating Neutral Particles Brighten Scientific Prospects) while rejecting the dominant backgrounds identified in XENON10. Livermore played a critical role as a founding member of LUX and in delivering major TPC detector parts for the experiment. In addition to increasing in volume to 370 kg, LUX’s location—1.5 kilometers underground at the Sanford Underground Research Facility (SURF) in South Dakota—protected the detector from high-energy cosmic rays that would bombard it above ground and pollute the data. The LUX detector was designed to collect light efficiently and to strongly separate nuclear recoils from electron recoils. LUX successfully suppressed backgrounds to the point where ordinary gamma-ray interactions were extremely unlikely to cause WIMP-like recoils. This outcome enabled the team to establish with high confidence that interactions identified in the detector might indicate WIMP signals, rather than ordinary physics interactions.
Upon completion of the LUX experiment, researchers established the larger LZ experiment—a combination of LUX and the European ZEPLIN-III experiments. As with LUX and XENON10, Livermore was a founding member of the LZ research team. With funding from the Department of Energy (DOE) Office of Science’s Office of High Energy Physics, LZ is a 38-institution project featuring the largest-ever detector using TPC technology at 10 metric tons of total liquid xenon volume. Also located at SURF, LZ expands on the background shielding that made LUX a success, and its unmatched size greatly increases its sensitivity for a dark matter interaction. “Going from LUX with a 370-kilogram volume to LZ, which is 10 metric tons, was a crucial increase,” says Mannino, who also serves as run manager on LZ. “I could hold the grids for LUX in my hands, and for LZ, at least three people are needed to lift them. This increase in size provides more sensitivity because we have more space and opportunities to have an interaction.” Adds Xu, “LZ is similar in principle to other TPCs, but the devil is in the details. We got the details right, which has enabled us to build the biggest detector in the world.”
Several Livermore efforts have simultaneously pushed the boundaries of what such a detector could observe, starting with a Laboratory Directed Research and Development Program Strategic Initiative grant to improve sensitivity for low-energy xenon nuclear recoil measurements. As part of this project, Xu mixed argon and xenon in a single detector to explore their shared benefits—heavier xenon nuclei are more likely to interact with incoming particles but are less visible, while lighter argon nuclei create a more visible signal but are less likely to interact with incoming particles. This Livermore effort aims to combine the benefits of both types of detection media in a single system for the prospect of enhanced dark matter sensitivity. In addition, Xu received an Early Career Research Program (ECRP) Award to push xenon-based detectors to their sensitivity boundary, the minimum interaction strength that the detectors can reliably distinguish from background noise. Under ECRP, Xu’s group explored the attainable dark matter sensitivities both in the lowest and the highest energy regions in a xenon TPC. The success of these projects amplified Livermore’s impact on LZ and on the broader field of dark matter search. “Dark matter is a crucial component of our fundamental understanding of the universe and is a high priority for DOE,” says Xu.
To XLZD and Beyond
To date, LZ holds the record for WIMP sensitivity, surpassing that of XENON10 by a factor of 10,000 and LUX by a factor of 100. The experiment will continue collecting data until 2028 with a goal of 1,000 live days of data collection, but the WIMP search will not end there.
The ever-increasing size and sensitivity of liquid xenon WIMP detectors will advance in a new project, XENON LUX ZEPLIN DARWIN (XLZD), which will merge the LZ experiment with the multi-institutional XENONnT and Darwin projects. XLZD, of which Livermore is also a founding member, is a global affair, with nearly every institution and researcher involved in xenon-based dark matter research already collaborating to propose and, eventually, build the new detector. The effort, expected to be about 10 times better than LZ and will require significantly more resources for success, is a major step in understanding the makeup of the universe. “We think an elegant solution exists to many of the problems in cosmology, which is dark matter, and WIMPs are still seen as a likely candidate,” says Xu. “We aim to find them, which will open the door to a new era of particle physics and a more complete understanding of the universe, rather than having to start over with a new theory. But who knows? We’ll do our best, and nature will tell us if we are right or wrong.”
—Lilly Ackerman
For further information contact Jingke Xu (925) 422-5002 (xu12 [at] llnl.gov (xu12[at]llnl[dot]gov)).
