By Jessica Jen
An initiative spearheaded by a team of SLAC National Accelerator Laboratory (SLAC) researchers seeks to produce a highly sensitive detector of dark matter particles.
Daniel Akerib, professor of particle physics and astrophysics, is working with his 27-person team made of two professors, several graduate students, technical staff and scientific staff at SLAC to build the detector, LUX-Zeplin (LZ). Akerib and his team collaborate with researchers from 37 other institutions, according to the experiment’s website. Ultimately, LZ will be placed about a mile underground at the Sanford Underground Research Facility (SURF) in South Dakota, the deepest underground US lab that also hosted LZ’s predecessor LUX, among other particle physics experiments. The depth reduces the amount of cosmic rays that reach LZ and cause scattering inside the xenon.
LZ will be detecting weakly interacting massive particles (WIMPs), elementary particles that make up dark matter, to confirm their existence and explain aspects of particle physics that cannot be explained with current scientific understanding.
“Even though a lot of this is turning wrenches and stuff, it’s all for searching for how nature works at its tiniest,” said Ryan Linehan, a doctoral candidate in physics.
Building the grids
Underground, researchers are hoping that WIMPs are the only material to scatter xenon, creating a scintillation signal and free electrons. The four grids inside LZ create the electric field needed to move the electrons to the liquid surface, extract the electrons into a gas above, and accelerate them so that the electrons form a second scintillation signal. Both signals are measured and analyzed to locate where the scattering occurred. This location is where the WIMP was inside LZ.
To prepare for the second phase, Linehan is developing the grids and evaluating their abilities to suppress background electron radiation caused by the strong electric fields. The grids are placed in a vessel with xenon gas to evaluate their effectiveness, and the results are applied to redesigning the grids. Linehan and other group members build, test and eventually ship the grids to SURF.
“[It’s] going from, you have a spool of wire and a big set of rings, let’s put them together in a way that makes sense. Let’s keep them very clean, let’s make sure they don’t get destroyed in the process, which we struggle with sometimes,” Linehan said. “We learn about how to build them, and how not to build them, and then once we do that we learn how to build them better by testing them in things like Phase 2 and Phase 1.”
Labib Rahman ’21, a physics major, joined the project as a summer undergraduate researcher assisting Linehan by developing algorithms to automate the LZ grid metrology so that the grid wires are in position. He is also involved in the krypton removal process and runs the sampling system, a machine that tests xenon purity.
“We use liquid nitrogen to cool down the xenon. When xenon freezes any contaminant in xenon doesn’t freeze at that temperature, so it will freeze first,” Rahman said. “Everything else will pass through and the residual gas analyzer analyzes what there is so we can get a sense of what other contaminants there are.”
At SLAC, researchers are responsible for purifying the noble gas xenon to circulate inside LZ, perfecting electrical wire mesh grids used to detect the WIMPs and developing the software for simulations and data analysis.
Xenon is used in the detector because it has fewer radioactive isotopes than other noble gases, which would interfere with the light signals, according to Linehan. Xenon’s large size also increases chances of scattering dark matter. LZ will be very sensitive to any radioactivity or cosmic rays in the same way that a Geiger counter detects radioactivity. According to Akerib, a Geiger counter has only about a gram of detecting substance, compared to the 10 tons of xenon that LZ needs. The LZ xenon must be purified so that all the electrons will be absorbed by the WIMPs, not by other substances present. An especially difficult contaminant to remove is krypton, which is similar to xenon in unreactivity but has isotopes that will still absorb electrons.
“If we don’t make [the xenon] super pure, low radioactivity, the whole detector will just light itself up from the inside,” Akerib said. “That’s what the krypton will do if we don’t take an extra step to remove it.”
Researchers run helium gas through the unpurified xenon inside large tubes filled with activated charcoal to flush out the krypton. Xenon, having more mass and more electrons than krypton, will have more brief, weak attractions to the surrounding charcoal, making its journey slower than krypton’s. The krypton is collected and discarded, and the xenon is run through again to ensure thorough purification.
This phase of the SLAC LZ segment tests xenon circulation in a small-scale model of LZ. According to Linehan, the main purpose of this phase is to understand the circulation system better and resolve any problems that arise. This way the full-sized system in LZ will be as efficient as possible.
The projected year for all the institutions involved to bring their respective parts to SURF and assemble LZ is 2020, according to Akerib. Since WIMPs have only weak nuclear forces and no electric charge, they pass through the planet as if it was transparent. Once LZ is installed, Akerib hopes that some of the 100 billion WIMPs passing through LZ each second will be detected. Even if LZ does not show evidence of WIMPs passing through, Akerib could quantify the non-observation. The effective cross sectional area of WIMPs, the area between two particles required for them to scatter, would have to a certain size since, if it is larger, the experiment would have shown interactions.
LZ was funded about two years ago, and the research and development was done a few years before that. It was a follow-up to LUX, which was a smaller-scale version of LZ. LUX was funded in 2007, but it first started collecting data in 2013. Akerib predicts that LZ will run for at least three years, perhaps more depending on results and what modifications the team can make to the detector.
“Now these experiments cost $50 million, $500 million, and science communities, because money is tight, have become more risk averse,” Akerib said. “And so it takes a lot more planning up front to propose a project. You have to have so much understanding of what you’re going to do before you’ve done it.”
LZ can also be used for other experiments besides searching for WIMPs. According to Linehan, LZ could determine if neutrinos are their own antiparticles and discover axions, possibly a type of dark matter that would solve problem in particle physics.
“There’s a whole bunch of scientific inquiries that we can make using a giant detector like this,” Linehan said. “And that’s one of the cool things about it. We have a big group, but there’s also a lot to study.” After the scientists are finished using LZ, its parts will be redistributed elsewhere for new research projects.
“[The puzzle of dark matter is] a giant gap in our understanding of nature,” Linehan said. “And it has been like that for almost 100 years now. So solving it is very alluring, providing evidence for what it might be is very alluring.”
Akerib echoed this sentiment. “We want to know. That’s the main thing. Did Newton and Einstein get it right? The gravity that they figured out on the scale of the solar system, is that enough to explain the galaxy and the entire cosmos?” Akerib said. “Or is there a wrinkle to gravity that we don’t understand? But really, we’re just curious. As physicists we want to know how the world works.”
Contact Jessica Jen at jessicajen23 ‘at’ gmail.com.